test(app): add manual performance diagnostics (#32937)

packages/app/AGENTS.md

@@ -1,3 +1,8 @@
+## Priorities
+
+- Prioritise, in this order: stability, simplicity, performance.
+- Before changing session or timeline code, record a production benchmark baseline and compare it after the change.
+
 ## Debugging
 
 - NEVER try to restart the app, or the server process, EVER.

packages/app/e2e/performance/AGENTS.md

@@ -0,0 +1,13 @@
+- Prioritize stability, then simplicity, then measurement overhead.
+- Use Playwright for scenario control, isolation, and completion checks.
+- Use Chrome Performance traces for generic browser profiling.
+- Use Electron `contentTracing` for packaged multi-process profiling.
+- Keep custom probes only for product-specific measurements.
+- Do not duplicate measurements across the harness, probes, and traces.
+- Run benchmarks serially to avoid cross-test contention.
+- Run benchmarks against production builds.
+- Keep detailed profiling opt-in when it changes workload behavior.
+- Preserve raw diagnostic data or use lossless representations.
+- Do not enforce machine-dependent performance thresholds.
+- Assert scenario completion and metric collection only.
+- Keep normal test discovery free of manual benchmarks.

packages/app/e2e/performance/README.md

@@ -0,0 +1,77 @@
+# Manual app performance suite
+
+The app's high-volume performance diagnostics live under `packages/app/e2e/performance` and are excluded from normal local and CI Playwright discovery. The benchmark config builds the app and serves the production bundle before running scenarios serially.
+
+Run the suite explicitly from `packages/app`:
+
+```sh
+bun run test:bench
+```
+
+PowerShell:
+
+```powershell
+$env:PLAYWRIGHT_WORKERS = "1"
+bun run test:bench
+```
+
+The suite contains:
+
+- cold and hot session-tab timing
+- cached session repaint and mutation tracing
+- streaming timeline throughput, RAF-gap, long-task, geometry, and remount diagnostics
+
+All benchmarks import the shared `benchmark` fixture. Pages created through Playwright's `page` fixture automatically capture main-frame navigation history and emit a Chrome trace when `OPENCODE_PERFORMANCE_TRACE_DIR` is set. Benchmarks that need isolated browser contexts use `withBenchmarkPage`, which owns the context and the same diagnostics lifecycle.
+
+New benchmarks should look like normal Playwright tests:
+
+```ts
+import { benchmark, expect } from "../benchmark"
+
+benchmark("measures one interaction", async ({ page, report }) => {
+  // Only scenario-specific setup and interaction belong here.
+  report({ durationMs: 42 })
+})
+```
+
+The fixture requires every benchmark to call `report()`, automatically names and closes traces, captures navigation history, attaches that history when a test fails, and emits metrics as a consistent `BENCHMARK` JSON line.
+
+```text
+BENCHMARK {"name":"...","context":{"project":"chromium","platform":"darwin"},"metrics":{...}}
+```
+
+Every observed page also emits `BENCHMARK_PAGE` with the same run ID, navigation history, and optional trace path before the final status-bearing `BENCHMARK` record. Chrome traces are browser-wide page-lifetime diagnostics; scenario metrics use narrower explicitly named observation windows.
+
+This follows the stack's own guidance: [Electron recommends repeated Chrome DevTools and Chrome Tracing measurement](https://www.electronjs.org/docs/latest/tutorial/performance), [Chrome DevTools recommends Performance recordings for runtime work](https://developer.chrome.com/docs/devtools/performance), and [Playwright uses traces for test debugging rather than renderer profiling](https://playwright.dev/docs/trace-viewer).
+
+These Playwright benchmarks profile the shared app renderer in Chromium. A future packaged Electron benchmark that needs main-process and multi-process attribution should use Electron's official [`contentTracing`](https://www.electronjs.org/docs/latest/api/content-tracing/) API rather than extending this renderer harness with bespoke process instrumentation.
+
+CPU and high-volume visual profiling are disabled by default. Set `TIMELINE_CPU_PROFILE=1` to enable both, or additionally set `TIMELINE_VISUAL_PROFILE=0` for CPU-only profiling.
+
+The streaming scenario's 30x CPU throttle is a deterministic stress profile, not a simulated end-user device.
+
+Benchmarks do not assert machine-dependent performance budgets. Streaming processes 160 deltas by default and reports renderer-observed completion time, throughput, RAF callback-gap distributions, frame-budget equivalents, and long tasks through final geometry settlement. Delta count and delivery batch are included in result context when overridden. These are main-thread callback diagnostics, not compositor presentation or dropped-frame measurements. Visual-only and geometry metrics are `null` when their probes are disabled. Tab metrics describe sampled DOM observations. Assertions verify scenario and metric collection completion. Repeated repaint states are run-length grouped, but every original observation timestamp is retained alongside raw mutation batches and layout shifts.
+
+Committed smoke and regression tests continue to own correctness coverage for pagination, tab paint, context resize, collapse state, and composer spacing.
+
+## Chrome traces
+
+Set `OPENCODE_PERFORMANCE_TRACE_DIR` to emit a standard Chrome DevTools trace for every benchmark page automatically:
+
+```sh
+OPENCODE_PERFORMANCE_TRACE_DIR=/tmp/opencode-performance-traces \
+bunx playwright test --config e2e/performance/playwright.config.ts \
+  timeline/session-tab-switch-benchmark.spec.ts
+```
+
+The emitted JSON is a standard Chrome trace and can be loaded directly into the Chrome DevTools Performance panel. `devtools-tracing` can optionally inspect it from the command line without adding package scripts or dependencies:
+
+Trace capture mirrors [Puppeteer's official tracing defaults and lifecycle](https://pptr.dev/api/puppeteer.tracing), using Chrome's `ReturnAsStream` transfer mode and failing when Chromium reports trace data loss.
+
+```sh
+bunx devtools-tracing stats <trace-path-from-BENCHMARK_PAGE>
+```
+
+INP analysis requires a trace with a supported navigation/interaction insight. Selector statistics require a trace captured with `OPENCODE_PERFORMANCE_SELECTOR_TRACE=1`.
+
+`e2e/performance/playwright.uncapped.config.ts` disables Chromium frame-rate limiting for explicit uncapped diagnostics. Native product benchmarks should use the default Playwright configuration.

packages/app/e2e/performance/benchmark.ts

@@ -0,0 +1,144 @@
+import { expect, test as base, type Browser, type Page, type TestInfo } from "@playwright/test"
+import { startChromeTrace } from "./chrome-trace"
+
+type BenchmarkFixtures = {
+  report: (metrics: Record<string, unknown>, context?: Record<string, unknown>) => void
+  reportState: { payload?: { metrics: Record<string, unknown>; context: Record<string, unknown> } }
+  benchmarkResult: void
+}
+
+export type PerformancePageDiagnostics = {
+  navigations: string[]
+  stop: () => Promise<string | undefined>
+}
+
+const pages = new WeakMap<Page, PerformancePageDiagnostics>()
+
+export const benchmark = base.extend<BenchmarkFixtures>({
+  reportState: async ({}, use) => use({}),
+  report: async ({ reportState }, use) => {
+    await use((metrics, context = {}) => {
+      if (reportState.payload) throw new Error("Benchmark reported metrics more than once")
+      reportState.payload = { metrics, context }
+    })
+  },
+  benchmarkResult: [
+    async ({ reportState }, use, testInfo) => {
+      await use()
+      const missing = !reportState.payload
+      console.log(
+        `BENCHMARK ${JSON.stringify({
+          schemaVersion: 2,
+          runID: process.env.OPENCODE_PERFORMANCE_RUN_ID,
+          name: benchmarkName(testInfo),
+          status: missing ? "failed" : testInfo.status,
+          expectedStatus: testInfo.expectedStatus,
+          retry: testInfo.retry,
+          repeatEachIndex: testInfo.repeatEachIndex,
+          context: {
+            project: testInfo.project.name,
+            platform: process.platform,
+            ...reportState.payload?.context,
+          },
+          metrics: reportState.payload?.metrics ?? null,
+          error: missing ? "Benchmark did not report metrics" : undefined,
+        })}`,
+      )
+      if (missing && testInfo.status === testInfo.expectedStatus)
+        throw new Error(`Benchmark did not report metrics: ${benchmarkName(testInfo)}`)
+    },
+    { auto: true },
+  ],
+  page: async ({ page }, use, testInfo) => {
+    const name = benchmarkName(testInfo)
+    const diagnostics = await observePerformancePage(page, name)
+    try {
+      await use(page)
+    } finally {
+      try {
+        await reportPerformancePage(name, diagnostics, testInfo)
+      } finally {
+        if (testInfo.status !== testInfo.expectedStatus) {
+          await testInfo.attach("performance-navigations", {
+            body: JSON.stringify(diagnostics.navigations, null, 2),
+            contentType: "application/json",
+          })
+        }
+      }
+    }
+  },
+})
+
+function benchmarkName(testInfo: TestInfo) {
+  return testInfo.titlePath.slice(1).join(" > ")
+}
+
+export { expect }
+
+async function observePerformancePage(page: Page, name: string) {
+  const navigations: string[] = []
+  const onNavigation = (frame: ReturnType<Page["mainFrame"]>) => {
+    if (frame === page.mainFrame()) navigations.push(frame.url())
+  }
+  page.on("framenavigated", onNavigation)
+  const stopTrace = await startChromeTrace(page, name).catch((error) => {
+    page.off("framenavigated", onNavigation)
+    throw error
+  })
+  let stopping: Promise<string | undefined> | undefined
+  const diagnostics: PerformancePageDiagnostics = {
+    navigations,
+    stop() {
+      page.off("framenavigated", onNavigation)
+      return (stopping ??= stopTrace?.() ?? Promise.resolve(undefined))
+    },
+  }
+  pages.set(page, diagnostics)
+  return diagnostics
+}
+
+export async function withBenchmarkPage<T>(
+  browser: Browser,
+  name: string,
+  run: (page: Page) => Promise<T>,
+  testInfo?: TestInfo,
+) {
+  const context = await browser.newContext()
+  try {
+    const page = await context.newPage()
+    const diagnostics = await observePerformancePage(page, name)
+    try {
+      return await run(page)
+    } finally {
+      await reportPerformancePage(name, diagnostics, testInfo)
+    }
+  } finally {
+    await context.close()
+  }
+}
+
+async function reportPerformancePage(name: string, diagnostics: PerformancePageDiagnostics, testInfo?: TestInfo) {
+  const trace = await diagnostics.stop()
+  console.log(
+    `BENCHMARK_PAGE ${JSON.stringify({
+      schemaVersion: 2,
+      runID: process.env.OPENCODE_PERFORMANCE_RUN_ID,
+      name,
+      test: testInfo ? benchmarkName(testInfo) : undefined,
+      retry: testInfo?.retry,
+      repeatEachIndex: testInfo?.repeatEachIndex,
+      context: {
+        platform: process.platform,
+        trace,
+        selectorTrace: process.env.OPENCODE_PERFORMANCE_SELECTOR_TRACE === "1",
+      },
+      navigations: diagnostics.navigations,
+    })}`,
+  )
+}
+
+export function benchmarkDiagnostics(page: Page) {
+  const diagnostics = pages.get(page)
+  if (!diagnostics) throw new Error("Performance diagnostics are not installed for this page")
+  return diagnostics
+}

packages/app/e2e/performance/chrome-trace.ts

@@ -0,0 +1,95 @@
+import type { CDPSession, Page } from "@playwright/test"
+import path from "node:path"
+import { mkdir, open, rename } from "node:fs/promises"
+import { Buffer } from "node:buffer"
+import { createHash, randomUUID } from "node:crypto"
+
+const categories = [
+  "-*",
+  "devtools.timeline",
+  "v8.execute",
+  "disabled-by-default-devtools.timeline",
+  "disabled-by-default-devtools.timeline.frame",
+  "toplevel",
+  "blink.console",
+  "blink.user_timing",
+  "latencyInfo",
+  "disabled-by-default-devtools.timeline.stack",
+  "disabled-by-default-v8.cpu_profiler",
+]
+
+export async function startChromeTrace(page: Page, name: string) {
+  const directory = process.env.OPENCODE_PERFORMANCE_TRACE_DIR
+  if (!directory) return
+
+  const selectors = process.env.OPENCODE_PERFORMANCE_SELECTOR_TRACE === "1"
+  const file = await prepareChromeTrace(directory, name, selectors)
+  const session = await page.context().newCDPSession(page)
+  try {
+    await session.send("Tracing.start", {
+      transferMode: "ReturnAsStream",
+      traceConfig: {
+        excludedCategories: categories
+          .filter((category) => category.startsWith("-"))
+          .map((category) => category.slice(1)),
+        includedCategories: [
+          ...categories.filter((category) => !category.startsWith("-")),
+          ...(selectors
+            ? ["disabled-by-default-blink.debug", "disabled-by-default-devtools.timeline.invalidationTracking"]
+            : []),
+        ],
+      },
+    })
+  } catch (error) {
+    await Promise.allSettled([session.detach()])
+    throw error
+  }
+  let stopping: Promise<string> | undefined
+
+  return () =>
+    (stopping ??= (async () => {
+      try {
+        const complete = new Promise<{ stream?: string; dataLossOccurred: boolean }>((resolve) =>
+          session.once("Tracing.tracingComplete", resolve),
+        )
+        await session.send("Tracing.end")
+        const result = await complete
+        if (!result.stream) throw new Error(`Chrome trace stream missing: ${file}`)
+        const partial = `${file}.partial`
+        await writeProtocolStream(session, result.stream, partial)
+        if (result.dataLossOccurred) throw new Error(`Chrome trace lost data; partial capture retained: ${partial}`)
+        await rename(partial, file)
+        return file
+      } finally {
+        await Promise.allSettled([session.detach()])
+      }
+    })())
+}
+
+export async function prepareChromeTrace(
+  directory: string,
+  name: string,
+  selectors: boolean,
+  nonce = randomUUID().slice(0, 8),
+) {
+  await mkdir(directory, { recursive: true })
+  const run = process.env.OPENCODE_PERFORMANCE_RUN_ID ?? "manual"
+  const hash = createHash("sha256").update(name).digest("hex").slice(0, 8)
+  return path.join(
+    directory,
+    `${run}-${name.replace(/[^a-zA-Z0-9_-]/g, "-")}-${hash}-${nonce}${selectors ? "-selectors" : ""}.json`,
+  )
+}
+
+async function writeProtocolStream(session: CDPSession, handle: string, file: string) {
+  const output = await open(file, "wx")
+  try {
+    while (true) {
+      const chunk = await session.send("IO.read", { handle })
+      await output.write(chunk.base64Encoded ? Buffer.from(chunk.data, "base64") : chunk.data)
+      if (chunk.eof) break
+    }
+  } finally {
+    await Promise.allSettled([output.close(), session.send("IO.close", { handle })])
+  }
+}

packages/app/e2e/performance/playwright.config.ts

@@ -0,0 +1,20 @@
+import config from "../../playwright.config"
+
+const port = Number(process.env.PLAYWRIGHT_PORT ?? 3000)
+process.env.PLAYWRIGHT_SERVER_PORT = String(port)
+process.env.OPENCODE_PERFORMANCE_RUN_ID ??= `${new Date().toISOString().replace(/[:.]/g, "-")}-${process.pid}`
+
+export default {
+  ...config,
+  testDir: ".",
+  testIgnore: "unit/**",
+  outputDir: "../test-results/performance",
+  fullyParallel: false,
+  workers: 1,
+  reporter: [["html", { outputFolder: "../playwright-report/performance", open: "never" }], ["line"]],
+  webServer: {
+    ...config.webServer,
+    command: `bun run build && bun run serve -- --host 0.0.0.0 --port ${port} --strictPort`,
+    reuseExistingServer: false,
+  },
+}

packages/app/e2e/performance/playwright.uncapped.config.ts

@@ -0,0 +1,13 @@
+import config from "./playwright.config"
+
+export default {
+  ...config,
+  outputDir: "../test-results/performance-uncapped",
+  reporter: [["html", { outputFolder: "../playwright-report/performance-uncapped", open: "never" }], ["line"]],
+  use: {
+    ...config.use,
+    launchOptions: {
+      args: ["--disable-frame-rate-limit", "--disable-gpu-vsync"],
+    },
+  },
+}

packages/app/e2e/performance/timeline/session-tab-flash.spec.ts

@@ -0,0 +1,49 @@
+import { benchmark, expect } from "../benchmark"
+import { expectSessionTitle } from "../../utils/waits"
+import { fixture } from "./session-timeline-stress.fixture"
+import {
+  collectCachedRepaintTrace,
+  compressCachedRepaintTrace,
+  installCachedRepaintProbe,
+  waitForCachedRepaintWindow,
+} from "./session-tab-repaint-probe"
+import { waitForStableTimeline } from "./session-tab-switch-probe"
+import {
+  installStressSessionTabs,
+  installTimelineSettings,
+  mockStressTimeline,
+  stressSessionHref,
+} from "./timeline-test-helpers"
+
+benchmark("samples cached session repaint after the click", async ({ page, report }) => {
+  benchmark.setTimeout(120_000)
+  await mockStressTimeline(page)
+  await installStressSessionTabs(page)
+  await installTimelineSettings(page)
+  await page.goto(stressSessionHref(fixture.targetID))
+  await expectSessionTitle(page, fixture.expected.targetTitle)
+  await waitForStableTimeline(page, fixture.expected.targetMessageIDs.at(-1)!)
+  await page
+    .locator(`[data-slot="titlebar-tabs"] a[href="${stressSessionHref(fixture.sourceID)}"]`)
+    .first()
+    .click()
+  await expectSessionTitle(page, fixture.expected.sourceTitle)
+  await waitForStableTimeline(page, fixture.expected.sourceMessageIDs.at(-1)!)
+
+  await installCachedRepaintProbe(page, {
+    targetHref: stressSessionHref(fixture.targetID),
+    destination: fixture.messages[fixture.targetID].map((message) => message.info.id),
+    source: fixture.messages[fixture.sourceID].map((message) => message.info.id),
+    last: fixture.expected.targetMessageIDs.at(-1)!,
+    windowMs: 1_000,
+  })
+
+  await page
+    .locator(`[data-slot="titlebar-tabs"] a[href="${stressSessionHref(fixture.targetID)}"]`)
+    .first()
+    .click()
+  await Promise.all([expectSessionTitle(page, fixture.expected.targetTitle), waitForCachedRepaintWindow(page, 1_000)])
+  const result = await collectCachedRepaintTrace(page)
+  report(compressCachedRepaintTrace(result))
+  expect(result.samples.length).toBeGreaterThan(0)
+})

packages/app/e2e/performance/timeline/session-tab-repaint-probe.ts

@@ -0,0 +1,251 @@
+import type { Page } from "@playwright/test"
+
+type CachedRepaintTrace = {
+  timeOriginEpochMs: number
+  startedAtPerformanceMs: number
+  samples: {
+    observedAtMs: number
+    root: number | undefined
+    scrollTop: number
+    scrollHeight: number
+    bottomErrorPx: number | undefined
+    last: boolean
+    rows: { key: string | undefined; node: number; top: number; bottom: number }[]
+    mounted: number
+    center: string | undefined
+    destination: string[]
+    source: string[]
+  }[]
+  mutations: { observedAtMs: number; changed: { type: string; node: number }[] }[]
+  shifts: { occurredAtMs: number; value: number }[]
+  windowMs: number
+  running: boolean
+  stop: () => void
+}
+
+export async function installCachedRepaintProbe(
+  page: Page,
+  input: { targetHref: string; destination: string[]; source: string[]; last: string; windowMs: number },
+) {
+  await page.evaluate(({ targetHref, destination, source, last, windowMs }) => {
+    const destinationIDs = new Set(destination)
+    const sourceIDs = new Set(source)
+    const nodeIDs = new WeakMap<Node, number>()
+    let nextNodeID = 1
+    const id = (node: Node) => {
+      const current = nodeIDs.get(node)
+      if (current) return current
+      nodeIDs.set(node, nextNodeID)
+      return nextNodeID++
+    }
+    const state: CachedRepaintTrace = {
+      timeOriginEpochMs: performance.timeOrigin,
+      startedAtPerformanceMs: 0,
+      samples: [],
+      mutations: [],
+      shifts: [],
+      windowMs,
+      running: false,
+      stop: () => {},
+    }
+    const recordShifts = (entries: PerformanceEntry[]) => {
+      if (!state.running) return
+      state.shifts.push(
+        ...entries
+          .map((entry) => {
+            if (
+              entry.startTime < state.startedAtPerformanceMs ||
+              entry.startTime > state.startedAtPerformanceMs + state.windowMs
+            )
+              return
+            return {
+              occurredAtMs: entry.startTime - state.startedAtPerformanceMs,
+              value: (entry as PerformanceEntry & { value: number }).value,
+            }
+          })
+          .filter((entry): entry is { occurredAtMs: number; value: number } => entry !== undefined),
+      )
+    }
+    const shiftObserver = new PerformanceObserver((entries) => recordShifts(entries.getEntries()))
+    shiftObserver.observe({ type: "layout-shift" })
+    const recordMutations = (entries: MutationRecord[]) => {
+      if (!state.running) return
+      const observedAtMs = performance.now() - state.startedAtPerformanceMs
+      if (observedAtMs > state.windowMs) return
+      const changed = entries.flatMap((entry) => [
+        ...[...entry.addedNodes].map((node) => ({ type: "add", node: id(node) })),
+        ...[...entry.removedNodes].map((node) => ({ type: "remove", node: id(node) })),
+      ])
+      if (changed.length) state.mutations.push({ observedAtMs, changed })
+    }
+    const mutationObserver = new MutationObserver(recordMutations)
+    mutationObserver.observe(document.documentElement, { childList: true, subtree: true })
+    state.stop = () => {
+      recordShifts(shiftObserver.takeRecords())
+      recordMutations(mutationObserver.takeRecords())
+      state.running = false
+      shiftObserver.disconnect()
+      mutationObserver.disconnect()
+    }
+    const sample = () => {
+      if (!state.running) return
+      setTimeout(() => {
+        if (!state.running) return
+        const observedAtMs = performance.now() - state.startedAtPerformanceMs
+        if (observedAtMs > state.windowMs) return
+        const root = [...document.querySelectorAll<HTMLElement>(".scroll-view__viewport")].find((element) =>
+          element.querySelector("[data-timeline-row]"),
+        )
+        if (root) {
+          const view = root.getBoundingClientRect()
+          const rows = [...root.querySelectorAll<HTMLElement>("[data-timeline-key]")]
+            .map((element) => ({
+              key: element.dataset.timelineKey,
+              node: id(element),
+              rect: element.getBoundingClientRect(),
+            }))
+            .filter((item) => item.rect.bottom > view.top && item.rect.top < view.bottom)
+            .map((item) => ({
+              key: item.key,
+              node: item.node,
+              top: item.rect.top - view.top,
+              bottom: item.rect.bottom - view.top,
+            }))
+          const messages = [...root.querySelectorAll<HTMLElement>("[data-message-id]")]
+            .filter((element) => {
+              const rect = element.getBoundingClientRect()
+              return rect.bottom > view.top && rect.top < view.bottom
+            })
+            .map((element) => element.dataset.messageId!)
+          const spacer = root.querySelector<HTMLElement>('[data-timeline-row="bottom-spacer"]')?.getBoundingClientRect()
+          state.samples.push({
+            observedAtMs,
+            root: id(root),
+            scrollTop: root.scrollTop,
+            scrollHeight: root.scrollHeight,
+            bottomErrorPx: spacer ? spacer.bottom - view.bottom : undefined,
+            last: messages.includes(last),
+            rows,
+            mounted: root.querySelectorAll("[data-timeline-key]").length,
+            center: document
+              .elementFromPoint(view.left + view.width / 2, view.top + view.height / 2)
+              ?.textContent?.slice(0, 80),
+            destination: messages.filter((messageID) => destinationIDs.has(messageID)),
+            source: messages.filter((messageID) => sourceIDs.has(messageID)),
+          })
+        } else {
+          state.samples.push({
+            observedAtMs,
+            root: undefined,
+            scrollTop: 0,
+            scrollHeight: 0,
+            bottomErrorPx: undefined,
+            last: false,
+            rows: [],
+            mounted: 0,
+            center: document.elementFromPoint(innerWidth / 2, innerHeight / 2)?.textContent?.slice(0, 80),
+            destination: [],
+            source: [],
+          })
+        }
+        requestAnimationFrame(sample)
+      }, 0)
+    }
+    document.addEventListener(
+      "click",
+      (event) => {
+        const link = event.target instanceof Element ? event.target.closest("a") : undefined
+        if (link?.getAttribute("href") !== targetHref) return
+        state.startedAtPerformanceMs = performance.now()
+        state.running = true
+        requestAnimationFrame(sample)
+      },
+      { capture: true, once: true },
+    )
+    ;(window as Window & { __cachedFlash?: CachedRepaintTrace }).__cachedFlash = state
+  }, input)
+}
+
+export function layoutShiftSample(entry: Pick<PerformanceEntry, "startTime"> & { value: number }, started: number) {
+  if (entry.startTime < started) return
+  return { occurredAtMs: entry.startTime - started, value: entry.value }
+}
+
+export async function waitForCachedRepaintWindow(page: Page, durationMs: number) {
+  await page.waitForFunction((durationMs) => {
+    const state = (window as Window & { __cachedFlash?: CachedRepaintTrace }).__cachedFlash
+    return !!state?.running && performance.now() - state.startedAtPerformanceMs >= durationMs
+  }, durationMs)
+}
+
+export async function collectCachedRepaintTrace(page: Page) {
+  return page.evaluate(() => {
+    const state = (window as Window & { __cachedFlash?: CachedRepaintTrace }).__cachedFlash!
+    state.stop()
+    return state
+  })
+}
+
+export function summarizeCachedRepaintTrace(trace: CachedRepaintTrace) {
+  const roots = trace.samples.map((sample) => sample.root)
+  const bottomErrors = trace.samples.flatMap((sample) =>
+    sample.bottomErrorPx === undefined ? [] : [Math.abs(sample.bottomErrorPx)],
+  )
+  const category = (sample: CachedRepaintTrace["samples"][number]) => {
+    if (sample.source.length) return "source"
+    if (sample.root === undefined || sample.rows.length === 0) return "blank"
+    if (!sample.destination.length) return "unknown"
+    if (sample.last && Math.abs(sample.bottomErrorPx ?? Infinity) <= 1) return "correct"
+    return "wrongDestination"
+  }
+  return {
+    samples: trace.samples.length,
+    durationMs: trace.samples.at(-1)?.observedAtMs ?? 0,
+    firstSampleObservedMs: trace.samples[0]?.observedAtMs,
+    firstSampleCorrect: trace.samples[0] ? category(trace.samples[0]) === "correct" : false,
+    blankSamples: trace.samples.filter((sample) => category(sample) === "blank").length,
+    sourceSamples: trace.samples.filter((sample) => category(sample) === "source").length,
+    wrongDestinationSamples: trace.samples.filter((sample) => category(sample) === "wrongDestination").length,
+    unknownSamples: trace.samples.filter((sample) => category(sample) === "unknown").length,
+    rootChanges: roots.slice(1).filter((root, index) => root !== roots[index]).length,
+    mountedMin: trace.samples.length ? Math.min(...trace.samples.map((sample) => sample.mounted)) : 0,
+    mountedMax: Math.max(...trace.samples.map((sample) => sample.mounted)),
+    maxBottomErrorPx: Math.max(0, ...bottomErrors),
+    mutationBatches: trace.mutations.length,
+    addedNodes: trace.mutations.reduce(
+      (sum, batch) => sum + batch.changed.filter((change) => change.type === "add").length,
+      0,
+    ),
+    removedNodes: trace.mutations.reduce(
+      (sum, batch) => sum + batch.changed.filter((change) => change.type === "remove").length,
+      0,
+    ),
+    layoutShiftValueSum: trace.shifts.reduce((sum, shift) => sum + shift.value, 0),
+    maxLayoutShiftValue: Math.max(0, ...trace.shifts.map((shift) => shift.value)),
+  }
+}
+
+export function compressCachedRepaintTrace(trace: CachedRepaintTrace) {
+  const samples: {
+    observedAtMs: number[]
+    state: Omit<CachedRepaintTrace["samples"][number], "observedAtMs">
+  }[] = []
+  for (const sample of trace.samples) {
+    const { observedAtMs, ...state } = sample
+    const previous = samples.at(-1)
+    if (previous && JSON.stringify(previous.state) === JSON.stringify(state)) {
+      previous.observedAtMs.push(observedAtMs)
+      continue
+    }
+    samples.push({ observedAtMs: [observedAtMs], state })
+  }
+  return {
+    timeOriginEpochMs: trace.timeOriginEpochMs,
+    startedAtPerformanceMs: trace.startedAtPerformanceMs,
+    windowMs: trace.windowMs,
+    summary: summarizeCachedRepaintTrace(trace),
+    samples,
+    mutations: trace.mutations,
+    shifts: trace.shifts,
+  }
+}

packages/app/e2e/performance/timeline/session-tab-switch-benchmark.spec.ts

@@ -0,0 +1,79 @@
+import type { Page } from "@playwright/test"
+import { expectSessionTitle } from "../../utils/waits"
+import { benchmark, expect, withBenchmarkPage } from "../benchmark"
+import { fixture } from "./session-timeline-stress.fixture"
+import { installStressSessionTabs, mockStressTimeline, stressSessionHref } from "./timeline-test-helpers"
+import { measureSessionSwitch, waitForStableTimeline } from "./session-tab-switch-probe"
+
+type Result = Awaited<ReturnType<typeof measureSessionSwitch>>
+
+benchmark("benchmarks cold and hot session tab switching", async ({ browser, report }, testInfo) => {
+  benchmark.setTimeout(180_000)
+  const results = { cold: [] as Result[], hot: [] as Result[] }
+  for (const mode of ["cold", "hot"] as const) {
+    for (let run = 0; run < 5; run++) {
+      results[mode].push(
+        await withBenchmarkPage(browser, `session-tab-switch-${mode}-${run}`, (page) => trial(page, mode), testInfo),
+      )
+    }
+  }
+  report({ results, summary: summarize(results) })
+})
+
+async function trial(page: Page, mode: "cold" | "hot") {
+  await mockStressTimeline(page)
+  await installStressSessionTabs(page)
+  if (mode === "hot") {
+    await page.goto(stressSessionHref(fixture.targetID))
+    await expectSessionTitle(page, fixture.expected.targetTitle)
+    await waitForStableTimeline(page, fixture.expected.targetMessageIDs.at(-1)!)
+    await switchSession(page, fixture.sourceID, fixture.expected.sourceTitle)
+  } else {
+    await page.goto(stressSessionHref(fixture.sourceID))
+    await expectSessionTitle(page, fixture.expected.sourceTitle)
+  }
+  await waitForStableTimeline(page, fixture.expected.sourceMessageIDs.at(-1)!)
+
+  const destinationIDs = fixture.messages[fixture.targetID].map((message) => message.info.id)
+  const sourceIDs = fixture.messages[fixture.sourceID].map((message) => message.info.id)
+  const lastID = fixture.expected.targetMessageIDs.at(-1)!
+  const href = stressSessionHref(fixture.targetID)
+  const result = await measureSessionSwitch(page, {
+    destinationIDs,
+    sourceIDs,
+    lastID,
+    href,
+    switch: () => switchSession(page, fixture.targetID, fixture.expected.targetTitle),
+  })
+  return result
+}
+
+function summarize(results: Record<"cold" | "hot", Result[]>) {
+  const stats = (values: (number | null)[]) => {
+    const sorted = values.filter((value): value is number => value !== null).sort((a, b) => a - b)
+    return {
+      min: sorted[0] ?? null,
+      median: sorted[Math.floor(sorted.length / 2)] ?? null,
+      max: sorted.at(-1) ?? null,
+      missing: values.length - sorted.length,
+    }
+  }
+  return Object.fromEntries(
+    Object.entries(results).map(([mode, values]) => [
+      mode,
+      {
+        firstDestinationObservedMs: stats(values.map((value) => value.firstDestinationObservedMs)),
+        firstCorrectObservedMs: stats(values.map((value) => value.firstCorrectObservedMs)),
+        stableObservedMs: stats(values.map((value) => value.stableObservedMs)),
+      },
+    ]),
+  )
+}
+
+async function switchSession(page: Page, sessionID: string, title: string) {
+  const href = stressSessionHref(sessionID)
+  const tab = page.locator(`[data-slot="titlebar-tabs"] a[href="${href}"]`).first()
+  await expect(tab).toBeVisible()
+  await tab.click()
+  await expectSessionTitle(page, title)
+}

packages/app/e2e/performance/timeline/session-tab-switch-metrics.ts

@@ -0,0 +1,46 @@
+export type SessionSwitchSample = {
+  observedAtMs: number
+  destination: string[]
+  source: string[]
+  hasVisibleRows: boolean
+  last: boolean
+  bottomErrorPx?: number
+}
+
+export function classifySessionSwitch(samples: SessionSwitchSample[]) {
+  const firstDestination = samples.findIndex((sample) => sample.destination.length > 0)
+  const firstCorrect = samples.findIndex(isCorrectDestination)
+  const stable = samples.findIndex((_, index) => isStableSessionSwitch(samples.slice(index, index + 3)))
+  return {
+    firstDestinationObservedMs: samples[firstDestination]?.observedAtMs ?? null,
+    firstCorrectObservedMs: samples[firstCorrect]?.observedAtMs ?? null,
+    stableObservedMs: samples[stable + 2]?.observedAtMs ?? null,
+    wrongDestinationSamples: samples
+      .slice(firstDestination)
+      .filter((sample) => sample.destination.length > 0 && !sample.last).length,
+    blankSamples: samples.filter((sample) => !sample.hasVisibleRows).length,
+    unknownSamples: samples.filter(
+      (sample) => sample.hasVisibleRows && sample.destination.length === 0 && sample.source.length === 0,
+    ).length,
+    sourceSamples: samples.filter((sample) => sample.source.length > 0).length,
+  }
+}
+
+export function isCorrectDestination(sample: SessionSwitchSample) {
+  return (
+    sample.destination.length > 0 &&
+    sample.source.length === 0 &&
+    sample.last &&
+    Math.abs(sample.bottomErrorPx ?? Infinity) <= 1
+  )
+}
+
+export function isStableSessionSwitch(samples: SessionSwitchSample[]) {
+  return samples.length === 3 && samples.every(isCorrectDestination)
+}
+
+export function isStableDestination(samples: Pick<SessionSwitchSample, "last" | "bottomErrorPx">[]) {
+  return (
+    samples.length === 3 && samples.every((sample) => sample.last && Math.abs(sample.bottomErrorPx ?? Infinity) <= 1)
+  )
+}

packages/app/e2e/performance/timeline/session-tab-switch-probe.ts

@@ -0,0 +1,152 @@
+import { expect, type Page } from "@playwright/test"
+import { classifySessionSwitch, isStableDestination, type SessionSwitchSample } from "./session-tab-switch-metrics"
+
+type SessionSwitchProbe = {
+  samples: SessionSwitchSample[]
+  stop: () => void
+}
+
+async function installSessionSwitchProbe(
+  page: Page,
+  input: { destinationIDs: string[]; sourceIDs: string[]; lastID: string; href: string },
+) {
+  await page.evaluate(({ destinationIDs, sourceIDs, lastID, href }) => {
+    const destination = new Set(destinationIDs)
+    const source = new Set(sourceIDs)
+    const samples: SessionSwitchSample[] = []
+    let started: number | undefined
+    let running = true
+    const sample = () => {
+      if (!running || started === undefined) return
+      setTimeout(() => {
+        if (!running || started === undefined) return
+        const observedAtMs = performance.now() - started
+        const root = [...document.querySelectorAll<HTMLElement>(".scroll-view__viewport")].find((element) =>
+          element.querySelector("[data-timeline-row]"),
+        )
+        if (root) {
+          const view = root.getBoundingClientRect()
+          const visible = [...root.querySelectorAll<HTMLElement>("[data-message-id]")]
+            .filter((element) => {
+              const rect = element.getBoundingClientRect()
+              return rect.bottom > view.top && rect.top < view.bottom
+            })
+            .map((element) => element.dataset.messageId!)
+          const hasVisibleRows = [...root.querySelectorAll<HTMLElement>("[data-timeline-key]")].some((element) => {
+            const rect = element.getBoundingClientRect()
+            return rect.bottom > view.top && rect.top < view.bottom
+          })
+          const spacer = root.querySelector<HTMLElement>('[data-timeline-row="bottom-spacer"]')?.getBoundingClientRect()
+          samples.push({
+            observedAtMs,
+            destination: visible.filter((id) => destination.has(id)),
+            source: visible.filter((id) => source.has(id)),
+            hasVisibleRows,
+            last: visible.includes(lastID),
+            bottomErrorPx: spacer ? spacer.bottom - view.bottom : undefined,
+          })
+        } else {
+          samples.push({ observedAtMs, destination: [], source: [], hasVisibleRows: false, last: false })
+        }
+        requestAnimationFrame(sample)
+      }, 0)
+    }
+    document.addEventListener(
+      "click",
+      (event) => {
+        const link = event.target instanceof Element ? event.target.closest("a") : undefined
+        if (link?.getAttribute("href") !== href) return
+        started = performance.now()
+        requestAnimationFrame(sample)
+      },
+      { capture: true, once: true },
+    )
+    ;(window as Window & { __sessionSwitchProbe?: SessionSwitchProbe }).__sessionSwitchProbe = {
+      samples,
+      stop: () => {
+        running = false
+      },
+    }
+  }, input)
+}
+
+async function waitForStableSessionSwitch(page: Page) {
+  await page.waitForFunction(() => {
+    const samples = (window as Window & { __sessionSwitchProbe?: SessionSwitchProbe }).__sessionSwitchProbe?.samples
+    if (!samples) return false
+    return samples.some((_, index) => {
+      const stable = samples.slice(index, index + 3)
+      return (
+        stable.length === 3 &&
+        stable.every(
+          (sample) =>
+            sample.destination.length > 0 &&
+            sample.source.length === 0 &&
+            sample.last &&
+            Math.abs(sample.bottomErrorPx ?? Infinity) <= 1,
+        )
+      )
+    })
+  })
+}
+
+async function collectSessionSwitchResult(page: Page) {
+  const samples = await page.evaluate(() => {
+    const probe = (window as Window & { __sessionSwitchProbe?: SessionSwitchProbe }).__sessionSwitchProbe!
+    probe.stop()
+    return probe.samples
+  })
+  return classifySessionSwitch(samples)
+}
+
+export async function measureSessionSwitch(
+  page: Page,
+  input: { destinationIDs: string[]; sourceIDs: string[]; lastID: string; href: string; switch: () => Promise<void> },
+) {
+  const { switch: run, ...probe } = input
+  await installSessionSwitchProbe(page, probe)
+  await run()
+  await waitForStableSessionSwitch(page)
+  return collectSessionSwitchResult(page)
+}
+
+export async function waitForStableTimeline(page: Page, lastID: string) {
+  const samples: Pick<SessionSwitchSample, "last" | "bottomErrorPx">[] = []
+  await expect
+    .poll(
+      async () => {
+        samples.push(
+          await page.evaluate(
+            (lastID) =>
+              new Promise<Pick<SessionSwitchSample, "last" | "bottomErrorPx">>((resolve) => {
+                requestAnimationFrame(() =>
+                  setTimeout(() => {
+                    const root = [...document.querySelectorAll<HTMLElement>(".scroll-view__viewport")].find((element) =>
+                      element.querySelector("[data-timeline-row]"),
+                    )
+                    if (!root) {
+                      resolve({ last: false })
+                      return
+                    }
+                    const view = root.getBoundingClientRect()
+                    const last = [...root.querySelectorAll<HTMLElement>("[data-message-id]")].some((element) => {
+                      if (element.dataset.messageId !== lastID) return false
+                      const rect = element.getBoundingClientRect()
+                      return rect.bottom > view.top && rect.top < view.bottom
+                    })
+                    const spacer = root
+                      .querySelector<HTMLElement>('[data-timeline-row="bottom-spacer"]')
+                      ?.getBoundingClientRect()
+                    resolve({ last, bottomErrorPx: spacer ? spacer.bottom - view.bottom : undefined })
+                  }, 0),
+                )
+              }),
+            lastID,
+          ),
+        )
+        return isStableDestination(samples.slice(-3))
+      },
+      { timeout: 30_000, intervals: [0] },
+    )
+    .toBe(true)
+}

packages/app/e2e/performance/timeline/session-timeline-benchmark.fixture.ts

@@ -0,0 +1,488 @@
+import { base64Encode } from "@opencode-ai/core/util/encode"
+import type { Page } from "@playwright/test"
+import { mockOpenCodeServer } from "../../utils/mock-server"
+import { expectAppVisible, expectSessionTitle } from "../../utils/waits"
+import { expect } from "../benchmark"
+
+const directory = "C:/OpenCode/TimelineStateRegression"
+const projectID = "proj_timeline_state_regression"
+const sessionID = "ses_timeline_state_regression"
+const userMessageID = "msg_user_regression"
+const assistantMessageID = "msg_assistant_regression"
+const editPartID = "prt_0001_edit"
+export const textPartID = "prt_9999_text"
+const title = "Timeline collapse state regression"
+const model = { providerID: "opencode", modelID: "claude-opus-4-6", variant: "max" }
+
+type EventPayload = {
+  directory: string
+  payload: Record<string, unknown>
+}
+
+const userMessage = {
+  info: {
+    id: userMessageID,
+    sessionID,
+    role: "user",
+    time: { created: 1700000000000 },
+    summary: { diffs: [] },
+    agent: "build",
+    model,
+  },
+  parts: [
+    {
+      id: "prt_user_text",
+      sessionID,
+      messageID: userMessageID,
+      type: "text",
+      text: "Please edit the file.",
+    },
+  ],
+}
+
+const editPart = {
+  id: editPartID,
+  sessionID,
+  messageID: assistantMessageID,
+  type: "tool",
+  callID: "call_edit_regression",
+  tool: "edit",
+  state: {
+    status: "completed",
+    input: { filePath: "src/regression.ts" },
+    output: "Edited src/regression.ts",
+    title: "src/regression.ts",
+    metadata: {
+      filediff: {
+        file: "src/regression.ts",
+        additions: 1,
+        deletions: 1,
+        before: "export const value = 'before'\n",
+        after: "export const value = 'after'\n",
+      },
+      diff: "diff --git a/src/regression.ts b/src/regression.ts\n-export const value = 'before'\n+export const value = 'after'\n",
+    },
+    time: { start: 1700000001000, end: 1700000002000 },
+  },
+}
+
+const streamedTextPart = {
+  id: textPartID,
+  sessionID,
+  messageID: assistantMessageID,
+  type: "text",
+  text: "Streaming added a later assistant text part.",
+}
+
+const assistantMessage = {
+  info: {
+    id: assistantMessageID,
+    sessionID,
+    role: "assistant",
+    time: { created: 1700000001000 },
+    parentID: userMessageID,
+    modelID: model.modelID,
+    providerID: model.providerID,
+    mode: "build",
+    agent: "build",
+    path: { cwd: directory, root: directory },
+    cost: 0.01,
+    tokens: { input: 100, output: 200, reasoning: 0, cache: { read: 0, write: 0 } },
+    variant: "max",
+  },
+  parts: [editPart],
+}
+
+export async function setupTimelineBenchmark(page: Page, options: { historyTurns: number; eventBatch: number }) {
+  const events: EventPayload[] = []
+  let eventBatch = options.eventBatch
+  await mockOpenCodeServer(page, {
+    directory,
+    project: project(),
+    provider: provider(),
+    sessions: [session()],
+    pageMessages: () => ({
+      items: [
+        ...Array.from({ length: options.historyTurns }, (_, index) => performanceTurn(index)).flat(),
+        userMessage,
+        assistantMessage,
+      ],
+    }),
+    events: () => events.splice(0, eventBatch),
+    eventRetry: 16,
+  })
+  await page.addInitScript(() => {
+    localStorage.setItem(
+      "settings.v3",
+      JSON.stringify({
+        general: {
+          editToolPartsExpanded: true,
+          shellToolPartsExpanded: true,
+          showReasoningSummaries: true,
+          showSessionProgressBar: true,
+        },
+      }),
+    )
+  })
+  await page.setViewportSize({ width: 1366, height: 768 })
+  const scroller = page.locator(".scroll-view__viewport", { has: page.locator("[data-timeline-row]") })
+  const text = page.locator(`[data-timeline-part-id="${textPartID}"]`).first()
+  await page.goto(`/${base64Encode(directory)}/session/${sessionID}`)
+  await expectSessionTitle(page, title)
+  await expectAppVisible(scroller)
+  return {
+    scroller,
+    text,
+    transport: {
+      enqueue(payload: EventPayload | EventPayload[]) {
+        events.push(...(Array.isArray(payload) ? payload : [payload]))
+      },
+      pendingCount() {
+        return events.length
+      },
+      releaseAll() {
+        eventBatch = events.length
+      },
+    },
+    async scrollToBottom() {
+      await scroller.evaluate((element) => {
+        element.scrollTop = element.scrollHeight
+      })
+    },
+    async waitForStableGeometry() {
+      await expect
+        .poll(() => scroller.evaluate((element) => element.scrollHeight - element.clientHeight - element.scrollTop))
+        .toBeLessThanOrEqual(1)
+      await page.waitForFunction((partID) => {
+        const root = [...document.querySelectorAll<HTMLElement>(".scroll-view__viewport")].find((element) =>
+          element.querySelector(`[data-timeline-part-id="${partID}"]`),
+        )
+        if (!root) return false
+        return new Promise<boolean>((resolve) => {
+          const height = root.scrollHeight
+          requestAnimationFrame(() =>
+            requestAnimationFrame(() =>
+              resolve(root.scrollHeight === height && root.scrollHeight - root.clientHeight - root.scrollTop <= 1),
+            ),
+          )
+        })
+      }, textPartID)
+    },
+  }
+}
+
+export function buildInitialStreamEvent(deltaCount: number): EventPayload {
+  return {
+    directory,
+    payload: {
+      type: "message.part.updated",
+      properties: {
+        part: {
+          ...streamedTextPart,
+          text: `Streaming${streamChunk(0, deltaCount + 1)}\n\n\`\`\`ts\nconst initial = true\n\`\`\``,
+        },
+      },
+    },
+  }
+}
+
+export function buildStreamDeltaEvents(deltaCount: number): EventPayload[] {
+  return Array.from({ length: deltaCount }, (_, index) => ({
+    directory,
+    payload: {
+      type: "message.part.delta",
+      properties: {
+        messageID: assistantMessageID,
+        partID: textPartID,
+        field: "text",
+        delta: streamChunk(index + 1, deltaCount + 1),
+      },
+    },
+  }))
+}
+
+function performanceTurn(index: number) {
+  const suffix = String(index).padStart(4, "0")
+  const userID = `msg_0000_${suffix}_a_user`
+  const assistantID = `msg_0000_${suffix}_b_assistant`
+  const before = historicalSource(index, false)
+  const after = historicalSource(index, true)
+  const parts = [
+    ...(index % 5 === 0
+      ? [
+          {
+            id: `prt_0000_${suffix}_reasoning`,
+            sessionID,
+            messageID: assistantID,
+            type: "reasoning",
+            text: `Reviewing the existing implementation. ${"constraint analysis ".repeat(20)}`,
+            time: { start: 1690000001000 + index * 2_000, end: 1690000001200 + index * 2_000 },
+          },
+        ]
+      : []),
+    {
+      id: `prt_0000_${suffix}_assistant`,
+      sessionID,
+      messageID: assistantID,
+      type: "text",
+      text: historicalMarkdown(index),
+    },
+    ...(index % 8 === 0
+      ? [
+          {
+            id: `prt_0000_${suffix}_edit`,
+            sessionID,
+            messageID: assistantID,
+            type: "tool",
+            callID: `call_0000_${suffix}_edit`,
+            tool: "edit",
+            state: {
+              status: "completed",
+              input: { filePath: `src/history-${index}.ts` },
+              output: `Edited src/history-${index}.ts`,
+              title: `src/history-${index}.ts`,
+              metadata: {
+                filediff: { file: `src/history-${index}.ts`, additions: 48, deletions: 48, before, after },
+              },
+              time: { start: 1690000001200 + index * 2_000, end: 1690000001400 + index * 2_000 },
+            },
+          },
+        ]
+      : []),
+    ...(index % 12 === 0
+      ? [
+          {
+            id: `prt_0000_${suffix}_write`,
+            sessionID,
+            messageID: assistantID,
+            type: "tool",
+            callID: `call_0000_${suffix}_write`,
+            tool: "write",
+            state: {
+              status: "completed",
+              input: { filePath: `src/generated-${index}.tsx`, content: after },
+              output: `Wrote src/generated-${index}.tsx`,
+              title: `src/generated-${index}.tsx`,
+              metadata: {
+                filediff: { file: `src/generated-${index}.tsx`, additions: 32, deletions: 0, before: "", after },
+              },
+              time: { start: 1690000001400 + index * 2_000, end: 1690000001500 + index * 2_000 },
+            },
+          },
+        ]
+      : []),
+    ...(index % 16 === 0
+      ? [
+          {
+            id: `prt_0000_${suffix}_patch`,
+            sessionID,
+            messageID: assistantID,
+            type: "tool",
+            callID: `call_0000_${suffix}_patch`,
+            tool: "apply_patch",
+            state: {
+              status: "completed",
+              input: { patchText: realisticPatch(index) },
+              output: "Success. Updated src/components/SessionCard.tsx",
+              title: "src/components/SessionCard.tsx",
+              metadata: {
+                files: [
+                  {
+                    filePath: "src/components/SessionCard.tsx",
+                    relativePath: "src/components/SessionCard.tsx",
+                    type: "update",
+                    additions: 8,
+                    deletions: 3,
+                    patch: realisticPatch(index),
+                    before,
+                    after,
+                  },
+                ],
+              },
+              time: { start: 1690000001500 + index * 2_000, end: 1690000001700 + index * 2_000 },
+            },
+          },
+        ]
+      : []),
+  ]
+  return [
+    {
+      info: {
+        id: userID,
+        sessionID,
+        role: "user",
+        time: { created: 1690000000000 + index * 2_000 },
+        summary: { diffs: [] },
+        agent: "build",
+        model,
+      },
+      parts: [
+        {
+          id: `prt_0000_${suffix}_user`,
+          sessionID,
+          messageID: userID,
+          type: "text",
+          text: `Historical prompt ${index}`,
+        },
+      ],
+    },
+    {
+      info: {
+        id: assistantID,
+        sessionID,
+        role: "assistant",
+        time: { created: 1690000001000 + index * 2_000, completed: 1690000001500 + index * 2_000 },
+        parentID: userID,
+        modelID: model.modelID,
+        providerID: model.providerID,
+        mode: "build",
+        agent: "build",
+        path: { cwd: directory, root: directory },
+        cost: 0.01,
+        tokens: { input: 100, output: 200, reasoning: 0, cache: { read: 0, write: 0 } },
+        variant: "max",
+        finish: "stop",
+      },
+      parts,
+    },
+  ]
+}
+
+function historicalMarkdown(index: number) {
+  const code = `import { For, Show, createSignal } from "solid-js"
+
+type SessionRow = { id: string; title: string; active: boolean }
+
+export function SessionList(props: { rows: SessionRow[] }) {
+  const [selected, setSelected] = createSignal<string>()
+  return (
+    <section aria-label="Sessions">
+      <For each={props.rows}>{(row) => (
+        <button classList={{ active: row.active }} onClick={() => setSelected(row.id)}>
+          <Show when={selected() === row.id} fallback={row.title}>{row.title.toUpperCase()}</Show>
+        </button>
+      )}</For>
+    </section>
+  )
+}`
+  return `## Session renderer review ${index}
+
+The active session keeps **semantic row identity** while reconciling measured content. See [Solid documentation](https://docs.solidjs.com/) and the inline \`measureElement(node)\` call.
+
+| Concern | Current behavior | Verification |
+| --- | --- | --- |
+| streaming | appends Markdown blocks | painted frames |
+| geometry | anchors visible rows | DOM coordinates |
+| tools | preserves expanded state | keyed remount probe |
+
+> Long sessions combine Markdown, syntax highlighting, tool output, and asynchronously rendered diffs.
+
+${index % 4 === 0 ? `\`\`\`tsx\n${code}\n\`\`\`\n\n\`\`\`bash\nbun typecheck\nbun test --preload ./happydom.ts ./src/pages/session\ngit diff --check\n\`\`\`` : "- preserve the viewport anchor\n- avoid replacing stable Markdown nodes\n- process provider deltas without blocking input"}`
+}
+
+function historicalSource(index: number, updated: boolean) {
+  const method = updated ? "toLocaleUpperCase(props.locale)" : "toUpperCase()"
+  const limit = updated ? 24 : 20
+  return `import { createMemo, For } from "solid-js"
+
+type Message = {
+  id: string
+  role: "user" | "assistant"
+  text: string
+  tokens: { input: number; output: number }
+}
+
+export function MessageSummary(props: { messages: Message[]; locale: string }) {
+  const visible = createMemo(() => props.messages.filter((message) => message.text.trim()).slice(-${limit}))
+  const total = createMemo(() => visible().reduce((sum, message) => sum + message.tokens.output, 0))
+  return (
+    <article data-session-index="${index}">
+      <header>{total().toLocaleString(props.locale)} output tokens</header>
+      <For each={visible()}>{(message) => <p data-role={message.role}>{message.text.${method}}</p>}</For>
+    </article>
+  )
+}
+`
+}
+
+function realisticPatch(index: number) {
+  return `*** Begin Patch
+*** Update File: src/components/SessionCard.tsx
+@@
+-const title = props.session.title.toUpperCase()
+-const messages = props.messages.slice(-20)
++const title = props.session.title.toLocaleUpperCase(props.locale)
++const messages = props.messages.filter((message) => message.text.trim()).slice(-24)
++const outputTokens = messages.reduce((sum, message) => sum + message.tokens.output, 0)
+@@
+-  <h2>{title}</h2>
++  <h2 data-session-index="${index}">{title}</h2>
++  <span>{outputTokens.toLocaleString(props.locale)} output tokens</span>
+*** End Patch`
+}
+
+export function streamChunk(index: number, count: number) {
+  if (index === 0) return `\n\n## Implementation plan\n\nStreaming **bold analysis`
+  if (index === count - 1)
+    return `\n\`\`\`\n\n## Verification\n\n- **Typecheck:** passed\n- **Timeline geometry:** stable\n- **Streaming output:** benchmark-complete <!-- stream-${index} -->`
+
+  const section = Math.floor(index / 18) + 1
+  const fragments = [
+    ` continues across three`,
+    ` or four word`,
+    ` provider deltas and`,
+    ` closes in this fragment**. <!-- stream-${index} -->\n\n`,
+    `| Concern | State`,
+    ` | Verification |\n|`,
+    ` --- | ---`,
+    ` | --- |\n|`,
+    ` markdown | incremental |`,
+    ` painted frames | <!-- stream-${index} -->\n\n`,
+    `\`\`\`tsx\nconst row: SessionRow`,
+    ` = rows[index] ??`,
+    ` fallback\nconst title =`,
+    ` row.title.toLocaleUpperCase(locale)\n`,
+    `const selected = createMemo(()`,
+    ` => row.id ===`,
+    ` activeID()) // stream-${index}\n`,
+    `// stream-${index}\n\`\`\`\n\n### Iteration ${section}\n\nStreaming **bold analysis`,
+  ]
+  return fragments[(index - 1) % fragments.length]!
+}
+
+function project() {
+  return {
+    id: projectID,
+    worktree: directory,
+    vcs: "git",
+    name: "timeline-state-regression",
+    time: { created: 1700000000000, updated: 1700000000000 },
+    sandboxes: [],
+  }
+}
+
+function session() {
+  return {
+    id: sessionID,
+    slug: "timeline-state-regression",
+    projectID,
+    directory,
+    title,
+    version: "dev",
+    time: { created: 1700000000000, updated: 1700000000000 },
+  }
+}
+
+function provider() {
+  return {
+    all: [
+      {
+        id: "opencode",
+        name: "OpenCode",
+        models: { "claude-opus-4-6": { id: "claude-opus-4-6", name: "Claude Opus 4.6", limit: { context: 200_000 } } },
+      },
+    ],
+    connected: ["opencode"],
+    default: { providerID: "opencode", modelID: "claude-opus-4-6" },
+  }
+}

packages/app/e2e/performance/timeline/session-timeline-benchmark.spec.ts

@@ -0,0 +1,85 @@
+import { benchmark, benchmarkDiagnostics, expect } from "../benchmark"
+import {
+  buildInitialStreamEvent,
+  buildStreamDeltaEvents,
+  setupTimelineBenchmark,
+  textPartID,
+} from "./session-timeline-benchmark.fixture"
+import { startTimelineProfile } from "./session-timeline-profile"
+import {
+  collectTimelineStreamMetrics,
+  installTimelineStreamProbe,
+  startTimelineStreamProbe,
+} from "./session-timeline-stream-probe"
+
+benchmark.describe("performance: session timeline streaming", () => {
+  benchmark("streams assistant text without remounting or oscillating", async ({ page, report }) => {
+    benchmark.setTimeout(480_000)
+    const cpuThrottle = Number(process.env.TIMELINE_CPU_THROTTLE ?? 30)
+    const deltaCount = Number(process.env.TIMELINE_DELTA_COUNT ?? 160)
+    const historyTurns = Number(process.env.TIMELINE_HISTORY_TURNS ?? 320)
+    const eventBatch = Number(process.env.TIMELINE_EVENT_BATCH ?? 1)
+    const minimal = process.env.TIMELINE_MINIMAL === "1"
+    const profileCPU = process.env.TIMELINE_CPU_PROFILE === "1"
+    const profileVisual = !minimal && profileCPU && process.env.TIMELINE_VISUAL_PROFILE !== "0"
+    const fixture = await setupTimelineBenchmark(page, {
+      historyTurns,
+      eventBatch,
+    })
+
+    fixture.transport.enqueue(buildInitialStreamEvent(deltaCount))
+    const contentStart = performance.now()
+    await expect(fixture.text).toBeVisible()
+    await expect(fixture.text).toContainText("Implementation plan")
+    const initialContentObservedMs = performance.now() - contentStart
+    await fixture.scrollToBottom()
+    await fixture.waitForStableGeometry()
+
+    const profile = await startTimelineProfile(page, { cpuThrottle, profileCPU })
+    await installTimelineStreamProbe(page, { textPartID, finalIndex: deltaCount, profileVisual, minimal })
+    const deltas = buildStreamDeltaEvents(deltaCount)
+    await startTimelineStreamProbe(page)
+    fixture.transport.enqueue(deltas)
+
+    await page.waitForFunction(
+      (finalIndex) =>
+        (
+          window as Window & {
+            __timelineStreamBenchmark?: { applied: { index: number }[] }
+          }
+        ).__timelineStreamBenchmark?.applied.some((value) => value.index === finalIndex),
+      deltaCount,
+      { timeout: 420_000 },
+    )
+    await expect(fixture.text).toContainText("benchmark-complete")
+    await expect(fixture.text).toContainText("Streaming")
+    await fixture.waitForStableGeometry()
+    const metrics = await collectTimelineStreamMetrics(page, {
+      textPartID,
+      finalIndex: deltaCount,
+      navigations: benchmarkDiagnostics(page).navigations,
+    })
+    const delivered = deltas.length - fixture.transport.pendingCount()
+    await profile.stop()
+
+    report(
+      {
+        endToEndInitialContentObservedMs: initialContentObservedMs,
+        ...metrics,
+        deliveredDeltas: delivered,
+        pendingDeltas: fixture.transport.pendingCount(),
+      },
+      {
+        cpuThrottle,
+        profileCPU,
+        profileVisual,
+        minimal,
+        queuedDeltas: deltas.length,
+        historyTurns,
+        eventBatch,
+      },
+    )
+
+    await profile.reset()
+  })
+})

packages/app/e2e/performance/timeline/session-timeline-profile.ts

@@ -0,0 +1,40 @@
+import type { CDPSession, Page } from "@playwright/test"
+
+export async function startTimelineProfile(page: Page, options: { cpuThrottle: number; profileCPU: boolean }) {
+  const cdp = await page.context().newCDPSession(page)
+  if (options.cpuThrottle > 1) await cdp.send("Emulation.setCPUThrottlingRate", { rate: options.cpuThrottle })
+  if (options.profileCPU) {
+    await cdp.send("Profiler.enable")
+    await cdp.send("Profiler.setSamplingInterval", { interval: 100 })
+    await cdp.send("Profiler.start")
+  }
+  return {
+    async stop() {
+      if (!options.profileCPU) return
+      const result = await cdp.send("Profiler.stop")
+      const self = new Map<number, number>()
+      result.profile.samples?.forEach((id, index) => {
+        const duration = (result.profile.timeDeltas?.[index] ?? 0) / 1_000
+        self.set(id, (self.get(id) ?? 0) + duration)
+      })
+      console.log(
+        "timeline cpu profile",
+        JSON.stringify(
+          result.profile.nodes
+            .map((node) => ({
+              function: node.callFrame.functionName || "(anonymous)",
+              url: node.callFrame.url,
+              line: node.callFrame.lineNumber + 1,
+              selfMs: self.get(node.id) ?? 0,
+            }))
+            .filter((node) => node.selfMs > 1)
+            .sort((a, b) => b.selfMs - a.selfMs)
+            .slice(0, 40),
+        ),
+      )
+    },
+    async reset() {
+      if (options.cpuThrottle > 1) await cdp.send("Emulation.setCPUThrottlingRate", { rate: 1 })
+    },
+  }
+}

packages/app/e2e/performance/timeline/session-timeline-stream-probe.ts

@@ -0,0 +1,547 @@
+import type { Page } from "@playwright/test"
+
+const STREAM_MARKER_PATTERN = "stream-(\\d+)"
+const STREAM_FRAGMENT_COUNT = 18
+
+type TimelineProbeState = {
+  started: number
+  ended: number
+  profileVisual: boolean
+  minimal: boolean
+  frames: number[]
+  frameAt: number[]
+  applied: { at: number; index: number }[]
+  geometry: {
+    scrollTop: number
+    scrollHeight: number
+    clientHeight: number
+    distance: number
+    virtualHeight: number
+    headerHeight: number
+  }[]
+  blanks: number
+  longTasks: number[]
+  layoutShifts: number[]
+  visibleMounts: number
+  visibleUnmounts: number
+  visibleRows: Set<Element>
+  visibleSubtreeMounts: string[]
+  visibleSubtreeUnmounts: string[]
+  visibleSubtreeReplacements: number
+  visibleSubtreeDropouts: string[]
+  visibleSubtrees: Map<string, Element>
+  subtreeKeys: WeakMap<Element, string>
+  maxOverlap: number
+  maxGap: number
+  maxPartTopMovement: number
+  previousPartTop: number
+  slowFrames: {
+    duration: number
+    index: number
+    phase: "stream" | "boundary" | "complete" | "unknown"
+    tokenSpans: number
+    blocks: number
+    codeBlocks: number
+    height: number
+    distance: number
+  }[]
+  scroll: {
+    calls: number
+    callNoops: number
+    sameFrameCalls: number
+    assignments: number
+    assignmentNoops: number
+    lastCallFrame: number
+    frame: number
+  }
+  row: HTMLElement
+  markdown: HTMLElement
+  running: boolean
+  previous: number
+  cleanup: () => void
+  start: () => void
+}
+
+export async function installTimelineStreamProbe(
+  page: Page,
+  options: { textPartID: string; finalIndex: number; profileVisual: boolean; minimal: boolean },
+) {
+  await page.evaluate(
+    ({ textPartID, finalIndex, profileVisual, minimal, markerPattern, fragmentCount }) => {
+      const part = document.querySelector<HTMLElement>(`[data-timeline-part-id="${textPartID}"]`)
+      const row = part?.closest<HTMLElement>("[data-timeline-row]")
+      const markdown = part?.querySelector<HTMLElement>('[data-component="markdown"]')
+      const root = part?.closest<HTMLElement>(".scroll-view__viewport")
+      if (!part || !row || !markdown || !root) throw new Error("missing streaming benchmark nodes")
+      const viewport = root.getBoundingClientRect()
+      const state: TimelineProbeState = {
+        started: 0,
+        ended: Infinity,
+        profileVisual,
+        minimal,
+        frames: [],
+        frameAt: [],
+        applied: [],
+        geometry: [],
+        blanks: 0,
+        longTasks: [],
+        layoutShifts: [],
+        visibleMounts: 0,
+        visibleUnmounts: 0,
+        visibleRows: new Set(
+          [...root.querySelectorAll("[data-timeline-key]")].filter((element) => {
+            const rect = element.getBoundingClientRect()
+            return rect.bottom > viewport.top && rect.top < viewport.bottom
+          }),
+        ),
+        visibleSubtreeMounts: [],
+        visibleSubtreeUnmounts: [],
+        visibleSubtreeReplacements: 0,
+        visibleSubtreeDropouts: [],
+        visibleSubtrees: new Map<string, Element>(),
+        subtreeKeys: new WeakMap<Element, string>(),
+        maxOverlap: 0,
+        maxGap: 0,
+        maxPartTopMovement: 0,
+        previousPartTop: part.getBoundingClientRect().top,
+        slowFrames: [],
+        scroll: {
+          calls: 0,
+          callNoops: 0,
+          sameFrameCalls: 0,
+          assignments: 0,
+          assignmentNoops: 0,
+          lastCallFrame: -1,
+          frame: 0,
+        },
+        row,
+        markdown,
+        running: false,
+        previous: 0,
+        cleanup: () => {},
+        start: () => {},
+      }
+      ;(window as Window & { __timelineStreamBenchmark?: TimelineProbeState }).__timelineStreamBenchmark = state
+      const scrollTo = Element.prototype.scrollTo
+      const scrollTop = Object.getOwnPropertyDescriptor(Element.prototype, "scrollTop")!
+      if (profileVisual) {
+        Element.prototype.scrollTo = function (...args) {
+          state.scroll.calls += 1
+          const top = typeof args[0] === "object" ? args[0]?.top : args[1]
+          if (typeof top === "number") {
+            const target = Math.min(top, this.scrollHeight - this.clientHeight)
+            if (Math.abs(this.scrollTop - target) < 1) state.scroll.callNoops += 1
+          }
+          if (state.scroll.lastCallFrame === state.scroll.frame) state.scroll.sameFrameCalls += 1
+          state.scroll.lastCallFrame = state.scroll.frame
+          return scrollTo.apply(this, args)
+        }
+        Object.defineProperty(Element.prototype, "scrollTop", {
+          configurable: true,
+          get: scrollTop.get,
+          set(value) {
+            state.scroll.assignments += 1
+            if (Math.abs(this.scrollTop - value) < 1) state.scroll.assignmentNoops += 1
+            scrollTop.set!.call(this, value)
+          },
+        })
+      }
+
+      const recordLongTasks = (entries: PerformanceEntry[]) => {
+        if (!state.running) return
+        state.longTasks.push(
+          ...entries
+            .filter((entry) => entry.startTime >= state.started && entry.startTime <= state.ended)
+            .map((entry) => entry.duration),
+        )
+      }
+      const longTaskObserver = new PerformanceObserver((list) => recordLongTasks(list.getEntries()))
+      longTaskObserver.observe({ type: "longtask" })
+      const recordLayoutShifts = (entries: PerformanceEntry[]) => {
+        if (!state.running) return
+        state.layoutShifts.push(
+          ...entries
+            .map((entry) => {
+              const shift = entry as LayoutShiftEntry
+              if (shift.startTime < state.started || shift.hadRecentInput) return
+              return shift.value
+            })
+            .filter((value): value is number => value !== undefined),
+        )
+      }
+      const layoutShiftObserver = profileVisual
+        ? new PerformanceObserver((list) => recordLayoutShifts(list.getEntries()))
+        : undefined
+      layoutShiftObserver?.observe({ type: "layout-shift", buffered: true })
+
+      const visible = (element: Element) => {
+        const rect = element.getBoundingClientRect()
+        const viewport = root.getBoundingClientRect()
+        const style = getComputedStyle(element)
+        return (
+          element.isConnected &&
+          rect.width > 0 &&
+          rect.height > 0 &&
+          rect.bottom > viewport.top &&
+          rect.top < viewport.bottom &&
+          style.display !== "none" &&
+          style.visibility !== "hidden" &&
+          Number(style.opacity) > 0
+        )
+      }
+      const critical = [
+        "[data-timeline-part-id]",
+        '[data-component="edit-content"]',
+        '[data-component="apply-patch-file-diff"]',
+        '[data-component="file"]',
+        '[data-component="markdown-code"]',
+        "[data-markdown-block]",
+      ].join(",")
+      const describe = (element: Element) => {
+        const cached = state.subtreeKeys.get(element)
+        if (!element.isConnected && cached) return cached
+        const part = element.closest<HTMLElement>("[data-timeline-part-id]")?.dataset.timelinePartId ?? "unknown"
+        const block = element
+          .closest<HTMLElement>("[data-markdown-key]")
+          ?.dataset.markdownKey?.replace(/:(?:code|full|live)$/, "")
+        const component =
+          element.getAttribute("data-component") ?? element.getAttribute("data-markdown-block") ?? element.tagName
+        const key = `${part}:${block ?? "root"}:${component}`
+        state.subtreeKeys.set(element, key)
+        return key
+      }
+      const recordMutations = (records: MutationRecord[]) => {
+        if (!state.running) return
+        records.forEach((record) => {
+          record.addedNodes.forEach((node) => {
+            if (node instanceof HTMLElement && node.matches("[data-timeline-key]") && visible(node)) {
+              state.visibleMounts += 1
+              state.visibleRows.add(node)
+            }
+            if (!(node instanceof Element)) return
+            const added = [node, ...node.querySelectorAll(critical)].filter((element) => element.matches(critical))
+            added.forEach((element) => {
+              if (visible(element)) state.visibleSubtreeMounts.push(describe(element))
+            })
+          })
+          record.removedNodes.forEach((node) => {
+            if (node instanceof HTMLElement && node.matches("[data-timeline-key]") && state.visibleRows.delete(node))
+              state.visibleUnmounts += 1
+            if (!(node instanceof Element)) return
+            const removed = [node, ...node.querySelectorAll(critical)].filter((element) => element.matches(critical))
+            removed.forEach((element) => {
+              const key = describe(element)
+              if (state.visibleSubtrees.get(key) === element) state.visibleSubtreeUnmounts.push(key)
+            })
+          })
+        })
+      }
+      const mutationObserver = profileVisual ? new MutationObserver(recordMutations) : undefined
+      mutationObserver?.observe(root, { childList: true, subtree: true })
+      const currentPart = () => root.querySelector<HTMLElement>(`[data-timeline-part-id="${textPartID}"]`)
+      const observeProgress = (at: number) => {
+        if (!state.running) return
+        const content = currentPart()?.textContent ?? ""
+        const index = content.includes("benchmark-complete")
+          ? finalIndex
+          : Number(content.match(new RegExp(markerPattern, "g"))?.at(-1)?.match(/\d+/)?.[0] ?? -1)
+        if (index >= 0 && index !== state.applied.at(-1)?.index) state.applied.push({ at, index })
+      }
+      const progressObserver = new MutationObserver(() => observeProgress(performance.now()))
+      progressObserver.observe(root, { characterData: true, childList: true, subtree: true })
+      state.cleanup = () => {
+        recordLongTasks(longTaskObserver.takeRecords())
+        recordLayoutShifts(layoutShiftObserver?.takeRecords() ?? [])
+        recordMutations(mutationObserver?.takeRecords() ?? [])
+        if (progressObserver.takeRecords().length) observeProgress(performance.now())
+        longTaskObserver.disconnect()
+        layoutShiftObserver?.disconnect()
+        mutationObserver?.disconnect()
+        progressObserver.disconnect()
+        if (!profileVisual) return
+        Element.prototype.scrollTo = scrollTo
+        Object.defineProperty(Element.prototype, "scrollTop", scrollTop)
+      }
+
+      const sample = (now: number) => {
+        if (!state.running) return
+        state.frameAt.push(now)
+        observeProgress(now)
+        if (minimal) {
+          state.frames.push(now - state.previous)
+          state.previous = now
+          requestAnimationFrame(sample)
+          return
+        }
+        setTimeout(() => {
+          if (!state.running) return
+          state.scroll.frame += 1
+          const duration = now - state.previous
+          state.frames.push(duration)
+          state.previous = now
+          const virtualRoot = root.querySelector<HTMLElement>("[data-timeline-virtual-content]")
+          const header = root.querySelector<HTMLElement>("[data-session-title]")
+          state.geometry.push({
+            scrollTop: root.scrollTop,
+            scrollHeight: root.scrollHeight,
+            clientHeight: root.clientHeight,
+            distance: root.scrollHeight - root.clientHeight - root.scrollTop,
+            virtualHeight: virtualRoot?.getBoundingClientRect().height ?? 0,
+            headerHeight: header?.getBoundingClientRect().height ?? 0,
+          })
+          const viewport = root.getBoundingClientRect()
+          if (profileVisual) {
+            const visibleRows = [...root.querySelectorAll<HTMLElement>("[data-timeline-key]")]
+              .map((element) => ({ element, rect: element.getBoundingClientRect() }))
+              .filter((item) => item.rect.bottom > viewport.top && item.rect.top < viewport.bottom)
+              .sort((a, b) => a.rect.top - b.rect.top)
+            state.visibleRows = new Set(visibleRows.map((item) => item.element))
+            const rows = visibleRows.map((item) => item.rect)
+            rows.slice(1).forEach((rect, index) => {
+              const previous = rows[index]!
+              state.maxOverlap = Math.max(state.maxOverlap, previous.bottom - rect.top)
+              state.maxGap = Math.max(state.maxGap, rect.top - previous.bottom)
+            })
+            const partTop = part.getBoundingClientRect().top
+            state.maxPartTopMovement = Math.max(state.maxPartTopMovement, Math.abs(partTop - state.previousPartTop))
+            state.previousPartTop = partTop
+          }
+          const visibleRow = [...root.querySelectorAll<HTMLElement>("[data-timeline-row]")].some((element) => {
+            const rect = element.getBoundingClientRect()
+            return rect.bottom > viewport.top && rect.top < viewport.bottom
+          })
+          if (!visibleRow) state.blanks += 1
+          if (profileVisual) {
+            const subtrees = new Map<string, { element: Element; rendered: boolean }>()
+            const visibleSubtrees = new Map<string, Element>()
+            root.querySelectorAll(critical).forEach((element) => {
+              const key = describe(element)
+              const rect = element.getBoundingClientRect()
+              const style = getComputedStyle(element)
+              const rendered =
+                element.isConnected &&
+                rect.width > 0 &&
+                rect.height > 0 &&
+                style.display !== "none" &&
+                style.visibility !== "hidden" &&
+                Number(style.opacity) > 0
+              subtrees.set(key, { element, rendered })
+              if (rendered && rect.bottom > viewport.top && rect.top < viewport.bottom) {
+                const previous = state.visibleSubtrees.get(key)
+                if (previous && previous !== element && key.startsWith(`${textPartID}:`))
+                  state.visibleSubtreeReplacements += 1
+                visibleSubtrees.set(key, element)
+              }
+            })
+            state.visibleSubtrees.forEach((element, key) => {
+              const current = subtrees.get(key)
+              if (key.startsWith(`${textPartID}:`) && !current?.rendered) {
+                const markdown = part.querySelector<HTMLElement>('[data-component="markdown"]')
+                state.visibleSubtreeDropouts.push(
+                  `${key}:projection=${markdown?.dataset.markdownProjectionLength}/${markdown?.dataset.markdownProjectionBlocks}:result=${markdown?.dataset.markdownResultLength}/${markdown?.dataset.markdownResultBlocks}:applied=${markdown?.dataset.markdownAppliedBlocks}:dom=${markdown?.children.length}`,
+                )
+              }
+              if (element.matches('[data-component="file"]')) {
+                const hadLines = element.hasAttribute("data-profiler-had-lines")
+                const hasLines = element.shadowRoot?.querySelector("[data-line]") != null
+                if (hasLines) element.setAttribute("data-profiler-had-lines", "")
+                if (hadLines && !hasLines) state.visibleSubtreeDropouts.push(`${key}:shadow-lines`)
+              }
+            })
+            state.visibleSubtrees = visibleSubtrees
+          }
+          if (profileVisual && duration > 33.34) {
+            const livePart = currentPart()
+            const content = livePart?.textContent ?? ""
+            const complete = content.includes("benchmark-complete")
+            const index = complete
+              ? finalIndex
+              : Number(content.match(new RegExp(markerPattern, "g"))?.at(-1)?.match(/\d+/)?.[0] ?? -1)
+            state.slowFrames.push({
+              duration,
+              index,
+              phase: complete
+                ? "complete"
+                : index >= 0 && index % fragmentCount === 0
+                  ? "boundary"
+                  : index >= 0
+                    ? "stream"
+                    : "unknown",
+              tokenSpans: livePart?.querySelectorAll(".shiki span").length ?? 0,
+              blocks: livePart?.querySelectorAll("[data-markdown-block]").length ?? 0,
+              codeBlocks: livePart?.querySelectorAll('[data-component="markdown-code"]').length ?? 0,
+              height: livePart?.getBoundingClientRect().height ?? 0,
+              distance: root.scrollHeight - root.clientHeight - root.scrollTop,
+            })
+          }
+          requestAnimationFrame(sample)
+        }, 0)
+      }
+      state.start = () => {
+        state.started = performance.now()
+        state.previous = state.started
+        state.running = true
+        requestAnimationFrame(sample)
+      }
+    },
+    { ...options, markerPattern: STREAM_MARKER_PATTERN, fragmentCount: STREAM_FRAGMENT_COUNT },
+  )
+}
+
+export function startTimelineStreamProbe(page: Page) {
+  return page.evaluate(() => {
+    const state = (window as Window & { __timelineStreamBenchmark?: TimelineProbeState }).__timelineStreamBenchmark
+    if (!state) throw new Error("missing streaming benchmark state")
+    state.start()
+  })
+}
+
+type LayoutShiftEntry = PerformanceEntry & { value: number; hadRecentInput?: boolean }
+
+export function layoutShiftValue(
+  entry: Pick<LayoutShiftEntry, "startTime" | "value" | "hadRecentInput">,
+  start: number,
+) {
+  if (entry.startTime < start || entry.hadRecentInput) return
+  return entry.value
+}
+
+export function removeVisibleRow<T>(visible: Set<T>, row: T) {
+  return visible.delete(row)
+}
+
+export function streamProgress(content: string) {
+  const index = Number(content.match(new RegExp(STREAM_MARKER_PATTERN, "g"))?.at(-1)?.match(/\d+/)?.[0] ?? -1)
+  return {
+    index,
+    phase: content.includes("benchmark-complete")
+      ? ("complete" as const)
+      : index >= 0 && index % STREAM_FRAGMENT_COUNT === 0
+        ? ("boundary" as const)
+        : index >= 0
+          ? ("stream" as const)
+          : ("unknown" as const),
+  }
+}
+
+export async function collectTimelineStreamMetrics(
+  page: Page,
+  options: { textPartID: string; finalIndex: number; navigations: string[] },
+) {
+  return page.evaluate(({ textPartID, finalIndex, navigations }) => {
+    const state = (window as Window & { __timelineStreamBenchmark?: TimelineProbeState }).__timelineStreamBenchmark
+    if (!state) throw new Error(`missing streaming benchmark state after navigation: ${JSON.stringify(navigations)}`)
+    state.ended = performance.now()
+    state.cleanup()
+    state.running = false
+    const part = document.querySelector<HTMLElement>(`[data-timeline-part-id="${textPartID}"]`)
+    const row = part?.closest<HTMLElement>("[data-timeline-row]")
+    const markdown = part?.querySelector<HTMLElement>('[data-component="markdown"]')
+    const sorted = state.frames.slice().sort((a, b) => a - b)
+    const duration = state.frames.reduce((sum, value) => sum + value, 0)
+    const longestSlowStreak = state.frames.reduce(
+      (result, value) => {
+        const current = value > 33.34 ? result.current + 1 : 0
+        return { current, longest: Math.max(result.longest, current) }
+      },
+      { current: 0, longest: 0 },
+    ).longest
+    const busyStart = state.applied.at(0)?.at
+    const completion = state.applied.find((value) => value.index === finalIndex)
+    const busyEnd = completion?.at
+    const busyFrames =
+      busyStart === undefined || busyEnd === undefined
+        ? []
+        : state.frames.filter((_, index) => state.frameAt[index]! >= busyStart && state.frameAt[index]! <= busyEnd)
+    const busySorted = busyFrames.slice().sort((a, b) => a - b)
+    const busyDuration = busyFrames.reduce((sum, value) => sum + value, 0)
+    const completionObservedMs = (completion?.at ?? NaN) - state.started
+    const visual = state.profileVisual
+      ? {
+          layoutShiftValueSum: state.layoutShifts.reduce((sum, value) => sum + value, 0),
+          maxLayoutShiftValue: Math.max(0, ...state.layoutShifts),
+          visibleMounts: state.visibleMounts,
+          visibleUnmounts: state.visibleUnmounts,
+          visibleSubtreeMounts: state.visibleSubtreeMounts,
+          visibleSubtreeUnmounts: [...new Set(state.visibleSubtreeUnmounts)],
+          visibleSubtreeReplacements: state.visibleSubtreeReplacements,
+          visibleSubtreeDropouts: [...new Set(state.visibleSubtreeDropouts)],
+          maxOverlapPx: state.maxOverlap,
+          maxGapPx: state.maxGap,
+          maxPartTopMovementPx: state.maxPartTopMovement,
+          slowestRafGaps: state.slowFrames
+            .sort((a, b) => b.duration - a.duration)
+            .slice(0, 20)
+            .map((frame) => ({
+              durationMs: frame.duration,
+              index: frame.index,
+              phase: frame.phase,
+              tokenSpans: frame.tokenSpans,
+              blocks: frame.blocks,
+              codeBlocks: frame.codeBlocks,
+              heightPx: frame.height,
+              distancePx: frame.distance,
+            })),
+          slowRafGapPhases: Object.fromEntries(
+            ["stream", "boundary", "complete", "unknown"].map((phase) => {
+              const frames = state.slowFrames.filter((frame) => frame.phase === phase)
+              return [
+                phase,
+                {
+                  count: frames.length,
+                  totalMs: frames.reduce((sum, frame) => sum + frame.duration, 0),
+                  maxMs: Math.max(0, ...frames.map((frame) => frame.duration)),
+                },
+              ]
+            }),
+          ),
+          scroll: state.scroll,
+        }
+      : null
+    const geometry = state.minimal
+      ? null
+      : {
+          maxDistancePx: Math.max(0, ...state.geometry.map((sample) => sample.distance)),
+          finalDistancePx: state.geometry.at(-1)?.distance ?? 0,
+          final: state.geometry.at(-1),
+          distanceTransitionsPx: state.geometry
+            .map((sample) => Math.round(sample.distance))
+            .filter((value, index, values) => index === 0 || value !== values[index - 1]),
+          bottomDriftTransitions: state.geometry.slice(1).filter((value, index) => {
+            const previous = state.geometry[index]?.distance ?? 0
+            return previous <= 1 && value.distance > 1
+          }).length,
+          blankSamples: state.blanks,
+        }
+    return {
+      capabilities: { visual: state.profileVisual, geometry: !state.minimal },
+      completionObservedMs,
+      deltasPerSecond: Number.isFinite(completionObservedMs) ? finalIndex / (completionObservedMs / 1_000) : null,
+      rafGapSamples: state.frames.length,
+      rafCallbackRate: duration ? (state.frames.length * 1000) / duration : 0,
+      observedProgressWindowRafCallbackRate: busyDuration ? (busyFrames.length * 1000) / busyDuration : null,
+      observedProgressWindowRafGapP95Ms: busySorted[Math.floor(busySorted.length * 0.95)] ?? null,
+      observedProgressWindowRafGaps: busyFrames.length,
+      maxObservedProgressIndex: Math.max(-1, ...state.applied.map((value) => value.index)),
+      observedProgressTransitions: state.applied.length,
+      rafGapP50Ms: sorted[Math.floor(sorted.length * 0.5)] ?? 0,
+      rafGapP95Ms: sorted[Math.floor(sorted.length * 0.95)] ?? 0,
+      rafGapP99Ms: sorted[Math.floor(sorted.length * 0.99)] ?? 0,
+      maxRafGapMs: sorted.at(-1) ?? 0,
+      rafGapsOver33Ms: state.frames.filter((value) => value > 33.34).length,
+      rafGapsOver50Ms: state.frames.filter((value) => value > 50).length,
+      missedFrameBudgetEquivalents: state.frames.reduce(
+        (sum, value) => sum + Math.max(0, Math.round(value / 16.67) - 1),
+        0,
+      ),
+      longestRafGapOver33MsStreak: longestSlowStreak,
+      longTaskCount: state.longTasks.length,
+      longTaskTimeMs: state.longTasks.reduce((sum, value) => sum + value, 0),
+      visual,
+      geometry,
+      rowReplaced: row !== state.row,
+      markdownReplaced: markdown !== state.markdown,
+      domTextCharacters: part?.textContent?.length ?? 0,
+    }
+  }, options)
+}

packages/app/e2e/performance/timeline/session-timeline-stress.fixture.ts

@@ -0,0 +1,335 @@
+const words = [
+  "alpha",
+  "bravo",
+  "charlie",
+  "delta",
+  "echo",
+  "foxtrot",
+  "golf",
+  "hotel",
+  "india",
+  "juliet",
+  "kilo",
+  "lima",
+  "metro",
+  "nova",
+  "orbit",
+  "pixel",
+  "quartz",
+  "river",
+  "signal",
+  "vector",
+]
+
+const sourceID = "ses_smoke_source"
+const targetID = "ses_smoke_target"
+const directory = "C:/OpenCode/SmokeProject"
+const projectID = "proj_smoke_timeline"
+const model = { providerID: "opencode", modelID: "claude-opus-4-6", variant: "max" }
+
+type MessageInfo = Record<string, unknown> & { id: string; role: "user" | "assistant" }
+type MessagePart = Record<string, unknown> & { id: string; type: string; text?: string; tool?: string }
+type Message = { info: MessageInfo; parts: MessagePart[] }
+
+function lorem(seed: number, length: number) {
+  let out = ""
+  let i = seed
+  while (out.length < length) {
+    const word = words[i % words.length]
+    out += (out ? " " : "") + word
+    if (i % 17 === 0) out += ".\n\n"
+    i += 7
+  }
+  return out.slice(0, length)
+}
+
+function id(prefix: string, value: number) {
+  return `${prefix}_smoke_${String(value).padStart(4, "0")}`
+}
+
+function userMessage(sessionID: string, index: number, textLength: number, diffs: unknown[] = []): Message {
+  const messageID = id("msg_user", index)
+  return {
+    info: {
+      id: messageID,
+      sessionID,
+      role: "user",
+      time: { created: 1700000000000 + index * 10_000 },
+      summary: { diffs },
+      agent: "build",
+      model,
+    },
+    parts: [
+      {
+        id: id("prt_user_text", index),
+        sessionID,
+        messageID,
+        type: "text",
+        text: lorem(index, textLength),
+      },
+    ],
+  }
+}
+
+function assistantMessage(sessionID: string, index: number, parentID: string, parts: MessagePart[]): Message {
+  const messageID = id("msg_assistant", index)
+  return {
+    info: {
+      id: messageID,
+      sessionID,
+      role: "assistant",
+      time: { created: 1700000000000 + index * 10_000 + 1_000, completed: 1700000000000 + index * 10_000 + 8_000 },
+      parentID,
+      modelID: model.modelID,
+      providerID: model.providerID,
+      mode: "build",
+      agent: "build",
+      path: { cwd: directory, root: directory },
+      cost: 0.01,
+      tokens: { input: 100, output: 200, reasoning: 0, cache: { read: 0, write: 0 } },
+      variant: "max",
+      finish: "stop",
+    },
+    parts: parts.map((part) => ({
+      ...part,
+      sessionID,
+      messageID,
+    })),
+  }
+}
+
+function textPart(index: number, partIndex: number, length: number): MessagePart {
+  const prose = lorem(index * 13 + partIndex, length)
+  const text =
+    index % 12 === 0
+      ? `${prose}\n\n\`\`\`ts\n${code(index, 80)}\n\`\`\``
+      : index % 5 === 0
+        ? `${prose}\n\n\`\`\`ts\nexport const value = "${lorem(index, 220)}"\n\`\`\``
+        : index % 7 === 0
+          ? `${prose}\n\nThe wrapped inline value is \`${lorem(index, 180)}\`.`
+          : prose
+  return { id: id(`prt_text_${partIndex}`, index), type: "text", text }
+}
+
+function reasoningPart(index: number, partIndex: number, length: number): MessagePart {
+  return {
+    id: id(`prt_reasoning_${partIndex}`, index),
+    type: "reasoning",
+    text: lorem(index * 19 + partIndex, length),
+    time: { start: 1700000000000 + index * 10_000, end: 1700000000000 + index * 10_000 + 500 },
+  }
+}
+
+function toolPart(
+  index: number,
+  partIndex: number,
+  tool: string,
+  input: Record<string, unknown>,
+  outputLength = 160,
+): MessagePart {
+  const metadata =
+    tool === "apply_patch"
+      ? { files: [patchFile(index, "update"), patchFile(index + 1, index % 2 === 0 ? "add" : "delete")] }
+      : tool === "edit" || tool === "write"
+        ? {
+            filediff: fileDiff(String(input.filePath ?? `src/generated/file-${index}.ts`), index),
+            diff: patch(index, outputLength),
+            preview: patch(index + 1, 420),
+          }
+        : tool === "question"
+          ? { answers: [["Proceed"], ["Keep sample output"]] }
+          : {}
+  return {
+    id: id(`prt_tool_${tool}_${partIndex}`, index),
+    type: "tool",
+    callID: id("call", index * 10 + partIndex),
+    tool,
+    state: {
+      status: "completed",
+      input,
+      output: lorem(index * 23 + partIndex, outputLength),
+      title: tool === "bash" ? "Verify generated output" : input.filePath || input.path || input.pattern || "completed",
+      metadata,
+      time: { start: 1700000000000 + index * 10_000, end: 1700000000000 + index * 10_000 + 400 },
+    },
+  }
+}
+
+function patchFile(seed: number, type: "add" | "update" | "delete") {
+  return {
+    filePath: `src/generated/patch-${seed}.ts`,
+    relativePath: `src/generated/patch-${seed}.ts`,
+    type,
+    additions: (seed % 7) + 1,
+    deletions: type === "add" ? 0 : seed % 4,
+    patch: patch(seed, 520),
+    before: type === "add" ? undefined : code(seed, 18),
+    after: type === "delete" ? undefined : code(seed + 1, 24),
+  }
+}
+
+function fileDiff(file: string, seed: number) {
+  const lines = seed % 12 === 0 ? 300 : seed % 8 === 0 ? 2 : 38
+  const before = code(seed, lines, seed % 10 === 0 ? 280 : 32)
+  const after =
+    lines === 2
+      ? before.replace("value1", "updatedValue1")
+      : lines === 300
+        ? code(seed + 1, lines, seed % 10 === 0 ? 280 : 32)
+        : before.replace("value4", "updatedValue4").replace("value20", "updatedValue20")
+  return {
+    file,
+    additions: lines === 300 ? 300 : lines === 2 ? 1 : 2,
+    deletions: lines === 300 ? 300 : lines === 2 ? 1 : 2,
+    before,
+    after,
+  }
+}
+
+function patch(seed: number, length: number) {
+  return `diff --git a/src/generated/file-${seed}.ts b/src/generated/file-${seed}.ts\n+${lorem(seed, length).replace(/\n/g, "\n+")}`
+}
+
+function code(seed: number, lines: number, width = 32) {
+  return Array.from(
+    { length: lines },
+    (_, index) => `export const value${index} = "${lorem(seed + index, width)}"`,
+  ).join("\n")
+}
+
+function turn(index: number): Message[] {
+  const diff = index % 9 === 0 ? [fileDiff(`src/generated/summary-${index}.ts`, index)] : []
+  const user = userMessage(targetID, index, 100 + (index % 4) * 80, diff)
+  const parts = [
+    ...(index % 5 === 0 ? [reasoningPart(index, 0, 420)] : []),
+    ...(index % 3 === 0
+      ? [
+          toolPart(index, 0, "read", { filePath: `src/generated/file-${index}.ts`, offset: 0, limit: 80 }, 220),
+          toolPart(index, 5, "glob", { path: directory, pattern: `**/*sample-${index}*.ts` }, 140),
+          toolPart(index, 1, "grep", { path: directory, pattern: `sample-${index}`, include: "*.ts" }, 180),
+          toolPart(index, 6, "list", { path: `src/generated/${index}` }, 120),
+        ]
+      : []),
+    textPart(index, 2, 160 + (index % 6) * 90),
+    ...(index % 4 === 0 ? [toolPart(index, 3, "edit", { filePath: `src/generated/file-${index}.ts` }, 700)] : []),
+    ...(index % 6 === 0
+      ? [toolPart(index, 7, "write", { filePath: `src/generated/write-${index}.ts`, content: code(index, 28) }, 560)]
+      : []),
+    ...(index % 8 === 0
+      ? [toolPart(index, 8, "apply_patch", { files: [`src/generated/patch-${index}.ts`] }, 620)]
+      : []),
+    ...(index % 7 === 0
+      ? [toolPart(index, 4, "bash", { command: "bun typecheck", description: "Verify generated output" }, 620)]
+      : []),
+    ...(index % 10 === 0 ? [toolPart(index, 9, "webfetch", { url: "https://example.com/docs/sample" }, 120)] : []),
+    ...(index % 11 === 0 ? [toolPart(index, 10, "websearch", { query: "sample movement notes" }, 240)] : []),
+    ...(index % 13 === 0
+      ? [
+          toolPart(
+            index,
+            11,
+            "question",
+            { questions: [{ question: "Use generated fixture?" }, { question: "Keep same row shape?" }] },
+            120,
+          ),
+        ]
+      : []),
+    ...(index % 17 === 0
+      ? [toolPart(index, 12, "task", { description: "Inspect generated fixture", subagent_type: "explore" }, 160)]
+      : []),
+  ]
+  return [user, assistantMessage(targetID, index, user.info.id, parts)]
+}
+
+const targetMessages = Array.from({ length: 72 }, (_, index) => turn(index)).flat()
+const sourceMessages = Array.from({ length: 12 }, (_, index) => [
+  userMessage(sourceID, index + 1000, 120),
+  assistantMessage(sourceID, index + 1000, id("msg_user", index + 1000), [textPart(index + 1000, 0, 240)]),
+]).flat()
+
+function renderable(part: MessagePart) {
+  if (part.type === "tool" && part.tool === "todowrite") return false
+  if (part.type === "text") return !!part.text.trim()
+  if (part.type === "reasoning") return !!part.text.trim()
+  return part.type !== "step-start" && part.type !== "step-finish" && part.type !== "patch"
+}
+
+function orderedParts(message: Message) {
+  return message.parts.slice().sort((a, b) => a.id.localeCompare(b.id))
+}
+
+export const fixture = {
+  directory,
+  project: {
+    id: projectID,
+    worktree: directory,
+    vcs: "git",
+    name: "smoke-project",
+    time: { created: 1700000000000, updated: 1700000000000 },
+    sandboxes: [],
+  },
+  provider: {
+    all: [
+      {
+        id: "opencode",
+        name: "OpenCode",
+        models: { "claude-opus-4-6": { id: "claude-opus-4-6", name: "Claude Opus 4.6", limit: { context: 200_000 } } },
+      },
+    ],
+    connected: ["opencode"],
+    default: { providerID: "opencode", modelID: "claude-opus-4-6" },
+  },
+  sessions: [
+    {
+      id: sourceID,
+      slug: "source",
+      projectID,
+      directory,
+      title: "Uncommitted changes inquiry",
+      version: "dev",
+      time: { created: 1700000000000, updated: 1700000000000 },
+    },
+    {
+      id: targetID,
+      slug: "target",
+      projectID,
+      directory,
+      title: "Example Game: sample jump movement & sample physics analysis",
+      version: "dev",
+      time: { created: 1700000001000, updated: 1700000001000 },
+    },
+  ],
+  sourceID,
+  targetID,
+  messages: { [sourceID]: sourceMessages, [targetID]: targetMessages },
+  expected: {
+    sourceTitle: "Uncommitted changes inquiry",
+    targetTitle: "Example Game: sample jump movement & sample physics analysis",
+    sourceMessageIDs: sourceMessages
+      .filter((message) => message.info.role === "user")
+      .map((message) => message.info.id),
+    targetMessageIDs: targetMessages
+      .filter((message) => message.info.role === "user")
+      .map((message) => message.info.id),
+    targetPartIDs: targetMessages.flatMap((message) =>
+      orderedParts(message)
+        .filter(renderable)
+        .map((part) => part.id),
+    ),
+  },
+}
+
+export function pageMessages(sessionID: string, limit: number, before?: string) {
+  const messages = fixture.messages[sessionID as keyof typeof fixture.messages] ?? []
+  const end = before
+    ? Math.max(
+        0,
+        messages.findIndex((message) => message.info.id === before),
+      )
+    : messages.length
+  const start = Math.max(0, end - limit)
+  return {
+    items: messages.slice(start, end),
+    cursor: start > 0 ? messages[start]!.info.id : undefined,
+  }
+}

packages/app/e2e/performance/timeline/timeline-test-helpers.ts

@@ -0,0 +1,67 @@
+import type { Page } from "@playwright/test"
+import { base64Encode } from "@opencode-ai/core/util/encode"
+import { mockOpenCodeServer } from "../../utils/mock-server"
+import { fixture } from "./session-timeline-stress.fixture"
+
+export async function installTimelineSettings(page: Page) {
+  await page.addInitScript(() => {
+    localStorage.setItem(
+      "settings.v3",
+      JSON.stringify({
+        general: {
+          editToolPartsExpanded: true,
+          shellToolPartsExpanded: true,
+          showReasoningSummaries: true,
+          showSessionProgressBar: true,
+        },
+      }),
+    )
+  })
+}
+
+export function mockStressTimeline(page: Page) {
+  return mockOpenCodeServer(page, {
+    sessions: fixture.sessions,
+    provider: fixture.provider,
+    directory: fixture.directory,
+    project: fixture.project,
+    pageMessages: (sessionID) => ({ items: fixture.messages[sessionID as keyof typeof fixture.messages] ?? [] }),
+  })
+}
+
+export async function installStressSessionTabs(page: Page) {
+  const server = `http://${process.env.PLAYWRIGHT_SERVER_HOST ?? "127.0.0.1"}:${process.env.PLAYWRIGHT_SERVER_PORT ?? "4096"}`
+  await page.addInitScript(
+    ({ directory, sourceID, targetID, dirBase64, server }) => {
+      localStorage.setItem(
+        "opencode.global.dat:server",
+        JSON.stringify({
+          projects: { local: [{ worktree: directory, expanded: true }] },
+          lastProject: { local: directory },
+        }),
+      )
+      localStorage.setItem(
+        "opencode.global.dat:tabs",
+        JSON.stringify(
+          [sourceID, targetID].map((sessionId) => ({
+            type: "session",
+            server,
+            dirBase64,
+            sessionId,
+          })),
+        ),
+      )
+    },
+    {
+      directory: fixture.directory,
+      sourceID: fixture.sourceID,
+      targetID: fixture.targetID,
+      dirBase64: base64Encode(fixture.directory),
+      server,
+    },
+  )
+}
+
+export function stressSessionHref(sessionID: string) {
+  return `/${base64Encode(fixture.directory)}/session/${sessionID}`
+}

packages/app/e2e/performance/unit/chrome-trace-write.test.ts

@@ -0,0 +1,15 @@
+import { expect, test } from "bun:test"
+import { mkdtemp, rm } from "node:fs/promises"
+import path from "node:path"
+import os from "node:os"
+import { prepareChromeTrace } from "../chrome-trace"
+
+test("creates the configured trace directory", async () => {
+  const root = await mkdtemp(path.join(os.tmpdir(), "opencode-trace-"))
+  try {
+    const file = await prepareChromeTrace(path.join(root, "nested", "traces"), "session/tab", false, "test")
+    expect(file).toEndWith("-session-tab-458ed9e3-test.json")
+  } finally {
+    await rm(root, { recursive: true, force: true })
+  }
+})

packages/app/e2e/performance/unit/session-tab-repaint-probe.test.ts

@@ -0,0 +1,42 @@
+import { expect, test } from "bun:test"
+import { compressCachedRepaintTrace, layoutShiftSample } from "../timeline/session-tab-repaint-probe"
+
+test("compresses repeated repaint states without losing frame samples", () => {
+  const state = {
+    root: 1,
+    scrollTop: 10,
+    scrollHeight: 20,
+    bottomErrorPx: 0,
+    last: true,
+    rows: [{ key: "row", node: 2, top: 0, bottom: 10 }],
+    mounted: 1,
+    center: "content",
+  }
+  const trace = {
+    timeOriginEpochMs: 1_000,
+    startedAtPerformanceMs: 100,
+    samples: [
+      { observedAtMs: 16, ...state, destination: ["target"], source: [] },
+      { observedAtMs: 32, ...state, destination: ["target"], source: [] },
+      { observedAtMs: 48, ...state, scrollTop: 11, destination: ["target"], source: [] },
+    ],
+    mutations: [{ observedAtMs: 20, changed: [{ type: "add", node: 2 }] }],
+    shifts: [{ occurredAtMs: 24, value: 0.1 }],
+    windowMs: 1_000,
+    running: false,
+    stop() {},
+  }
+  const compressed = compressCachedRepaintTrace(trace)
+  const samples = compressed.samples.flatMap((group) =>
+    group.observedAtMs.map((observedAtMs) => ({ observedAtMs, ...group.state })),
+  )
+
+  expect(samples).toEqual(trace.samples)
+  expect(compressed.mutations).toEqual(trace.mutations)
+  expect(compressed.shifts).toEqual(trace.shifts)
+})
+
+test("records layout shifts at occurrence time within the probe window", () => {
+  expect(layoutShiftSample({ startTime: 99, value: 0.1 }, 100)).toBeUndefined()
+  expect(layoutShiftSample({ startTime: 124, value: 0.2 }, 100)).toEqual({ occurredAtMs: 24, value: 0.2 })
+})

packages/app/e2e/performance/unit/session-tab-switch-metrics.test.ts

@@ -0,0 +1,54 @@
+import { expect, test } from "bun:test"
+import { classifySessionSwitch } from "../timeline/session-tab-switch-metrics"
+
+test("counts source and blank samples before the destination is observed", () => {
+  const result = classifySessionSwitch([
+    { observedAtMs: 16, destination: [], source: ["source"], hasVisibleRows: true, last: false },
+    { observedAtMs: 32, destination: [], source: [], hasVisibleRows: false, last: false },
+    { observedAtMs: 48, destination: ["destination"], source: [], hasVisibleRows: true, last: true, bottomErrorPx: 0 },
+    { observedAtMs: 64, destination: ["destination"], source: [], hasVisibleRows: true, last: true, bottomErrorPx: 0 },
+    { observedAtMs: 80, destination: ["destination"], source: [], hasVisibleRows: true, last: true, bottomErrorPx: 0 },
+  ])
+
+  expect(result.blankSamples).toBe(1)
+  expect(result.sourceSamples).toBe(1)
+  expect(result.unknownSamples).toBe(0)
+  expect(result.firstDestinationObservedMs).toBe(48)
+  expect(result.stableObservedMs).toBe(80)
+})
+
+test("does not classify mixed source and destination content as correct", () => {
+  const result = classifySessionSwitch([
+    {
+      observedAtMs: 16,
+      destination: ["destination"],
+      source: ["source"],
+      hasVisibleRows: true,
+      last: true,
+      bottomErrorPx: 0,
+    },
+    { observedAtMs: 32, destination: ["destination"], source: [], hasVisibleRows: true, last: true, bottomErrorPx: 0 },
+    { observedAtMs: 48, destination: ["destination"], source: [], hasVisibleRows: true, last: true, bottomErrorPx: 0 },
+    { observedAtMs: 64, destination: ["destination"], source: [], hasVisibleRows: true, last: true, bottomErrorPx: 0 },
+  ])
+
+  expect(result.firstCorrectObservedMs).toBe(32)
+  expect(result.stableObservedMs).toBe(64)
+})
+
+test("reports missing correctness without throwing", () => {
+  const result = classifySessionSwitch([
+    {
+      observedAtMs: 16,
+      destination: ["destination"],
+      source: ["source"],
+      hasVisibleRows: true,
+      last: true,
+      bottomErrorPx: 0,
+    },
+  ])
+
+  expect(result.firstDestinationObservedMs).toBe(16)
+  expect(result.firstCorrectObservedMs).toBeNull()
+  expect(result.stableObservedMs).toBeNull()
+})

packages/app/e2e/performance/unit/session-timeline-stream-probe.test.ts

@@ -0,0 +1,14 @@
+import { expect, test } from "bun:test"
+import { streamChunk } from "../timeline/session-timeline-benchmark.fixture"
+import { streamProgress } from "../timeline/session-timeline-stream-probe"
+
+test("classifies emitted stream markers using the fixture cycle", () => {
+  expect(streamProgress("before stream-17 after stream-18")).toEqual({ index: 18, phase: "boundary" })
+  expect(streamProgress("before stream-18 after stream-19")).toEqual({ index: 19, phase: "stream" })
+  expect(streamProgress("benchmark-complete stream-36")).toEqual({ index: 36, phase: "complete" })
+  expect(streamProgress("no marker")).toEqual({ index: -1, phase: "unknown" })
+})
+
+test("emits progress markers at fixture boundaries", () => {
+  expect(streamProgress(streamChunk(18, 160))).toEqual({ index: 18, phase: "boundary" })
+})

packages/app/e2e/performance/unit/session-timeline-visual-tracking.test.ts

@@ -0,0 +1,16 @@
+import { expect, test } from "bun:test"
+import { layoutShiftValue, removeVisibleRow } from "../timeline/session-timeline-stream-probe"
+
+test("excludes layout shifts before the probe window and recent input", () => {
+  expect(layoutShiftValue({ startTime: 9, value: 0.1 }, 10)).toBeUndefined()
+  expect(layoutShiftValue({ startTime: 10, value: 0.2, hadRecentInput: true }, 10)).toBeUndefined()
+  expect(layoutShiftValue({ startTime: 11, value: 0.3 }, 10)).toBe(0.3)
+})
+
+test("classifies removed rows from their last painted visibility", () => {
+  const row = {}
+  const visible = new Set([row])
+
+  expect(removeVisibleRow(visible, row)).toBe(true)
+  expect(removeVisibleRow(visible, row)).toBe(false)
+})

packages/app/e2e/utils/mock-server.ts

@@ -44,7 +44,10 @@ export async function mockOpenCodeServer(page: Page, config: MockServerConfig) {
   await page.route("**/*", async (route) => {
     const url = new URL(route.request().url())
     const targetPort = process.env.PLAYWRIGHT_SERVER_PORT ?? "4096"
-    if (url.port !== targetPort) return route.fallback()
+    const appPort = new URL(
+      process.env.PLAYWRIGHT_BASE_URL ?? `http://127.0.0.1:${process.env.PLAYWRIGHT_PORT ?? "3000"}`,
+    ).port
+    if (url.port !== targetPort && url.port !== appPort) return route.fallback()
 
     const path = url.pathname
     if (path === "/global/event" || path === "/event") return sse(route, config.events?.(), config.eventRetry)
@@ -72,7 +75,8 @@ export async function mockOpenCodeServer(page: Page, config: MockServerConfig) {
       return json(route, pageData.items, pageData.cursor ? { "x-next-cursor": pageData.cursor } : undefined)
     }
 
-    return json(route, {})
+    if (url.port === targetPort && targetPort !== appPort) return json(route, {})
+    return route.fallback()
   })
 }
 

packages/app/package.json

@@ -24,7 +24,8 @@
     "test:e2e": "playwright test",
     "test:e2e:local": "playwright test",
     "test:e2e:ui": "playwright test --ui",
-    "test:e2e:report": "playwright show-report e2e/playwright-report"
+    "test:e2e:report": "playwright show-report e2e/playwright-report",
+    "test:bench": "bun test ./e2e/performance/unit && playwright test --config e2e/performance/playwright.config.ts"
   },
   "license": "MIT",
   "devDependencies": {

packages/app/playwright.config.ts

@@ -9,6 +9,7 @@ const reuse = !process.env.CI
 const workers = Number(process.env.PLAYWRIGHT_WORKERS ?? (process.env.CI ? 5 : 0)) || undefined
 export default defineConfig({
   testDir: "./e2e",
+  testIgnore: process.env.OPENCODE_PERFORMANCE === "1" ? "performance/**/*.test.ts" : "performance/**",
   outputDir: "./e2e/test-results",
   timeout: 60_000,
   expect: {