fix(session): delay jump-to-bottom button (#20853)

packages/app/src/pages/session.tsx

@@ -347,6 +347,7 @@ export default function Page() {
     scroll: {
       overflow: false,
       bottom: true,
+      jump: false,
     },
   })
 
@@ -1247,13 +1248,17 @@ export default function Page() {
   let scrollStateTarget: HTMLDivElement | undefined
   let fillFrame: number | undefined
 
+  const jumpThreshold = (el: HTMLDivElement) => Math.max(400, el.clientHeight)
+
   const updateScrollState = (el: HTMLDivElement) => {
     const max = el.scrollHeight - el.clientHeight
+    const distance = max - el.scrollTop
     const overflow = max > 1
-    const bottom = !overflow || el.scrollTop >= max - 2
+    const bottom = !overflow || distance <= 2
+    const jump = overflow && distance > jumpThreshold(el)
 
-    if (ui.scroll.overflow === overflow && ui.scroll.bottom === bottom) return
-    setUi("scroll", { overflow, bottom })
+    if (ui.scroll.overflow === overflow && ui.scroll.bottom === bottom && ui.scroll.jump === jump) return
+    setUi("scroll", { overflow, bottom, jump })
   }
 
   const scheduleScrollState = (el: HTMLDivElement) => {

packages/app/src/pages/session/message-timeline.tsx

@@ -200,7 +200,7 @@ export function MessageTimeline(props: {
   mobileChanges: boolean
   mobileFallback: JSX.Element
   actions?: UserActions
-  scroll: { overflow: boolean; bottom: boolean }
+  scroll: { overflow: boolean; bottom: boolean; jump: boolean }
   onResumeScroll: () => void
   setScrollRef: (el: HTMLDivElement | undefined) => void
   onScheduleScrollState: (el: HTMLDivElement) => void
@@ -568,10 +568,9 @@ export function MessageTimeline(props: {
         <div
           class="absolute left-1/2 -translate-x-1/2 bottom-6 z-[60] pointer-events-none transition-all duration-200 ease-out"
           classList={{
-            "opacity-100 translate-y-0 scale-100":
-              props.scroll.overflow && !props.scroll.bottom && !staging.isStaging(),
+            "opacity-100 translate-y-0 scale-100": props.scroll.overflow && props.scroll.jump && !staging.isStaging(),
             "opacity-0 translate-y-2 scale-95 pointer-events-none":
-              !props.scroll.overflow || props.scroll.bottom || staging.isStaging(),
+              !props.scroll.overflow || !props.scroll.jump || staging.isStaging(),
           }}
         >
           <button