STUPID SEXY TIMELINE (#16420)

packages/app/src/pages/session.tsx

@@ -121,13 +121,9 @@ function createSessionHistoryWindow(input: SessionHistoryWindowInput) {
       return
     }
     const beforeTop = el.scrollTop
-    const beforeHeight = el.scrollHeight
     fn()
-    requestAnimationFrame(() => {
-      const delta = el.scrollHeight - beforeHeight
-      if (!delta) return
-      el.scrollTop = beforeTop + delta
-    })
+    void el.scrollHeight
+    el.scrollTop = beforeTop
   }
 
   const backfillTurns = () => {
@@ -210,7 +206,7 @@ function createSessionHistoryWindow(input: SessionHistoryWindowInput) {
     if (!input.userScrolled()) return
     const el = input.scroller()
     if (!el) return
-    if (el.scrollTop >= turnScrollThreshold) return
+    if (el.scrollHeight - el.clientHeight + el.scrollTop >= turnScrollThreshold) return
 
     const start = turnStart()
     if (start > 0) {
@@ -1110,7 +1106,7 @@ export default function Page() {
   const updateScrollState = (el: HTMLDivElement) => {
     const max = el.scrollHeight - el.clientHeight
     const overflow = max > 1
-    const bottom = !overflow || el.scrollTop >= max - 2
+    const bottom = !overflow || Math.abs(el.scrollTop) <= 2 || !autoScroll.userScrolled()
 
     if (ui.scroll.overflow === overflow && ui.scroll.bottom === bottom) return
     setUi("scroll", { overflow, bottom })
@@ -1133,7 +1129,7 @@ export default function Page() {
 
   const resumeScroll = () => {
     setStore("messageId", undefined)
-    autoScroll.forceScrollToBottom()
+    autoScroll.smoothScrollToBottom()
     clearMessageHash()
 
     const el = scroller
@@ -1201,13 +1197,11 @@ export default function Page() {
 
       const el = scroller
       const delta = next - dockHeight
-      const stick = el
-        ? !autoScroll.userScrolled() || el.scrollHeight - el.clientHeight - el.scrollTop < 10 + Math.max(0, delta)
-        : false
+      const stick = el ? Math.abs(el.scrollTop) < 10 + Math.max(0, delta) : false
 
       dockHeight = next
 
-      if (stick) autoScroll.forceScrollToBottom()
+      if (stick) autoScroll.smoothScrollToBottom()
 
       if (el) scheduleScrollState(el)
       scrollSpy.markDirty()
@@ -1293,6 +1287,7 @@ export default function Page() {
                     onScrollSpyScroll={scrollSpy.onScroll}
                     onTurnBackfillScroll={historyWindow.onScrollerScroll}
                     onAutoScrollInteraction={autoScroll.handleInteraction}
+                    onPreserveScrollAnchor={autoScroll.preserve}
                     centered={centered()}
                     setContentRef={(el) => {
                       content = el