fix(app): scroll jitter/loop

packages/app/src/pages/session.tsx

@@ -37,7 +37,6 @@ import { createOpenReviewFile, createSizing } from "@/pages/session/helpers"
 import { MessageTimeline } from "@/pages/session/message-timeline"
 import { type DiffStyle, SessionReviewTab, type SessionReviewTabProps } from "@/pages/session/review-tab"
 import { resetSessionModel, syncSessionModel } from "@/pages/session/session-model-helpers"
-import { createScrollSpy } from "@/pages/session/scroll-spy"
 import { SessionMobileTabs } from "@/pages/session/session-mobile-tabs"
 import { SessionSidePanel } from "@/pages/session/session-side-panel"
 import { TerminalPanel } from "@/pages/session/terminal-panel"
@@ -486,20 +485,49 @@ export default function Page() {
     return "main"
   })
 
-  const activeMessage = createMemo(() => {
-    if (!store.messageId) return lastUserMessage()
-    const found = visibleUserMessages()?.find((m) => m.id === store.messageId)
-    return found ?? lastUserMessage()
-  })
   const setActiveMessage = (message: UserMessage | undefined) => {
+    messageMark = scrollMark
     setStore("messageId", message?.id)
   }
 
+  const anchor = (id: string) => `message-${id}`
+
+  const cursor = () => {
+    const root = scroller
+    if (!root) return store.messageId
+
+    const box = root.getBoundingClientRect()
+    const line = box.top + 100
+    const list = [...root.querySelectorAll<HTMLElement>("[data-message-id]")]
+      .map((el) => {
+        const id = el.dataset.messageId
+        if (!id) return
+
+        const rect = el.getBoundingClientRect()
+        return { id, top: rect.top, bottom: rect.bottom }
+      })
+      .filter((item): item is { id: string; top: number; bottom: number } => !!item)
+
+    const shown = list.filter((item) => item.bottom > box.top && item.top < box.bottom)
+    const hit = shown.find((item) => item.top <= line && item.bottom >= line)
+    if (hit) return hit.id
+
+    const near = [...shown].sort((a, b) => {
+      const da = Math.abs(a.top - line)
+      const db = Math.abs(b.top - line)
+      if (da !== db) return da - db
+      return a.top - b.top
+    })[0]
+    if (near) return near.id
+
+    return list.filter((item) => item.top <= line).at(-1)?.id ?? list[0]?.id ?? store.messageId
+  }
+
   function navigateMessageByOffset(offset: number) {
     const msgs = visibleUserMessages()
     if (msgs.length === 0) return
 
-    const current = store.messageId
+    const current = store.messageId && messageMark === scrollMark ? store.messageId : cursor()
     const base = current ? msgs.findIndex((m) => m.id === current) : msgs.length
     const currentIndex = base === -1 ? msgs.length : base
     const targetIndex = currentIndex + offset
@@ -572,6 +600,8 @@ export default function Page() {
   let dockHeight = 0
   let scroller: HTMLDivElement | undefined
   let content: HTMLDivElement | undefined
+  let scrollMark = 0
+  let messageMark = 0
 
   const scrollGestureWindowMs = 250
 
@@ -616,6 +646,7 @@ export default function Page() {
       () => {
         setStore("messageId", undefined)
         setStore("changes", "session")
+        setUi("pendingMessage", undefined)
       },
       { defer: true },
     ),
@@ -1110,12 +1141,6 @@ export default function Page() {
 
   let scrollStateFrame: number | undefined
   let scrollStateTarget: HTMLDivElement | undefined
-  const scrollSpy = createScrollSpy({
-    onActive: (id) => {
-      if (id === store.messageId) return
-      setStore("messageId", id)
-    },
-  })
 
   const updateScrollState = (el: HTMLDivElement) => {
     const max = el.scrollHeight - el.clientHeight
@@ -1163,31 +1188,21 @@ export default function Page() {
     ),
   )
 
-  createEffect(
-    on(
-      sessionKey,
-      () => {
-        scrollSpy.clear()
-      },
-      { defer: true },
-    ),
-  )
-
-  const anchor = (id: string) => `message-${id}`
-
   const setScrollRef = (el: HTMLDivElement | undefined) => {
     scroller = el
     autoScroll.scrollRef(el)
-    scrollSpy.setContainer(el)
     if (el) scheduleScrollState(el)
   }
 
+  const markUserScroll = () => {
+    scrollMark += 1
+  }
+
   createResizeObserver(
     () => content,
     () => {
       const el = scroller
       if (el) scheduleScrollState(el)
-      scrollSpy.markDirty()
     },
   )
 
@@ -1220,7 +1235,6 @@ export default function Page() {
       if (stick) autoScroll.forceScrollToBottom()
 
       if (el) scheduleScrollState(el)
-      scrollSpy.markDirty()
     },
   )
 
@@ -1248,7 +1262,6 @@ export default function Page() {
 
   onCleanup(() => {
     document.removeEventListener("keydown", handleKeyDown)
-    scrollSpy.destroy()
     if (reviewFrame !== undefined) cancelAnimationFrame(reviewFrame)
     if (scrollStateFrame !== undefined) cancelAnimationFrame(scrollStateFrame)
   })
@@ -1280,7 +1293,7 @@ export default function Page() {
           <div class="flex-1 min-h-0 overflow-hidden">
             <Switch>
               <Match when={params.id}>
-                <Show when={activeMessage()}>
+                <Show when={lastUserMessage()}>
                   <MessageTimeline
                     mobileChanges={mobileChanges()}
                     mobileFallback={reviewContent({
@@ -1300,8 +1313,7 @@ export default function Page() {
                     onAutoScrollHandleScroll={autoScroll.handleScroll}
                     onMarkScrollGesture={markScrollGesture}
                     hasScrollGesture={hasScrollGesture}
-                    isDesktop={isDesktop()}
-                    onScrollSpyScroll={scrollSpy.onScroll}
+                    onUserScroll={markUserScroll}
                     onTurnBackfillScroll={historyWindow.onScrollerScroll}
                     onAutoScrollInteraction={autoScroll.handleInteraction}
                     centered={centered()}
@@ -1320,8 +1332,6 @@ export default function Page() {
                     }}
                     renderedUserMessages={historyWindow.renderedUserMessages()}
                     anchor={anchor}
-                    onRegisterMessage={scrollSpy.register}
-                    onUnregisterMessage={scrollSpy.unregister}
                   />
                 </Show>
               </Match>