fix(history): preserve manual timeline browsing during pending jumps

Cancel deferred active-marker reveals when the reader scrolls or interacts with the global rail, and reveal without a smooth animation. A previous selection must not move or unmount the next click target while its distant window is still loading.

Extend the Chromium latest-intent regression to keep the next destination visible beyond the reveal delay. Update the isolated native pruning UI fixture to proxy history routes and provide the same service-location inventory used by whole-session cleanup.

Validated all five browser navigation cases, the UI typecheck, packaged-resource smoke checks, and the complete isolated OpenCode 2.0.5 native/UI pruning fixture, including whole-session confirmation and persistence after reload.

packages/ui/src/components/message-timeline.tsx

@@ -383,22 +383,27 @@ const MessageTimeline: Component<MessageTimelineProps> = (props) => {
 
   const [scrollElement, setScrollElement] = createSignal<HTMLDivElement | undefined>()
   const [virtualizerHandle, setVirtualizerHandle] = createSignal<VirtualizerHandle | undefined>()
+  let revealTimer: ReturnType<typeof setTimeout> | undefined
+  const cancelReveal = () => {
+    if (revealTimer !== undefined) clearTimeout(revealTimer)
+    revealTimer = undefined
+  }
 
   const handleScroll = () => {
+    cancelReveal()
     if (hoveredSegment()) clearHoverPreview()
   }
 
   createEffect(on(() => props.activeSegmentId, (activeId) => {
     if (!activeId) return
-    const timer = typeof window !== "undefined" ? window.setTimeout(() => {
+    revealTimer = setTimeout(() => {
+      revealTimer = undefined
       const index = segmentIndexById().get(activeId)
-      if (index !== undefined) virtualizerHandle()?.scrollToIndex(index, { align: "nearest", smooth: true })
-    }, 120) : null
-    onCleanup(() => {
-      if (timer !== null && typeof window !== "undefined") {
-        window.clearTimeout(timer)
-      }
-    })
+      // The rail is globally virtualized: an animated reveal can keep moving
+      // under the pointer while the reader chooses another distant destination.
+      if (index !== undefined) virtualizerHandle()?.scrollToIndex(index, { align: "nearest" })
+    }, 120)
+    onCleanup(cancelReveal)
   }))
 
   createEffect(() => {
@@ -569,6 +574,9 @@ const MessageTimeline: Component<MessageTimelineProps> = (props) => {
         role="navigation"
         aria-label={t("messageTimeline.ariaLabel")}
         onScroll={handleScroll}
+        onWheel={cancelReveal}
+        onPointerDown={cancelReveal}
+        onKeyDown={cancelReveal}
       >
         <Virtualizer ref={setVirtualizerHandle} data={props.segments} scrollRef={scrollElement()} bufferSize={TIMELINE_VIRTUALIZER_BUFFER_PX}>
           {(segment) => {

packages/ui/tests/browser/history-navigation.test.ts

@@ -74,10 +74,18 @@ async function fixture() {
   }
 }
 
-async function clickTimeline(page: Page, index: number) {
+async function clickTimeline(page: Page, index: number, checkReaderPosition = false) {
   await page.locator(".message-timeline").evaluate((element, index) => {
     element.scrollTop = element.scrollHeight * index / 1500
   }, index)
+  if (checkReaderPosition) {
+    // Let the prior selection's 120ms reveal expire while the reader is
+    // browsing elsewhere. It must not reclaim the rail or animate the target.
+    await page.waitForTimeout(250)
+    const target = await page.locator(`.message-timeline-segment[data-message-id="${navigationMessageId(index)}"]`).boundingBox()
+    const rail = await page.locator(".message-timeline").boundingBox()
+    assert(target && rail && target.y >= rail.y && target.y < rail.y + rail.height, "reader's destination remains visible before clicking")
+  }
   await page.locator(`.message-timeline-segment[data-message-id="${navigationMessageId(index)}"]`).click()
 }
 const snapshot = (page: Page) => page.evaluate(() => (window as any).fixture.snapshot())
@@ -119,7 +127,7 @@ test("latest timeline intent supersedes a slow far jump, including a resident de
     const release = f.hold(250)
     await clickTimeline(f.page, 250)
     await f.page.locator(".history-navigation-status").waitFor()
-    await clickTimeline(f.page, 1450)
+    await clickTimeline(f.page, 1450, true)
     await f.page.locator(".history-navigation-status").waitFor({ state: "hidden" })
     release()
     await f.page.waitForTimeout(150)

scripts/test-session-pruning-ui.mjs

@@ -29,6 +29,8 @@ export async function testPruningUI({ client, baseUrl, root, location, generate,
     ...(connection ? { getSharedServiceConnection: async () => connection } : {}),
     getInstanceAuthorizationHeader: () => `Basic ${Buffer.from("opencode:isolated-pruning-fixture").toString("base64")}`,
     getServiceDirectory: () => location.directory,
+    getServiceLocation: () => location,
+    getWorktrees: async () => ({ worktrees: [] }),
     getSharedServiceClient: async () => client,
     getSessionEnvironment: async () => {
       const { sessionEnvironment } = await tsImport("../packages/server/src/workspaces/session-environment.ts", import.meta.url)
@@ -71,6 +73,7 @@ export async function testPruningUI({ client, baseUrl, root, location, generate,
     server: { host: "127.0.0.1", port: 0, hmr: false, watch: null, proxy: {
       "/workspaces/pruning-ui/instance": { target: `http://127.0.0.1:${broker.server.address().port}` },
       "/api/workspaces/pruning-ui/session-pruning": { target: `http://127.0.0.1:${broker.server.address().port}` },
+      "/api/workspaces/pruning-ui/session-history": { target: `http://127.0.0.1:${broker.server.address().port}` },
       "/fixture-events": { target: `http://127.0.0.1:${broker.server.address().port}` },
     } },
   })