fix(desktop): settle channel opens at latest message

Wait for the virtualized timeline API before consuming initial channel
positioning, and do not mistake measurement-driven offsets for reader
input while the virtualizer is settling at the floor.

Co-authored-by: Carl <c7ebe626f000404285d3686e1dc74cc07cc60a9754a150041ba132e14bd3e2ec@sprout-oss.stage.blox.sqprod.co>
Signed-off-by: Wes <wesbillman@users.noreply.github.com>

desktop/src/features/messages/ui/TimelineMessageList.tsx

@@ -670,7 +670,12 @@ function VirtualizedTimelineRows({
       if (!list || !(scroller instanceof HTMLDivElement)) return;
       onVirtualizerRangeChanged?.();
       const distanceFromBottom = list.scrollSize - list.viewportSize - offset;
-      if (distanceFromBottom > 32) cancelBottomSettle();
+      // Do not infer reader intent from an intermediate virtualizer offset.
+      // Initial channel positioning deliberately chases the floor while rows
+      // are measured; those measurements can briefly report a large gap and
+      // emit `onScroll` without any user input. Cancelling here strands the
+      // channel above its newest message. The settle hook's wheel, pointer,
+      // touch, and key listeners are the authoritative user-interaction gate.
       onAtBottomStateChange?.(distanceFromBottom <= 32);
       if (
         prependAnchorRef.current !== null ||
@@ -686,7 +691,6 @@ function VirtualizedTimelineRows({
     },
     [
       armUpwardMomentum,
-      cancelBottomSettle,
       capturePrependAnchor,
       onAtBottomStateChange,
       onStartReached,

desktop/src/features/messages/ui/useAnchoredScroll.ts

@@ -607,6 +607,11 @@ export function useAnchoredScroll({
     // to the requested target message, or to the bottom by default.
     if (!hasInitializedRef.current) {
       if (isLoading) return;
+      // The virtualized list owns the actual scroll node. Its API registers in
+      // a child layout effect, after this parent hook's first pass; treating
+      // that API-less pass as initialized writes to the inert outer wrapper
+      // and permanently consumes the channel's initial bottom pin.
+      if (virtualizerOwnsPrependAnchoring && !virtualScrollToBottom) return;
       // Establish the initial position before the browser paints. The follow-up
       // frame is a settling pass for content whose measurements land with the
       // commit (fonts, deferred rows, media), not the first bottom pin. Keeping

desktop/tests/e2e/scroll-history.spec.ts

@@ -68,6 +68,46 @@ async function getMessagePosition(
   }, messageId);
 }
 
+test("channel switch settles at the newest message after virtualized rows measure", async ({
+  page,
+}) => {
+  await installMockBridge(page);
+  await page.goto("/");
+  await page.waitForFunction(
+    () => typeof window.__BUZZ_E2E_EMIT_MOCK_MESSAGE__ === "function",
+  );
+
+  await page.evaluate(() => {
+    const base = Math.floor(Date.now() / 1000);
+    for (let index = 0; index < 80; index += 1) {
+      window.__BUZZ_E2E_EMIT_MOCK_MESSAGE__?.({
+        channelName: "general",
+        content: `switch-bottom ${index} ${"variable-height ".repeat(
+          index % 7,
+        )}`,
+        createdAt: base + index,
+      });
+    }
+  });
+
+  // Open another channel first so this exercises a real channel switch, where
+  // the virtualizer API is temporarily null while the keyed list remounts.
+  await page.getByTestId("channel-random").click();
+  await expect(page.getByTestId("chat-title")).toHaveText("random");
+  await page.getByTestId("channel-general").click();
+  await expect(page.getByTestId("chat-title")).toHaveText("general");
+
+  const timeline = page.getByTestId("message-timeline");
+  await expect(timeline).toContainText("switch-bottom 79");
+  await expect
+    .poll(async () => {
+      const metrics = await getTimelineMetrics(page);
+      return metrics.scrollHeight - metrics.clientHeight - metrics.scrollTop;
+    })
+    .toBeLessThanOrEqual(1);
+  await expect(page.getByTestId("message-scroll-to-latest")).toHaveCount(0);
+});
+
 test("first channel load paints the first window without waiting for the row-floor top-up", async ({
   page,
 }) => {