fix(chat): eliminate conversation switch flicker

apps/webview/e2e/browser-smoke.e2e.mts

@@ -96,7 +96,65 @@ async function verifyLongThreadVirtualization(page: Page): Promise<void> {
     { flags: RE_LONG_THREAD_TURN.flags, source: RE_LONG_THREAD_TURN.source },
   );
 
-  await page.locator('[data-thread-title]', { hasText: longThreadTitle }).click();
+  const firstPaint = await page.evaluate(
+    ({ lastTurn, title }) =>
+      new Promise<{
+        bottomGap: number | null;
+        conversationTitle: string | null;
+        tailMounted: boolean;
+      }>((resolve, reject) => {
+        const frameId = requestAnimationFrame(() => {
+          try {
+            const thread = [...document.querySelectorAll('[data-thread-title]')].find((element) =>
+              element.textContent.includes(title),
+            );
+            if (!(thread instanceof HTMLElement)) throw new Error(`Missing thread: ${title}`);
+            thread.click();
+            queueMicrotask(() => {
+              try {
+                const conversationTitle =
+                  document.querySelector('[data-conversation-title]')?.textContent ?? null;
+                const scroll = document.querySelector('[role="log"]')?.firstElementChild;
+                const content = scroll?.firstElementChild;
+                if (!(scroll instanceof HTMLElement) || !(content instanceof HTMLElement)) {
+                  throw new TypeError('Missing long-thread scroll surface');
+                }
+
+                const observer = new IntersectionObserver(
+                  ([entry]) => {
+                    observer.disconnect();
+                    resolve({
+                      bottomGap: entry.rootBounds
+                        ? entry.boundingClientRect.bottom - entry.rootBounds.bottom
+                        : null,
+                      conversationTitle,
+                      tailMounted: content.textContent.includes(`Turn ${lastTurn} —`),
+                    });
+                  },
+                  { root: scroll },
+                );
+                observer.observe(content);
+              } catch (error) {
+                reject(new Error('Could not observe first conversation paint', { cause: error }));
+              }
+            });
+          } catch (error) {
+            reject(new Error('Could not select the long thread', { cause: error }));
+          }
+        });
+        void frameId;
+      }),
+    { lastTurn: longThreadTurns, title: longThreadTitle },
+  );
+  assert.ok(
+    firstPaint.conversationTitle?.includes(longThreadTitle),
+    `First paint still showed ${JSON.stringify(firstPaint.conversationTitle)}`,
+  );
+  assert.ok(firstPaint.tailMounted, 'First paint did not show the long-thread tail');
+  assert.ok(
+    firstPaint.bottomGap !== null && Math.abs(firstPaint.bottomGap) <= 2,
+    `First paint opened ${String(firstPaint.bottomGap)}px above bottom`,
+  );
   await page.locator('[data-conversation-title]', { hasText: longThreadTitle }).waitFor();
   await page.waitForFunction((lastTurn) => {
     const scroll = document.querySelector('[role="log"]')?.firstElementChild;

packages/client/workbench/src/surface/use-workbench-sessions.ts

@@ -17,7 +17,7 @@ import { withoutAutomationSessions } from '@linkcode/ui';
 import { toastManager } from 'coss-ui/components/toast';
 import { noop } from 'foxact/noop';
 import { useEffect } from 'foxact/use-abortable-effect';
-import { useDeferredValue, useMemo, useRef } from 'react';
+import { useMemo, useRef } from 'react';
 import { useTranslations } from 'use-intl';
 import { captureProductEvent } from '../analytics/product-analytics';
 import type { NavLocation } from '../navigation/history';
@@ -94,18 +94,15 @@ export function useWorkbenchSessions(onError: (err: unknown) => void): Workbench
   // Session page: every thread stays one click away in the sidebar, so we skip straight to the
   // new-thread draft instead of auto-opening an arbitrary recent session (an all-automation list
   // has nothing to open either; an explicit automation selection resolves against the full list).
-  // Deferred for the render path only: the outgoing thread stays painted while the incoming tree
-  // renders, instead of flashing through an empty surface. Mutations use the live store values.
-  const deferredSelectedId = useDeferredValue(selectedId);
-  const draft = explicitDraft ?? (deferredSelectedId === null ? LANDING_DRAFT : null);
+  const draft = explicitDraft ?? (selectedId === null ? LANDING_DRAFT : null);
 
   const active = useMemo(() => {
     if (draft) return null;
     // Only an explicit selection resolves a conversation. One absent from the loaded list must NOT
     // fall back to a different thread (wrong conversation); hold null while the effect below
     // refreshes the list so a click-through to a not-yet-listed session resolves.
-    return sessionById(sessions, deferredSelectedId);
-  }, [draft, deferredSelectedId, sessions]);
+    return sessionById(sessions, selectedId);
+  }, [draft, selectedId, sessions]);
   const activeId = active?.sessionId ?? null;
 
   const recordNavigation = useNavigationHistoryStore((state) => state.record);

packages/presentation/ui/src/chat/conversation.tsx

@@ -78,12 +78,14 @@ export function ConversationContent<T>({
       return;
     }
 
-    // The library's instant path still waits for rAF; snap before paint while virtua settles.
-    const snapToBottom = (): void => {
-      if (state.isAtBottom) scroll.scrollTop = scroll.scrollHeight;
+    // Keep the DOM and virtua's external-scroll offset aligned while rows settle before paint.
+    const snapToBottom = (syncVirtualizer = false): void => {
+      if (!state.isAtBottom) return;
+      scroll.scrollTop = scroll.scrollHeight;
+      if (syncVirtualizer) scroll.dispatchEvent(new Event('scroll'));
     };
     snapToBottom();
-    const observer = new ResizeObserver(snapToBottom);
+    const observer = new ResizeObserver(() => snapToBottom(true));
     observer.observe(content);
     return () => observer.disconnect();
   }, [contentRef, reduceMotion, scrollRef, smoothConversationScrolling, state]);