fix(ui): eliminate flash when auto-folding work sections on page load

Replace useEffect with synchronous state derivation during render so
the browser never paints the unfolded intermediate state. This prevents
the visible "jump" when loading an issue page with already-completed
work sections like "worked for 4 minutes".

Co-Authored-By: Paperclip <noreply@paperclip.ing>

ui/src/components/IssueChatThread.tsx

@@ -820,23 +820,24 @@ function IssueChatAssistantMessage() {
   const hasCoT = message.content.some((p) => p.type === "reasoning" || p.type === "tool-call");
   const isFoldable = !isRunning && hasCoT && !!chainOfThoughtLabel;
   const [folded, setFolded] = useState(isFoldable);
-  const previousMessageIdRef = useRef<string | null>(message.id);
-  const previousIsFoldableRef = useRef(isFoldable);
+  const [prevFoldKey, setPrevFoldKey] = useState({ messageId: message.id, isFoldable });
 
-  useEffect(() => {
+  // Derive fold state synchronously during render (not in useEffect) so the
+  // browser never paints the un-folded intermediate state — prevents the
+  // visible "jump" when loading a page with already-folded work sections.
+  if (message.id !== prevFoldKey.messageId || isFoldable !== prevFoldKey.isFoldable) {
     const nextFolded = resolveAssistantMessageFoldedState({
       messageId: message.id,
       currentFolded: folded,
       isFoldable,
-      previousMessageId: previousMessageIdRef.current,
-      previousIsFoldable: previousIsFoldableRef.current,
+      previousMessageId: prevFoldKey.messageId,
+      previousIsFoldable: prevFoldKey.isFoldable,
     });
-    previousMessageIdRef.current = message.id;
-    previousIsFoldableRef.current = isFoldable;
+    setPrevFoldKey({ messageId: message.id, isFoldable });
     if (nextFolded !== folded) {
       setFolded(nextFolded);
     }
-  }, [folded, isFoldable, message.id]);
+  }
 
   const handleVote = async (
     vote: FeedbackVoteValue,