perf: isolate chat render caches by pane

ui/src/pages/chat/chat-thread.test.ts

@@ -23,6 +23,7 @@ const SENDER_METADATA_BLOCK =
 
 function createProps(overrides: Partial<CachedChatItemsProps> = {}): CachedChatItemsProps {
   return {
+    paneId: "pane-a",
     sessionKey: "main",
     runId: null,
     messages: [],
@@ -2454,6 +2455,31 @@ describe("thread item cache", () => {
       }),
     ).not.toBe(first);
   });
+
+  it("keeps same-session render caches isolated between panes", () => {
+    resetChatThreadState();
+    const messages = [
+      { role: "assistant", content: "needle" },
+      { role: "user", content: "other" },
+    ];
+    const paneA = createProps({
+      paneId: "pane-a",
+      messages,
+      searchOpen: true,
+      searchQuery: "needle",
+    });
+    const paneB = createProps({ paneId: "pane-b", messages });
+
+    const paneAItems = buildCachedChatItems(paneA);
+    const paneBItems = buildCachedChatItems(paneB);
+
+    expect(buildCachedChatItems({ ...paneA })).toBe(paneAItems);
+    expect(buildCachedChatItems({ ...paneB })).toBe(paneBItems);
+
+    resetChatThreadState("pane-a");
+    expect(buildCachedChatItems({ ...paneA })).not.toBe(paneAItems);
+    expect(buildCachedChatItems({ ...paneB })).toBe(paneBItems);
+  });
 });
 
 function canvasBlocksIn(group: MessageGroup): unknown[] {

ui/src/pages/chat/chat-thread.ts

@@ -46,6 +46,7 @@ import { getOrCreateSessionCacheValue } from "./session-cache.ts";
 import { buildUserChatMessageContentBlocks } from "./user-message-content.ts";
 
 type BuildChatItemsProps = {
+  paneId: string;
   sessionKey: string;
   runId?: string | null;
   /** Invalidates cached display copy when the active UI language changes. */
@@ -77,13 +78,17 @@ type StreamRunRenderItem = {
   parts: Array<Extract<ChatItem, { kind: "stream" } | { kind: "reading-indicator" }>>;
 };
 
-const chatItemsBySession = new Map<string, CachedChatItems>();
+const chatItemsByPane = new Map<string, Map<string, CachedChatItems>>();
 const expandedToolCardsBySession = new Map<string, Map<string, boolean>>();
 const initializedToolCardsBySession = new Map<string, Set<string>>();
 const lastAutoExpandPrefBySession = new Map<string, boolean>();
 
-export function resetChatThreadState(): void {
-  chatItemsBySession.clear();
+export function resetChatThreadState(paneId?: string): void {
+  if (paneId) {
+    chatItemsByPane.delete(paneId);
+    return;
+  }
+  chatItemsByPane.clear();
   resetWorkingStartedAt();
   expandedToolCardsBySession.clear();
   initializedToolCardsBySession.clear();
@@ -1546,7 +1551,12 @@ function sameChatItemsInput(previous: BuildChatItemsProps, next: BuildChatItemsP
 export function buildCachedChatItems(
   input: BuildChatItemsProps,
 ): ReturnType<typeof buildChatItems> {
-  const cached = getOrCreateSessionCacheValue(chatItemsBySession, input.sessionKey, () => ({
+  let paneCache = chatItemsByPane.get(input.paneId);
+  if (!paneCache) {
+    paneCache = new Map();
+    chatItemsByPane.set(input.paneId, paneCache);
+  }
+  const cached = getOrCreateSessionCacheValue(paneCache, input.sessionKey, () => ({
     input: null,
     items: [],
   }));

ui/src/pages/chat/components/chat-thread.ts

@@ -189,6 +189,7 @@ export function resetChatThreadPresentationState(paneId?: string) {
   removeChatSelectionPopup();
   if (paneId) {
     threadStates.delete(paneId);
+    resetChatThreadState(paneId);
   } else {
     threadStates.clear();
     resetChatThreadState();
@@ -598,6 +599,7 @@ export function renderChatThread(props: ChatThreadProps) {
   const deleted = getDeletedMessages(props.sessionKey);
   const locale = i18n.getLocale();
   const chatItems = buildCachedChatItems({
+    paneId: props.paneId,
     sessionKey: props.sessionKey,
     runId:
       props.sessions?.sessions.find((row) => areUiSessionKeysEquivalent(row.key, props.sessionKey))