From 69b2c8bd159b785042ecba74ca9588122fd2c957 Mon Sep 17 00:00:00 2001
From: Vincent Koc <vincentkoc@ieee.org>
Date: Mon, 1 Jun 2026 07:42:24 +0100
Subject: [PATCH] perf(ui): record pending send paint timing (#88960)

---
 ui/src/ui/app-chat.test.ts          | 37 +++++++++++++++++++++++++++
 ui/src/ui/app-chat.ts               | 39 +++++++++++++++++++++++++++++
 ui/src/ui/control-ui-performance.ts |  9 +++++++
 3 files changed, 85 insertions(+)

diff --git a/ui/src/ui/app-chat.test.ts b/ui/src/ui/app-chat.test.ts
index 257048cd4dfa..1dbbccf4f532 100644
--- a/ui/src/ui/app-chat.test.ts
+++ b/ui/src/ui/app-chat.test.ts
@@ -1057,6 +1057,7 @@ describe("handleSendChat", () => {
   });
 
   afterEach(() => {
+    vi.restoreAllMocks();
     vi.unstubAllGlobals();
   });
 
@@ -1223,6 +1224,42 @@ describe("handleSendChat", () => {
     expect(ack?.requestDurationMs).toEqual(expect.any(Number));
   });
 
+  it("records pending send paint timing before a delayed chat.send ACK", async () => {
+    vi.spyOn(window, "requestAnimationFrame").mockImplementation((callback) => {
+      queueMicrotask(() => callback(0));
+      return 1;
+    });
+    const chatSend = createDeferred<{ status: "started" }>();
+    const request = vi.fn((method: string) => {
+      if (method === "chat.send") {
+        return chatSend.promise;
+      }
+      throw new Error(`Unexpected request: ${method}`);
+    });
+    const host = makeHost({
+      client: { request } as unknown as ChatHost["client"],
+      chatMessage: "measure painted pending send",
+      eventLogBuffer: [],
+      tab: "debug",
+    });
+
+    const send = handleSendChat(host);
+
+    await vi.waitFor(() =>
+      expect(eventPayloads(host, "control-ui.chat.send").map((payload) => payload.phase)).toEqual(
+        expect.arrayContaining(["pending-visible", "request-start", "pending-painted"]),
+      ),
+    );
+
+    chatSend.resolve({ status: "started" });
+    await send;
+
+    const phasesAfterAck = eventPayloads(host, "control-ui.chat.send").map(
+      (payload) => payload.phase,
+    );
+    expect(phasesAfterAck).toEqual(expect.arrayContaining(["ack"]));
+  });
+
   it("waits for an in-flight model picker update before sending chat", async () => {
     const switchUpdate = createDeferred<boolean>();
     const request = vi.fn(async (method: string) => {
diff --git a/ui/src/ui/app-chat.ts b/ui/src/ui/app-chat.ts
index 93748209cd24..7ed70a5858e4 100644
--- a/ui/src/ui/app-chat.ts
+++ b/ui/src/ui/app-chat.ts
@@ -31,6 +31,7 @@ import {
   controlUiNowMs,
   recordControlUiPerformanceEvent,
   roundedControlUiDurationMs,
+  scheduleControlUiAfterPaint,
 } from "./control-ui-performance.ts";
 import { resolveControlUiAuthHeader } from "./control-ui-auth.ts";
 import {
@@ -417,6 +418,7 @@ function enqueuePendingSendMessage(
   };
   host.chatQueue = [...host.chatQueue, pending];
   recordChatSendTiming(host, pending, "pending-visible", submittedAtMs);
+  schedulePendingSendPaintTiming(host, pending, submittedAtMs);
   scheduleChatScroll(host as unknown as Parameters<typeof scheduleChatScroll>[0], true);
   return pending;
 }
@@ -563,6 +565,7 @@ type QueuedChatSendResult = "sent" | "pending" | "failed";
 
 type ChatSendTimingPhase =
   | "pending-visible"
+  | "pending-painted"
   | "request-start"
   | "ack"
   | "queued-busy"
@@ -600,6 +603,42 @@ function recordChatSendTiming(
   );
 }
 
+function shouldRecordPendingSendPaint(item: ChatQueueItem): boolean {
+  return (
+    typeof item.sendSubmittedAtMs === "number" &&
+    (item.sendState === "waiting-model" ||
+      item.sendState === "sending" ||
+      item.sendState === "waiting-reconnect")
+  );
+}
+
+function schedulePendingSendPaintTiming(
+  host: ChatHost,
+  item: ChatQueueItem,
+  startedAtMs = item.sendSubmittedAtMs,
+) {
+  const sessionKey = item.sessionKey ?? host.sessionKey;
+  const sendRunId = item.sendRunId;
+  if (!sendRunId || startedAtMs == null) {
+    return;
+  }
+  scheduleControlUiAfterPaint(
+    host as Parameters<typeof scheduleControlUiAfterPaint>[0],
+    () => {
+      if (!visibleSessionMatches(host, sessionKey, item.agentId)) {
+        return;
+      }
+      const queued = readChatQueueForSession(host, sessionKey).find(
+        (entry) => entry.id === item.id && entry.sendRunId === sendRunId,
+      );
+      if (!queued || !shouldRecordPendingSendPaint(queued)) {
+        return;
+      }
+      recordChatSendTiming(host, queued, "pending-painted", startedAtMs);
+    },
+  );
+}
+
 function ensureQueuedSendState(
   host: ChatHost,
   item: ChatQueueItem,
diff --git a/ui/src/ui/control-ui-performance.ts b/ui/src/ui/control-ui-performance.ts
index 83f6bf53f771..9205c87edac5 100644
--- a/ui/src/ui/control-ui-performance.ts
+++ b/ui/src/ui/control-ui-performance.ts
@@ -159,6 +159,15 @@ export function scheduleControlUiTabVisibleTiming(
     .then(() => runAfterPaint(record));
 }
 
+export function scheduleControlUiAfterPaint(
+  host: Pick<ControlUiPerformanceHost, "updateComplete">,
+  callback: () => void,
+) {
+  void Promise.resolve(host.updateComplete)
+    .catch(() => undefined)
+    .then(() => runAfterPaint(callback));
+}
+
 export function beginControlUiRefresh(
   host: ControlUiPerformanceHost,
   tab: Tab,