fix(webui): correct activity timer duration (#4649)

webui/src/tests/thread-messages.test.tsx

@@ -1,5 +1,5 @@
 import { render, screen } from "@testing-library/react";
-import { describe, expect, it } from "vitest";
+import { afterEach, describe, expect, it, vi } from "vitest";
 
 import {
   assistantCopyFlags,
@@ -9,6 +9,10 @@ import {
 } from "@/components/thread/ThreadMessages";
 import type { UIMessage } from "@/lib/types";
 
+afterEach(() => {
+  vi.useRealTimers();
+});
+
 describe("ThreadMessages", () => {
   it("groups consecutive reasoning and tool rows into one timeline before the answer", () => {
     const messages: UIMessage[] = [
@@ -293,6 +297,45 @@ describe("ThreadMessages", () => {
     expect(screen.queryByLabelText(/editing foo\.txt/i)).not.toBeInTheDocument();
   });
 
+  it("times live activity from the user turn start", () => {
+    vi.useFakeTimers();
+    const startedAt = 1_700_000_000_000;
+    vi.setSystemTime(startedAt + 230_000);
+    const messages: UIMessage[] = [
+      {
+        id: "u1",
+        role: "user",
+        content: "run it",
+        turnId: "turn-1",
+        turnPhase: "user",
+        turnSeq: 1,
+        createdAt: startedAt,
+      },
+      {
+        id: "t1",
+        role: "tool",
+        kind: "trace",
+        content: "exec()",
+        traces: ["exec()"],
+        turnId: "turn-1",
+        turnPhase: "activity",
+        turnSeq: 2,
+        createdAt: startedAt + 220_000,
+      },
+    ];
+
+    const units = buildDisplayUnits(messages, true);
+
+    expect(
+      units[1].type === "activity" ? units[1].startedAtMs : undefined,
+    ).toBe(startedAt);
+
+    render(<ThreadMessages messages={messages} isStreaming />);
+
+    expect(screen.getByText("Working for 3m 50s")).toBeInTheDocument();
+    expect(screen.queryByText("Working for 10s")).not.toBeInTheDocument();
+  });
+
   it("folds final answer reasoning into the preceding activity timeline", () => {
     const messages: UIMessage[] = [
       {