fix(webui): auto-collapse completed activity

webui/src/components/thread/AgentActivityCluster.tsx

@@ -225,13 +225,18 @@ export function AgentActivityCluster({
 
   const [userToggledOuter, setUserToggledOuter] = useState(false);
   const [outerOpenLocal, setOuterOpenLocal] = useState(false);
+  const [completionHoldOpen, setCompletionHoldOpen] = useState(false);
   const [now, setNow] = useState(() => Date.now());
   const activityScrollRef = useRef<HTMLDivElement>(null);
   const activityContentRef = useRef<HTMLDivElement>(null);
   const autoFollowActivityRef = useRef(true);
   const scrollFrameRef = useRef<number | null>(null);
-  /** Live work and the trace directly attached to an answer read like a visible trail. */
+  const wasTurnStreamingRef = useRef(isTurnStreaming);
-  const outerExpanded = userToggledOuter ? outerOpenLocal : isTurnStreaming || hasBodyBelow;
+  const wasTurnStreaming = wasTurnStreamingRef.current;
+  /** Live work stays open; completed work briefly shows the done state, then tucks away. */
+  const outerExpanded = userToggledOuter
+    ? outerOpenLocal
+    : isTurnStreaming || completionHoldOpen || (wasTurnStreaming && !isTurnStreaming);
 
   const hasLiveEditingFiles = isTurnStreaming && hasEditingFiles;
   const singleFilePath = fileCount === 1 ? primaryFilePath : undefined;
@@ -377,6 +382,19 @@ export function AgentActivityCluster({
     return () => window.clearInterval(interval);
   }, [isTurnStreaming]);
 
+  useEffect(() => {
+    const wasStreaming = wasTurnStreamingRef.current;
+    wasTurnStreamingRef.current = isTurnStreaming;
+    if (isTurnStreaming) {
+      setCompletionHoldOpen(false);
+      return undefined;
+    }
+    if (!wasStreaming || userToggledOuter) return undefined;
+    setCompletionHoldOpen(true);
+    const timeout = window.setTimeout(() => setCompletionHoldOpen(false), 900);
+    return () => window.clearTimeout(timeout);
+  }, [isTurnStreaming, userToggledOuter]);
+
   const onActivityScroll = useCallback(() => {
     const el = activityScrollRef.current;
     if (!el) return;

webui/src/tests/agent-activity-cluster.test.tsx

@@ -1,5 +1,5 @@
 import { act, fireEvent, render, screen, waitFor } from "@testing-library/react";
-import { describe, expect, it } from "vitest";
+import { describe, expect, it, vi } from "vitest";
 
 import { AgentActivityCluster } from "@/components/thread/AgentActivityCluster";
 import type { CliAppInfo, McpPresetInfo, UIMessage } from "@/lib/types";
@@ -293,6 +293,53 @@ describe("AgentActivityCluster", () => {
     await waitFor(() => expect(marker).toHaveClass("animate-in"));
   });
 
+  it("briefly shows completed activity, then auto-collapses before the answer", () => {
+    vi.useFakeTimers();
+    const liveReasoning: UIMessage = {
+      id: "r-collapse",
+      role: "assistant",
+      content: "",
+      reasoning: "checking files",
+      reasoningStreaming: true,
+      isStreaming: true,
+      createdAt: 1,
+    };
+    try {
+      const { rerender } = render(
+        <AgentActivityCluster
+          messages={[liveReasoning]}
+          isTurnStreaming
+          hasBodyBelow
+        />,
+      );
+      expect(screen.getByTestId("agent-activity-scroll")).toBeInTheDocument();
+
+      rerender(
+        <AgentActivityCluster
+          messages={[{
+            ...liveReasoning,
+            reasoningStreaming: false,
+            isStreaming: false,
+          }]}
+          isTurnStreaming={false}
+          hasBodyBelow
+        />,
+      );
+
+      expect(screen.getByTestId("agent-activity-scroll")).toBeInTheDocument();
+      act(() => {
+        vi.advanceTimersByTime(901);
+      });
+      expect(screen.queryByTestId("agent-activity-scroll")).not.toBeInTheDocument();
+      expect(screen.getByRole("button", { name: /1 steps/i })).toHaveAttribute(
+        "aria-expanded",
+        "false",
+      );
+    } finally {
+      vi.useRealTimers();
+    }
+  });
+
   it("renders file edit totals and a compact expanded file list", async () => {
     const restoreMotion = installReducedMotion();
     try {
@@ -583,6 +630,8 @@ describe("AgentActivityCluster", () => {
       />,
     );
 
+    fireEvent.click(screen.getByRole("button", { name: /1 tool calls/i }));
+
     expect(screen.getByText("Shell")).toBeInTheDocument();
     expect(screen.getByText(/cat << 'EOF' \| bash · script, 6 lines/)).toBeInTheDocument();
     expect(screen.queryByText(/SECRET_TOKEN/)).not.toBeInTheDocument();