fix: clear stale agent activity on cancel

Clear only still-active side-panel agent states when a run ends as cancelled or error, while preserving completed agent statuses. Add a renderer regression test for cancelled runs.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/renderer/lib/sessionActivity.ts

@@ -48,6 +48,14 @@ export function applySessionEventActivity(
     }
   }
 
+  if (
+    event.kind === 'run-updated'
+    && event.run
+    && (event.run.status === 'cancelled' || event.run.status === 'error')
+  ) {
+    return clearActiveSessionActivity(current, event.sessionId);
+  }
+
   return current;
 }
 
@@ -137,6 +145,38 @@ function removeSessionActivity(
   return next;
 }
 
+function clearActiveSessionActivity(
+  current: SessionActivityMap,
+  sessionId: string,
+): SessionActivityMap {
+  const sessionActivity = current[sessionId];
+  if (!sessionActivity) {
+    return current;
+  }
+
+  let changed = false;
+  const nextSessionActivity = Object.fromEntries(
+    Object.entries(sessionActivity).filter(([, activity]) => {
+      const keep = !isAgentActivityActive(activity);
+      changed ||= !keep;
+      return keep;
+    }),
+  );
+
+  if (!changed) {
+    return current;
+  }
+
+  if (Object.keys(nextSessionActivity).length === 0) {
+    return removeSessionActivity(current, sessionId);
+  }
+
+  return {
+    ...current,
+    [sessionId]: nextSessionActivity,
+  };
+}
+
 function resolveAgentKey(event: SessionEventRecord): string | undefined {
   return event.agentId?.trim() || event.agentName?.trim();
 }

tests/renderer/sessionActivity.test.ts

@@ -179,6 +179,55 @@ describe('session activity helpers', () => {
     ).toEqual(current);
   });
 
+  test('clears only active agent states when a run is cancelled', () => {
+    const current: SessionActivityMap = {
+      'session-1': {
+        architect: {
+          agentId: 'architect',
+          agentName: 'Architect',
+          activityType: 'completed',
+        },
+        reviewer: {
+          agentId: 'reviewer',
+          agentName: 'Reviewer',
+          activityType: 'thinking',
+        },
+      },
+    };
+
+    expect(
+      applySessionEventActivity(current, {
+        sessionId: 'session-1',
+        kind: 'run-updated',
+        occurredAt: '2026-03-23T00:00:01.000Z',
+        run: {
+          id: 'run-1',
+          requestId: 'turn-1',
+          projectId: 'project-1',
+          projectPath: 'C:\\workspace\\project',
+          workspaceKind: 'project',
+          patternId: 'pattern-1',
+          patternName: 'Pattern',
+          patternMode: 'single',
+          triggerMessageId: 'msg-1',
+          startedAt: '2026-03-23T00:00:00.000Z',
+          completedAt: '2026-03-23T00:00:01.000Z',
+          status: 'cancelled',
+          agents: [],
+          events: [],
+        },
+      }),
+    ).toEqual({
+      'session-1': {
+        architect: {
+          agentId: 'architect',
+          agentName: 'Architect',
+          activityType: 'completed',
+        },
+      },
+    });
+  });
+
   test('builds rows for all agents with sensible defaults', () => {
     expect(buildAgentActivityRows(undefined, agents)).toEqual([
       {