fix(agent-hooks): keep a displaced completion unread in the sidebar badge (STA-3386)

Review round 3: sidebar-badge mode counts only the live entry, so a
session boundary landing on an unacknowledged completion silently
dropped the sidebar badge while the agent-events count kept it. Count
the displaced completion from history for boundary rows, and pin the
behavior with countActivityUnread tests.

src/renderer/src/components/activity/useActivityUnreadCount.test.ts

@@ -0,0 +1,66 @@
+import { describe, expect, it } from 'vitest'
+import type { AgentStatusEntry } from '../../../../shared/agent-status-types'
+import { countActivityUnread } from './useActivityUnreadCount'
+
+const PANE = 'tab-1:11111111-1111-4111-8111-111111111111'
+
+function makeEntry(overrides: Partial<AgentStatusEntry>): AgentStatusEntry {
+  return {
+    state: 'done',
+    prompt: '',
+    updatedAt: 2_000,
+    stateStartedAt: 2_000,
+    paneKey: PANE,
+    agentType: 'claude',
+    stateHistory: [],
+    ...overrides
+  }
+}
+
+function makeSource(entry: AgentStatusEntry, ackAt = 0) {
+  return {
+    acknowledgedAgentsByPaneKey: { [PANE]: ackAt },
+    agentStatusByPaneKey: { [PANE]: entry },
+    migrationUnsupportedByPtyId: {},
+    retainedAgentsByPaneKey: {},
+    worktreesByRepo: {}
+  }
+}
+
+describe('countActivityUnread session-boundary rows (STA-3386)', () => {
+  it('does not count a session-boundary done as unread in either mode', () => {
+    const source = makeSource(makeEntry({ sessionBoundary: true }))
+    expect(countActivityUnread(source, 'sidebar-badge')).toBe(0)
+    expect(countActivityUnread(source, 'agent-events')).toBe(0)
+  })
+
+  it('keeps counting a real completion displaced into history by a session boundary', () => {
+    // Why: agent finished (unacknowledged), then the user resumed the session — the
+    // boundary row replaces the live done but the finish must stay unread in both badges.
+    const source = makeSource(
+      makeEntry({
+        sessionBoundary: true,
+        stateHistory: [{ state: 'done', prompt: 'fix bug', startedAt: 1_000 }]
+      })
+    )
+    expect(countActivityUnread(source, 'sidebar-badge')).toBe(1)
+    expect(countActivityUnread(source, 'agent-events')).toBe(1)
+  })
+
+  it('stops counting the displaced completion once acknowledged', () => {
+    const source = makeSource(
+      makeEntry({
+        sessionBoundary: true,
+        stateHistory: [{ state: 'done', prompt: 'fix bug', startedAt: 1_000 }]
+      }),
+      1_500
+    )
+    expect(countActivityUnread(source, 'sidebar-badge')).toBe(0)
+    expect(countActivityUnread(source, 'agent-events')).toBe(0)
+  })
+
+  it('still counts an ordinary unacknowledged done in sidebar-badge mode', () => {
+    const source = makeSource(makeEntry({}))
+    expect(countActivityUnread(source, 'sidebar-badge')).toBe(1)
+  })
+})

src/renderer/src/components/activity/useActivityUnreadCount.ts

@@ -60,14 +60,22 @@ export function countActivityUnread(
         }
       }
     }
-    // Why: a session-boundary done is an idle connect (STA-3386), not an event to read;
-    // history never contains one (the status slice skips the push), so only the live entry needs the gate.
+    // Why: a session-boundary done is an idle connect (STA-3386), not an event to read.
+    // History never contains a boundary, but it DOES keep the real completion a boundary
+    // displaced (the slice pushes it on done→done), so sidebar-badge mode — which skips the
+    // history loop above — must still count that displaced completion or the badge silently
+    // drops an unacknowledged finish the moment its session is resumed.
     if (
       isUnreadAgentState(entry.state) &&
       entry.sessionBoundary !== true &&
       ackAt < entry.stateStartedAt
     ) {
       count += 1
+    } else if (mode === 'sidebar-badge' && entry.state === 'done' && entry.sessionBoundary) {
+      const displaced = entry.stateHistory.at(-1)
+      if (displaced && isUnreadAgentState(displaced.state) && ackAt < displaced.startedAt) {
+        count += 1
+      }
     }
   }