fix(ui): never reuse a stale live-turn skeleton and poll lagging traces

A finished turn whose persisted JSONL had not flushed yet left the live
turn on screen; the next prompt then patched its content into the stale
skeleton, so the new message rendered under the previous user bubble.
Key each live turn and rebuild the skeleton on mismatch, and poll the
persisted trace after a lagging read so the conversation converges
without waiting for a user action. Module nav rows also collapse to one
line at the primary-action control height.

packages/ui/desktop/src/app.ts

@@ -69,6 +69,8 @@ interface SessionUpdatePayload {
 
 /** One in-flight turn rendered incrementally while ACP updates stream in. */
 interface LiveTurn {
+  /** Distinguishes turns so a new prompt never reuses the previous turn's DOM skeleton. */
+  key: number
   userText?: string
   thinking: string
   answer: string
@@ -154,6 +156,9 @@ const state = {
   graph: buildTraceGraph('', []),
   expandedActivityIds: new Set<string>(),
   traceLoadRevision: 0,
+  liveTurnCounter: 0,
+  traceCatchupTimer: undefined as number | undefined,
+  traceCatchupAttempts: 0,
 }
 
 function t(key: I18nKey): string {
@@ -212,8 +217,8 @@ function buildShell(): void {
           <button class="primary-action" data-action="new-session"><span data-text="app.newChat"></span><kbd>⌘N</kbd></button>
         </section>
         <nav class="product-nav" aria-label="DeepSeek Harness modules">
-          <button class="module-button selected" data-module="sessions"><span><strong data-text="app.sessions"></strong><small data-text="app.sessionsSubtitle"></small></span></button>
-          <button class="module-button" data-module="develop"><span><strong data-text="app.develop"></strong><small data-text="app.developSubtitle"></small></span></button>
+          <button class="module-button selected" data-module="sessions" data-title="app.sessionsSubtitle"><strong data-text="app.sessions"></strong></button>
+          <button class="module-button" data-module="develop" data-title="app.developSubtitle"><strong data-text="app.develop"></strong></button>
         </nav>
         <label class="session-search">
           <span data-text="app.sessions"></span>
@@ -340,6 +345,9 @@ function applyStaticText(): void {
   for (const node of appEl.querySelectorAll<HTMLElement>('[data-text]')) {
     node.textContent = t(node.dataset.text as I18nKey)
   }
+  for (const node of appEl.querySelectorAll<HTMLElement>('[data-title]')) {
+    node.title = t(node.dataset.title as I18nKey)
+  }
   el.searchInput.placeholder = t('app.searchPlaceholder')
   el.composerInput.placeholder = state.selectedSessionId === undefined
     ? t('composer.placeholderDraft')
@@ -436,7 +444,7 @@ async function refreshSessions(preferredId?: string): Promise<void> {
   }
 }
 
-async function loadTrace(sessionId: string): Promise<void> {
+async function loadTrace(sessionId: string, resetExpansion = true): Promise<void> {
   captureFeedbackDraft()
   const revision = ++state.traceLoadRevision
   state.selectedSessionId = sessionId
@@ -450,7 +458,7 @@ async function loadTrace(sessionId: string): Promise<void> {
     showError(String(error))
   }
   if (revision !== state.traceLoadRevision || state.selectedSessionId !== sessionId) return
-  applyTrace(trace, true)
+  applyTrace(trace, resetExpansion)
 }
 
 function applyTrace(trace: TracePayload, resetExpansion: boolean): void {
@@ -476,6 +484,32 @@ function applyTrace(trace: TracePayload, resetExpansion: boolean): void {
   renderWaterfall()
   renderInspector()
   scrollChatToBottom(true)
+  scheduleTraceCatchup(sessionId)
+}
+
+/**
+ * Persisted JSONL can lag a finished turn (the runtime flushes asynchronously),
+ * which leaves the live turn on screen next to a stale conversation. Poll the
+ * trace briefly until the expected turn lands instead of waiting for a user
+ * action to trigger the next read.
+ */
+function scheduleTraceCatchup(sessionId: string): void {
+  if (state.traceCatchupTimer !== undefined) {
+    window.clearTimeout(state.traceCatchupTimer)
+    state.traceCatchupTimer = undefined
+  }
+  const live = state.live.get(sessionId)
+  if (live === undefined || state.busySessionId !== undefined || live.status === 'error') {
+    state.traceCatchupAttempts = 0
+    return
+  }
+  if (state.traceCatchupAttempts >= 12) return
+  state.traceCatchupAttempts += 1
+  state.traceCatchupTimer = window.setTimeout(() => {
+    state.traceCatchupTimer = undefined
+    if (state.selectedSessionId !== sessionId || state.busySessionId !== undefined) return
+    void loadTrace(sessionId, false)
+  }, 500)
 }
 
 function updateSessionSummaryFromTrace(trace: TracePayload): void {
@@ -515,7 +549,7 @@ function completedTurnCount(trace: TracePayload | undefined): number {
 function handleSessionUpdate(payload: SessionUpdatePayload): void {
   const update = payload.update
   const kind = String(update.sessionUpdate ?? '')
-  const live = state.live.get(payload.sessionId) ?? { thinking: '', answer: '', tools: [], expectedCompletedTurns: completedTurnCount(state.trace) + 1, status: 'streaming' as const }
+  const live = state.live.get(payload.sessionId) ?? { key: ++state.liveTurnCounter, thinking: '', answer: '', tools: [], expectedCompletedTurns: completedTurnCount(state.trace) + 1, status: 'streaming' as const }
   if (kind === 'agent_message_chunk') {
     live.answer += contentText(update.content)
     live.status = 'streaming'
@@ -593,6 +627,13 @@ function renderLiveTurn(): void {
 function ensureLiveSkeleton(live: LiveTurn): void {
   el.conversation.querySelector('.empty-thread')?.remove()
   let root = el.liveTurn.querySelector<HTMLElement>('.message.live')
+  // A stale skeleton from an earlier turn (persisted trace still lagging) must
+  // never be patched with the new turn's content — its user bubble would keep
+  // showing the previous prompt.
+  if (root !== null && root.dataset.liveKey !== String(live.key)) {
+    el.liveTurn.innerHTML = ''
+    root = null
+  }
   if (root === null) {
     el.liveTurn.innerHTML = `
       ${live.userText === undefined ? '' : `
@@ -600,7 +641,7 @@ function ensureLiveSkeleton(live: LiveTurn): void {
           <div class="message-card"><div class="user-bubble">${escapeHtml(live.userText)}</div></div>
         </article>
       `}
-      <article class="message assistant live">
+      <article class="message assistant live" data-live-key="${live.key}">
         <div class="avatar">A</div>
         <div class="message-card">
           <div class="activity-list">
@@ -663,6 +704,7 @@ async function sendPrompt(prompt: string): Promise<void> {
   }
   const draftKey = state.selectedSessionId ?? DRAFT_SESSION_ID
   state.live.set(draftKey, {
+    key: ++state.liveTurnCounter,
     userText: prompt,
     thinking: '',
     answer: '',

packages/ui/desktop/src/styles.css

@@ -246,9 +246,12 @@ kbd {
   transition: background 120ms var(--ease-out);
 }
 
+/* Module nav rows match the primary action exactly: one control height. */
 .module-button {
-  min-height: var(--row-list);
-  padding: 6px 10px;
+  display: flex;
+  align-items: center;
+  height: var(--control-lg);
+  padding: 0 10px;
 }
 
 .module-button:not(.selected) {

packages/ui/desktop/tests/renderer.spec.ts

@@ -234,4 +234,91 @@ describe('desktop renderer chat lifecycle', () => {
     document.querySelector<HTMLButtonElement>('[data-action="jump-traj"]')!.click()
     expect(document.querySelector('#trajView')?.classList.contains('active')).toBe(true)
   })
+
+  it('starts a fresh live skeleton per turn and converges without user action when the persisted trace lags', async () => {
+    const prompts: Deferred<unknown>[] = [deferred(), deferred(), deferred()]
+    const promptQueue = [...prompts]
+    let traceRead: unknown
+    const turnEvents = (turn: number, userText: string, answer: string, base: number): unknown[] => [
+      { type: 'turn/start', seq: base, time: base + 1, data: { turn, trigger: { kind: 'message' } } },
+      { type: 'user/message', seq: base + 1, time: base + 2, data: { content: [{ type: 'text', text: userText }] } },
+      { type: 'step/start', seq: base + 2, time: base + 3, data: { turn, step: 1 } },
+      { type: 'assistant/message', seq: base + 3, time: base + 4, data: { turn, step: 1, content: [{ type: 'text', text: answer }] } },
+      { type: 'step/end', seq: base + 4, time: base + 5, data: { turn, step: 1 } },
+      { type: 'turn/end', seq: base + 5, time: base + 6, data: { turn, reason: { kind: 'completed' } } },
+    ]
+    const trace = (events: unknown[]): unknown => ({ found: true, sessionId: 's-lag', header: { id: 's-lag' }, rawText: '', feedback: [], events })
+    const turn1Trace = trace(turnEvents(1, 'first', 'first answer', 0))
+    const fullTrace = trace([
+      ...turnEvents(1, 'first', 'first answer', 0),
+      ...turnEvents(2, 'second', 'second answer', 10),
+      ...turnEvents(3, 'third', 'third answer', 20),
+    ])
+    traceRead = turn1Trace
+
+    window.dshDesktop = {
+      runtime: {
+        start: async () => ({}),
+        stop: async () => ({}),
+        restart: async () => ({}),
+        status: async () => ({ state: 'running', repoRoot: '/repo' }),
+        onStatus: () => () => {},
+        onStderr: () => () => {},
+      },
+      sessions: {
+        list: async () => ({ sessions: [] }),
+        create: async () => ({ sessionId: 's-lag', trace: trace([]) }),
+        load: async () => ({}),
+        prompt: async () => promptQueue.shift()!.promise,
+        cancel: async () => ({}),
+        reveal: async () => ({}),
+        onUpdate: () => () => {},
+      },
+      trace: { read: async () => traceRead },
+      feedback: { list: async () => [], add: async () => ({}) },
+      dev: { status: async () => ({ git: {} }) },
+    }
+
+    await import('../src/app.ts')
+    await vi.waitFor(() => {
+      expect(document.querySelector('#composerInput')).not.toBeNull()
+    })
+    const composer = document.querySelector<HTMLTextAreaElement>('#composerInput')!
+    const form = document.querySelector<HTMLFormElement>('#composerForm')!
+    const send = (text: string): void => {
+      composer.value = text
+      composer.dispatchEvent(new Event('input', { bubbles: true }))
+      form.dispatchEvent(new Event('submit', { bubbles: true, cancelable: true }))
+    }
+
+    document.querySelector<HTMLButtonElement>('[data-action="new-session"]')!.click()
+    send('first')
+    prompts[0]!.resolve({ response: {}, trace: turn1Trace })
+    await vi.waitFor(() => {
+      expect(document.querySelector('#conversation')?.textContent).toContain('first answer')
+      expect(document.querySelector('#liveTurn')?.innerHTML).toBe('')
+    })
+
+    // Turn 2 finishes but the persisted trace still lags behind: the live turn stays.
+    send('second')
+    prompts[1]!.resolve({ response: {}, trace: turn1Trace })
+    await vi.waitFor(() => {
+      expect(document.querySelector<HTMLButtonElement>('#cancelButton')?.hidden).toBe(true)
+    })
+    expect(document.querySelector('#liveTurn .user-bubble')?.textContent).toBe('second')
+
+    // A third prompt must never reuse the previous turn's skeleton (swallowed-message regression).
+    send('third')
+    await vi.waitFor(() => {
+      expect(document.querySelector('#liveTurn .user-bubble')?.textContent).toBe('third')
+    })
+
+    // Once the persisted log catches up, the view converges with no user action.
+    prompts[2]!.resolve({ response: {}, trace: turn1Trace })
+    traceRead = fullTrace
+    await vi.waitFor(() => {
+      expect(document.querySelector('#conversation')?.textContent).toContain('third answer')
+      expect(document.querySelector('#liveTurn')?.innerHTML).toBe('')
+    }, { timeout: 4000 })
+  })
 })