[UI] Subagent status line flickers and causes layout jumping

[randomm]

Problem

When background agents are running, the status line showing current activity (e.g., → Fuzu-Production-Db_query:) flashes very briefly and then disappears. This causes:

1. Flickering - Status line appears for ~1 second then vanishes
2. Layout jumping - Screen jumps when status line appears/disappears
3. Worse with multiple agents - More agents = more jumping

Screenshots

Without status line (most of the time):

• Shows only: ◉ Research-Specialist Task "Analyze PhD user recommendations"

With status line (brief flash):

• Shows: ◉ Research-Specialist Task "Analyze PhD user recommendations"
• Plus: → Fuzu-Production-Db_query:

Root Cause Analysis

THE BUG: Missing initial session registration causes race condition

When a task spawns a subagent:

1. TaskTool creates child session (task.ts:169-199)

   • Returns sessionId in metadata: ctx.metadata({ metadata: { sessionId: session.id } })
   • Session is persisted to storage via Session.createNext

2. Task component renders activity (index.tsx:1805-1807)

   const childMessages = createMemo(() =>
     props.metadata.sessionId ? (sync.data.message[props.metadata.sessionId] ?? []) : [],
   )

   • Immediately looks up sync.data.message[props.metadata.sessionId]

3. Sync context message store is EMPTY for new session (sync.tsx:228-266)

   • sync.data.message[childSessionID] is undefined initially
   • Child session's messages arrive via "message.updated" events after a delay
   • During this delay, childMessages() returns [], activity() returns null

4. Sync.session.sync() only runs on parent session (index.tsx:173-187)

   • The createEffect in Session component calls sync.session.sync(route.sessionID) for parent only
   • Child session is never synced, so its messages never populate the store via sync

5. Result: Activity line flashes → vanishes → reappears

   • T0: Task component renders with sessionId but no messages → no activity line
   • T1 (~1 sec): First "message.updated" event arrives → activity line appears briefly
   • T2 (~2 sec): State update clears sync.data.message[childSessionId] temporarily
   • T3: When stable, activity line stays visible (if at all)

Key Files

File	Lines	Purpose
packages/opencode/src/cli/cmd/tui/routes/session/index.tsx	1790-1888	Task component renders subagent status
packages/opencode/src/cli/cmd/tui/routes/session/index.tsx	1805-1807	childMessages memo lookup
packages/opencode/src/cli/cmd/tui/routes/session/index.tsx	1810-1838	activity memo calculation
packages/opencode/src/cli/cmd/tui/context/sync.tsx	228-266	"message.updated" event handler
packages/opencode/src/tool/task.ts	163-199, 225-250	Child session creation and metadata

Suggested Fix Approaches

Option 1: Fetch child session messages on demand

// In packages/opencode/src/cli/cmd/tui/routes/session/index.tsx
function Task(props: ToolProps<typeof TaskTool>) {
  const sync = useSync()
  
  // NEW: Ensure child session data is loaded
  createEffect(async () => {
    const childSessionId = props.metadata.sessionId
    if (!childSessionId) return
    const hasMessages = !!sync.data.message[childSessionId]?.length
    if (!hasMessages) {
      await sync.session.sync(childSessionId)
    }
  })
  
  // ... rest of component ...
}

Option 2: Add activity placeholder state

const activity = createMemo(() => {
  const msgs = childMessages()
  const hasPending = props.part.state.status === "running" || props.part.state.status === "pending"
  
  if (msgs.length === 0 && hasPending && props.metadata.sessionId) {
    return "Starting..."
  }
  
  // ... existing logic ...
})

Option 3: Reserve layout space

Prevent jumping by always reserving space for the status line when a task is running.

Related Issues

• [UI] Restore Subagent Activity Streaming #22 Restore Subagent Activity Streaming (may share root cause)

Priority

Medium - UX issue affecting usability when running parallel agents

[randomm]

Fixed in PR #39. Added error handling to child session sync to prevent UI flicker.