fix: resync session state after SSE reconnect to prevent stuck subagent UI (#817)

* fix: resync session state after SSE reconnect to prevent stuck subagent UI

When a subagent completes while the page is in the background (common on
mobile PWA and desktop webview), the final SSE events are lost. The UI
then stays stuck on 'Waiting for subagent activity...' because:

- part.state.status never transitions to 'completed'
- session_status is never updated to 'idle'
- activeLatched remains true indefinitely

Fix:
- Add onReconnect callback to event pipeline, fired after SSE reconnect
- Add pageshow listener for bfcache restores (mobile PWA back-forward)
- On reconnect, re-fetch session list for directories with active sessions
- Pass explicit directory to useSessionActivity in ToolPart for subagents
  to ensure the correct child store is queried

Closes #810

* fix(chat): resolve pending subagent task binding before metadata arrives

* fix: restore subagent activity and tool visibility after reconnect

- Resyncs session status and child session data after SSE reconnect
- Ensures child task tool messages are read from the correct directory
- Prevents stale assistant fallback from keeping sessions stuck as active

---------

Co-authored-by: Bohdan Triapitsyn <artmore@protonmail.com>

packages/ui/src/sync/event-pipeline.ts

@@ -34,12 +34,17 @@ const HEARTBEAT_TIMEOUT_MS = 15_000
 // Pipeline factory
 // ---------------------------------------------------------------------------
 
-export function createEventPipeline(input: {
+export type EventPipelineInput = {
   sdk: OpencodeClient
   onEvent: (directory: string, payload: Event) => void
-}) {
-  const { sdk, onEvent } = input
+  /** Called after SSE reconnects (visibility restore or heartbeat timeout). */
+  onReconnect?: () => void
+}
+
+export function createEventPipeline(input: EventPipelineInput) {
+  const { sdk, onEvent, onReconnect } = input
   const abort = new AbortController()
+  let hasConnected = false
 
   // Queue state
   let queue: QueuedEvent[] = []
@@ -149,6 +154,12 @@ export function createEventPipeline(input: {
           },
         })
 
+        if (hasConnected) {
+          onReconnect?.()
+        } else {
+          hasConnected = true
+        }
+
         let yielded = Date.now()
         resetHeartbeat()
 
@@ -197,21 +208,32 @@ export function createEventPipeline(input: {
     }
   })().finally(flush)
 
-  // Visibility handler — flush immediately when tab becomes visible
+  // Visibility handler — abort SSE on heartbeat timeout so the loop reconnects.
+  // The reconnect triggers onReconnect above, which lets consumers resync state.
   const onVisibility = () => {
     if (typeof document === "undefined") return
     if (document.visibilityState !== "visible") return
     if (Date.now() - lastEventAt < HEARTBEAT_TIMEOUT_MS) return
     attempt?.abort()
   }
+
+  // pageshow handler — fires on back-forward cache restore (common on mobile PWA).
+  // bfcache restores the page without a fresh load, so SSE state may be stale.
+  const onPageShow = (event: PageTransitionEvent) => {
+    if (!event.persisted) return
+    attempt?.abort()
+  }
+
   if (typeof document !== "undefined") {
     document.addEventListener("visibilitychange", onVisibility)
+    window.addEventListener("pageshow", onPageShow)
   }
 
   // Cleanup — abort SSE, flush remaining events, remove listeners
   const cleanup = () => {
     if (typeof document !== "undefined") {
       document.removeEventListener("visibilitychange", onVisibility)
+      window.removeEventListener("pageshow", onPageShow)
     }
     abort.abort()
     flush()