fix(desktop): rebind sessions after websocket reconnect (salvage of #41740) (#43004)

* fix(desktop): rebind sessions after websocket reconnect

* docs(desktop): explain the reconnect-resume guard in use-route-resume

The reconnect fix turns on two subtle conditions with no inline rationale:
`seenGatewayStateRef` suppresses a spurious "became open" on the first effect
run (so a session mounting with the gateway already open doesn't double-resume),
and the `gatewayBecameOpen ||` arm forces a re-resume even when the route looks
`alreadyActive` because the cached runtime id can be stale after the gateway
rebinds/reaps the session. Comment both so the next reader doesn't "simplify"
them back into the original bug. No behavior change.

---------

Co-authored-by: Josh Dow <josh.dow@prepad.io>

apps/desktop/src/app/session/hooks/use-route-resume.test.tsx

@@ -187,4 +187,72 @@ describe('useRouteResume', () => {
     expect(resumeSession).toHaveBeenCalledTimes(1)
     expect(resumeSession).toHaveBeenCalledWith('session-2', true)
   })
+
+  it('resumes the selected route again when the gateway reconnects', () => {
+    const resumeSession = vi.fn(async () => undefined)
+    const startFreshSessionDraft = vi.fn()
+    const activeSessionIdRef: MutableRefObject<null | string> = { current: 'runtime-1' }
+    const creatingSessionRef = { current: false }
+    const runtimeIdByStoredSessionIdRef = { current: new Map([['session-1', 'runtime-1']]) }
+    const selectedStoredSessionIdRef: MutableRefObject<null | string> = { current: 'session-1' }
+
+    const { rerender } = render(
+      <RouteResumeHarness
+        activeSessionId="runtime-1"
+        activeSessionIdRef={activeSessionIdRef}
+        creatingSessionRef={creatingSessionRef}
+        currentView="chat"
+        freshDraftReady={false}
+        gatewayState="open"
+        locationPathname="/session-1"
+        resumeSession={resumeSession}
+        routedSessionId="session-1"
+        runtimeIdByStoredSessionIdRef={runtimeIdByStoredSessionIdRef}
+        selectedStoredSessionId="session-1"
+        selectedStoredSessionIdRef={selectedStoredSessionIdRef}
+        startFreshSessionDraft={startFreshSessionDraft}
+      />
+    )
+
+    expect(resumeSession).not.toHaveBeenCalled()
+
+    rerender(
+      <RouteResumeHarness
+        activeSessionId="runtime-1"
+        activeSessionIdRef={activeSessionIdRef}
+        creatingSessionRef={creatingSessionRef}
+        currentView="chat"
+        freshDraftReady={false}
+        gatewayState="closed"
+        locationPathname="/session-1"
+        resumeSession={resumeSession}
+        routedSessionId="session-1"
+        runtimeIdByStoredSessionIdRef={runtimeIdByStoredSessionIdRef}
+        selectedStoredSessionId="session-1"
+        selectedStoredSessionIdRef={selectedStoredSessionIdRef}
+        startFreshSessionDraft={startFreshSessionDraft}
+      />
+    )
+
+    rerender(
+      <RouteResumeHarness
+        activeSessionId="runtime-1"
+        activeSessionIdRef={activeSessionIdRef}
+        creatingSessionRef={creatingSessionRef}
+        currentView="chat"
+        freshDraftReady={false}
+        gatewayState="open"
+        locationPathname="/session-1"
+        resumeSession={resumeSession}
+        routedSessionId="session-1"
+        runtimeIdByStoredSessionIdRef={runtimeIdByStoredSessionIdRef}
+        selectedStoredSessionId="session-1"
+        selectedStoredSessionIdRef={selectedStoredSessionIdRef}
+        startFreshSessionDraft={startFreshSessionDraft}
+      />
+    )
+
+    expect(resumeSession).toHaveBeenCalledTimes(1)
+    expect(resumeSession).toHaveBeenCalledWith('session-1', true)
+  })
 })

apps/desktop/src/app/session/hooks/use-route-resume.ts

@@ -56,13 +56,19 @@ export function useRouteResume({
   startFreshSessionDraft
 }: RouteResumeOptions) {
   const lastPathnameRef = useRef<string | null>(null)
+  const seenGatewayStateRef = useRef(false)
   const wasGatewayOpenRef = useRef(false)
 
   useEffect(() => {
     const gatewayOpen = gatewayState === 'open'
     const pathnameChanged = lastPathnameRef.current !== locationPathname
-    const gatewayBecameOpen = !wasGatewayOpenRef.current && gatewayOpen
+    // Fire only on a genuine closed->open transition (a reconnect). seenGatewayStateRef
+    // stays false until the first effect run, so a session that mounts with the gateway
+    // already open is not mistaken for "became open" and does not double-resume with the
+    // pathname-driven initial resume below.
+    const gatewayBecameOpen = seenGatewayStateRef.current && !wasGatewayOpenRef.current && gatewayOpen
     lastPathnameRef.current = locationPathname
+    seenGatewayStateRef.current = true
     wasGatewayOpenRef.current = gatewayOpen
 
     if (currentView !== 'chat' || !gatewayOpen) {
@@ -99,7 +105,11 @@ export function useRouteResume({
       // before the pathname updates from /:sid -> /.
       const shouldResume = pathnameChanged || gatewayBecameOpen || stuckOnRoutedSession
 
-      if (!alreadyActive && shouldResume && !creatingSessionRef.current) {
+      // On a reconnect (gatewayBecameOpen) re-resume even when the route looks
+      // `alreadyActive`: the cached runtime id can be stale once the gateway
+      // rebinds/reaps the session on its side, and trusting it strands Desktop on
+      // a dead id ("session not found"). Otherwise keep skipping when already active.
+      if ((gatewayBecameOpen || !alreadyActive) && shouldResume && !creatingSessionRef.current) {
         void resumeSession(routedSessionId, true)
       }