fix(queue): drain correct session queue after cross-session stream completion (#964)

When a session finishes streaming while the user has switched to a different
session, setBusy(false) was draining S.session.session_id (the currently
*viewed* session) instead of the session that actually finished. Queued
follow-up messages were silently dropped.

Root cause: setBusy() has no context about which session triggered it.
The activeSid closure variable inside attachLiveStream() knew the right
session but was not propagated.

Fix: add _queueDrainSid module global (null by default). Stream done and
error handlers set it to activeSid immediately before calling setBusy(false).
setBusy(false) reads and clears _queueDrainSid, falling back to S.session if
it is unset (the common case where the user hasn't switched away).

Handlers patched: done event, start-call error handler, stream_end/stream_stop
reconnection fallback, and max-retry error exit.

Co-authored with Claude Sonnet 4.6 / Anthropic.

static/messages.js

@@ -153,7 +153,7 @@ async function send(){
     if(!_approvalSessionId || _approvalSessionId===activeSid) hideApprovalCard(true);removeThinking();
     if(!_clarifySessionId || _clarifySessionId===activeSid) hideClarifyCard(true);
     S.messages.push({role:'assistant',content:`**Error:** ${errMsg}`});
-    renderMessages();setBusy(false);setComposerStatus(`Error: ${errMsg}`);
+    _queueDrainSid=activeSid;renderMessages();setBusy(false);setComposerStatus(`Error: ${errMsg}`);
     return;
   }
 
@@ -726,7 +726,7 @@ function attachLiveStream(activeSid, streamId, uploaded=[], options={}){
         _markSessionViewed(activeSid, d.session.message_count ?? S.messages.length);
         syncTopbar();renderMessages();loadDir('.');
       }
-      renderSessionList();setBusy(false);setStatus('');
+      _queueDrainSid=activeSid;renderSessionList();setBusy(false);setStatus('');
       setComposerStatus('');
       playNotificationSound();
       sendBrowserNotification('Response complete',assistantText?assistantText.slice(0,100):'Task finished');
@@ -900,7 +900,7 @@ function attachLiveStream(activeSid, streamId, uploaded=[], options={}){
         _markSessionViewed(activeSid, session.message_count ?? S.messages.length);
         syncTopbar();renderMessages();
       }
-      renderSessionList();setBusy(false);setComposerStatus('');
+      _queueDrainSid=activeSid;renderSessionList();setBusy(false);setComposerStatus('');
       return true;
     }catch(_){
       return false;
@@ -951,7 +951,7 @@ function attachLiveStream(activeSid, streamId, uploaded=[], options={}){
             const _cbe=$('btnCancel');if(_cbe)_cbe.style.display='none';
             clearLiveToolCards();
             removeThinking();
-            setBusy(false);
+            _queueDrainSid=activeSid;setBusy(false);
             setComposerStatus('');
             renderMessages();
             renderSessionList();

static/ui.js

@@ -1,6 +1,13 @@
 const S={session:null,messages:[],entries:[],busy:false,pendingFiles:[],toolCalls:[],activeStreamId:null,currentDir:'.',activeProfile:'default'};
 const INFLIGHT={};  // keyed by session_id while request in-flight
 const SESSION_QUEUES={};  // keyed by session_id for queued follow-up turns
+// Tracks which session's queue to drain in setBusy(false).
+// Set to activeSid just before setBusy(false) in done/error handlers so the
+// queue drains the session that *finished*, not the one currently viewed.
+// Single-shot: setBusy() reads and clears this on every call. Concurrent
+// back-to-back stream completions would overwrite it, but HTTPServer is
+// single-threaded so only one done event fires at a time in practice.
+let _queueDrainSid=null;
 const $=id=>document.getElementById(id);
 function _getSessionQueue(sid, create=false){
   if(!sid) return [];
@@ -946,10 +953,12 @@ function setBusy(v){
     setComposerStatus('');
     // Always hide Cancel button when not busy
     const _cb=$('btnCancel');if(_cb)_cb.style.display='none';
-    const sid=S.session&&S.session.session_id;
+    const sid=_queueDrainSid||(S.session&&S.session.session_id);
+    _queueDrainSid=null;
     updateQueueBadge(sid);
-    // Drain one queued message for the currently viewed session after UI settles
-    const next=sid?shiftQueuedSessionMessage(sid):null;
+    // Drain one queued message for the finished session after UI settles
+    const _isViewedSid=!S.session||sid===S.session.session_id;
+    const next=sid&&_isViewedSid?shiftQueuedSessionMessage(sid):null;
     if(next){
       updateQueueBadge(sid);
       setTimeout(()=>{

tests/test_regressions.py

@@ -501,8 +501,9 @@ def test_session_scoped_message_queue_frontend_wiring(cleanup_test_sessions):
     assert "const SESSION_QUEUES" in ui_src
     assert "function queueSessionMessage" in ui_src
     assert "function shiftQueuedSessionMessage" in ui_src
-    assert "const sid=S.session&&S.session.session_id;" in ui_src
-    assert "const next=sid?shiftQueuedSessionMessage(sid):null;" in ui_src
+    # _queueDrainSid tracks which session's queue to drain even after session switches
+    assert "_queueDrainSid" in ui_src
+    assert "shiftQueuedSessionMessage(sid)" in ui_src
     assert "queueSessionMessage(S.session.session_id" in messages_src
     assert "updateQueueBadge(S.session.session_id);" in messages_src
     assert "updateQueueBadge(sid);" in sessions_src