fix: sync active session across tabs (#1359)

Adds a 'storage' event listener for the hermes-webui-session localStorage key. Idle tabs auto-load the new active session and re-render the sidebar; busy tabs show a toast and do not interrupt the active turn.

Co-authored-by: Dennis Soong <dso2ng@gmail.com>

static/sessions.js

@@ -1628,6 +1628,22 @@ function renderSessionListFromCache(){
   }
 }
 
+async function _handleActiveSessionStorageEvent(e){
+  if(!e || e.key !== 'hermes-webui-session') return;
+  const sid = e.newValue;
+  if(!sid || (S.session && S.session.session_id === sid)) return;
+  if(S.busy){
+    if(typeof showToast==='function') showToast('Active session changed in another tab. Finish the current turn before switching.',3000);
+    return;
+  }
+  await loadSession(sid);
+  renderSessionListFromCache();
+}
+
+if(typeof window!=='undefined'){
+  window.addEventListener('storage', (e) => { void _handleActiveSessionStorageEvent(e); });
+}
+
 async function deleteSession(sid){
   const ok=await showConfirmDialog({
     message:'Delete this conversation?',

tests/test_session_cross_tab_sync.py

@@ -0,0 +1,16 @@
+"""Regression tests for cross-tab active session synchronization."""
+from pathlib import Path
+
+REPO_ROOT = Path(__file__).parent.parent.resolve()
+SESSIONS_JS = (REPO_ROOT / "static" / "sessions.js").read_text(encoding="utf-8")
+
+
+def test_sessions_js_listens_for_active_session_storage_changes():
+    assert "addEventListener('storage'" in SESSIONS_JS or 'addEventListener("storage"' in SESSIONS_JS
+    assert "hermes-webui-session" in SESSIONS_JS
+    assert "_handleActiveSessionStorageEvent" in SESSIONS_JS
+
+
+def test_storage_sync_does_not_switch_while_busy():
+    marker = "if(S.busy)"
+    assert marker in SESSIONS_JS, "cross-tab storage sync must not switch sessions during an active turn"