diff --git a/static/panels.js b/static/panels.js
index 75f77605..1f7e3e9d 100644
--- a/static/panels.js
+++ b/static/panels.js
@@ -72,6 +72,10 @@ function syncAppTitlebar() {
   if (_renamingAppTitlebar) return;
 
   titleEl.textContent = mainText;
+  if (panel !== 'chat') {
+    const bot = typeof assistantDisplayName === 'function' ? assistantDisplayName() : '';
+    document.title = bot ? mainText + ' \u2014 ' + bot : mainText;
+  }
   if (subEl) {
     if (subText) {
       subEl.textContent = subText;
@@ -272,7 +276,8 @@ async function switchPanel(name, opts = {}) {
     if (overlay) overlay.classList.add('visible');
   }
   _resyncChatSidebarAfterPanelSwitch();
-  syncAppTitlebar();
+  if (nextPanel === 'chat' && typeof syncTopbar === 'function') syncTopbar();
+  else syncAppTitlebar();
   return true;
 }
 
diff --git a/tests/test_issue4039_panel_document_title.py b/tests/test_issue4039_panel_document_title.py
new file mode 100644
index 00000000..97540b65
--- /dev/null
+++ b/tests/test_issue4039_panel_document_title.py
@@ -0,0 +1,31 @@
+"""Regression coverage for panel-driven document.title ownership (#4039)."""
+
+from __future__ import annotations
+
+
+def _src(name: str) -> str:
+    with open(f"static/{name}", encoding="utf-8") as f:
+        return f.read()
+
+
+def test_sync_app_titlebar_stamps_non_chat_document_title():
+    src = _src("panels.js")
+
+    assert "if (panel !== 'chat') {" in src
+    assert "const bot = typeof assistantDisplayName === 'function' ? assistantDisplayName() : '';" in src
+    assert "document.title = bot ? mainText + ' \\u2014 ' + bot : mainText;" in src
+
+
+def test_switch_panel_restores_chat_title_via_sync_topbar():
+    src = _src("panels.js")
+
+    assert "if (nextPanel === 'chat' && typeof syncTopbar === 'function') syncTopbar();" in src
+    assert "else syncAppTitlebar();" in src
+
+
+def test_chat_title_format_stays_owned_by_sync_topbar():
+    panels_src = _src("panels.js")
+    ui_src = _src("ui.js")
+
+    assert "document.title=sessionTitle+' \\u2014 '+assistantDisplayName();" in ui_src
+    assert "document.title=sessionTitle+' \\u2014 '+assistantDisplayName();" not in panels_src