fix tab title clobber during bot name refresh

CHANGELOG.md

@@ -3,6 +3,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- **Desktop tab titles keep the active session name when profile/settings boot paths refresh the assistant name (#4086).** `applyBotName()` now leaves `document.title` alone while a chat session is active, so `syncTopbar()` remains the owner of the per-session `"<session> — <assistant>"` title and desktop tabs no longer collapse to the same fallback name.
+
 ## [v0.51.382] — 2026-06-13 — Release MU (Stable Assistant Turn Anchors: activity-scene projection, inert, #4093)
 
 ### Added

static/boot.js

@@ -1761,7 +1761,7 @@ function applyBotName(){
   // The saved assistant name applies to the default profile only.
   // Non-default profiles use their own profile names.
   const name=assistantDisplayName();
-  document.title=name;
+  if(!S.session) document.title=name;
   const sidebarH1=document.querySelector('.sidebar-header h1');
   if(sidebarH1) sidebarH1.textContent=name;
   const logo=document.querySelector('.sidebar-header .logo');

tests/test_issue4086_document_title_owner.py

@@ -0,0 +1,40 @@
+from pathlib import Path
+
+
+ROOT = Path(__file__).resolve().parents[1]
+BOOT_JS = ROOT / "static" / "boot.js"
+UI_JS = ROOT / "static" / "ui.js"
+
+
+def _extract_function(src: str, signature: str) -> str:
+    start = src.find(signature)
+    assert start != -1, f"{signature} not found"
+    depth = 0
+    for idx in range(start, len(src)):
+        ch = src[idx]
+        if ch == "{":
+            depth += 1
+        elif ch == "}":
+            depth -= 1
+            if depth == 0:
+                return src[start : idx + 1]
+    raise AssertionError(f"{signature} body did not terminate")
+
+
+def test_apply_bot_name_does_not_overwrite_active_session_document_title():
+    """Session titles belong to syncTopbar() while a chat session is active."""
+    src = BOOT_JS.read_text()
+    body = _extract_function(src, "function applyBotName(){")
+
+    assert "if(!S.session) document.title=name;" in body
+    assert "document.title=name;" not in body.replace(
+        "if(!S.session) document.title=name;",
+        "",
+    )
+
+
+def test_sync_topbar_remains_session_document_title_owner():
+    src = UI_JS.read_text()
+    body = _extract_function(src, "function syncTopbar(){")
+
+    assert "document.title=sessionTitle+' \\u2014 '+assistantDisplayName();" in body