fix(window): stop the app panicking when windows close and open together (#2124)

Closing a window posts its full-screen notification while the runtime still
holds its window table; the title bar observer emitted from there and the emit
re-entered the table ("RefCell already mutably borrowed"). The emit now goes
out after the notification returns.

Opening several project windows at once could also hand the same project-N
label to each request, so label choice and window creation now take turns.

Co-authored-by: Claude <[email protected]>

src-tauri/src/titlebar.rs

@@ -89,7 +89,12 @@ fn track(window: &NSWindow, app: &tauri::AppHandle) -> Tracked {
                 crate::macos::hide_tab_bar(&window);
             }
             // The web title bar releases the room kept for the window buttons in full screen.
-            app.state::<crate::state::AppState>().sink.emit(ide_core::IdeEvent::WindowFullscreenChanged);
+            // Closing a window posts this while the runtime still holds its window table, and an
+            // emit from here evaluates script in that table; send it from outside the notification.
+            let app = app.clone();
+            tauri::async_runtime::spawn(async move {
+                app.state::<crate::state::AppState>().sink.emit(ide_core::IdeEvent::WindowFullscreenChanged);
+            });
         });
         let observer = unsafe {
             NSNotificationCenter::defaultCenter().addObserverForName_object_queue_usingBlock(

src-tauri/src/windows.rs

@@ -286,6 +286,9 @@ pub fn settings_label(parent: &str) -> String {
     format!("{}-{parent}", AuxKind::Settings.name())
 }
 
+/// Held from choosing a project window's label until the window exists.
+static CREATING: Mutex<()> = Mutex::new(());
+
 /// Windows being built to join a window group, which takes their frame from the group.
 #[cfg(target_os = "macos")]
 static JOINING_GROUP: Mutex<Vec<String>> = Mutex::new(Vec::new());
@@ -665,6 +668,8 @@ fn create_window_for(
 ) -> IdeResult<WebviewWindow> {
     #[cfg(target_os = "macos")]
     let tab_target = if _restoring { None } else { crate::macos::new_project_tab_target(app, &launch)? };
+    // The label is free only until the window exists, so concurrent opens take turns.
+    let creating = CREATING.lock();
     let windows = app.webview_windows();
     let label = next_label(windows.keys().map(String::as_str));
     let mut config = window_config(app)?;
@@ -691,6 +696,7 @@ fn create_window_for(
         config.visible = false;
     }
     let window = build_project_window(app, config);
+    drop(creating);
     // Plugins hear of the window from the main thread's queue; forget it after them.
     #[cfg(target_os = "macos")]
     if tab_target.is_some() {