macOS: Close windows automatically when exiting (#4154)

This disallows carrying over open windows between calls of `run_app_on_demand`
(which wasn't intended to be supported anyhow).

src/changelog/unreleased.md

@@ -241,3 +241,4 @@ changelog entry.
 - On Windows, fixed ~500 ms pause when clicking the title bar during continuous redraw.
 - On macos, `WindowExtMacOS::set_simple_fullscreen` now honors `WindowExtMacOS::set_borderless_game`
 - On X11 and Wayland, fixed pump_events with `Some(Duration::Zero)` blocking with `Wait` polling mode
+- On macOS, fixed `run_app_on_demand` returning without closing open windows.

src/platform_impl/apple/appkit/app_state.rs

@@ -11,7 +11,7 @@ use objc2_foundation::NSNotification;
 
 use super::super::event_handler::EventHandler;
 use super::super::event_loop_proxy::EventLoopProxy;
-use super::event_loop::{stop_app_immediately, ActiveEventLoop, PanicInfo};
+use super::event_loop::{notify_windows_of_exit, stop_app_immediately, ActiveEventLoop, PanicInfo};
 use super::menu;
 use super::observer::{EventLoopWaker, RunLoop};
 use crate::application::ApplicationHandler;
@@ -156,7 +156,8 @@ impl AppState {
 
     pub fn will_terminate(self: &Rc<Self>, _notification: &NSNotification) {
         trace_scope!("NSApplicationWillTerminateNotification");
-        // TODO: Notify every window that it will be destroyed, like done in iOS?
+        let app = NSApplication::sharedApplication(self.mtm);
+        notify_windows_of_exit(&app);
         self.event_handler.terminate();
         self.internal_exit();
     }
@@ -365,6 +366,7 @@ impl AppState {
         if self.exiting() {
             let app = NSApplication::sharedApplication(self.mtm);
             stop_app_immediately(&app);
+            notify_windows_of_exit(&app);
         }
 
         if self.stop_before_wait.get() {

src/platform_impl/apple/appkit/event_loop.rs

@@ -407,6 +407,22 @@ pub(super) fn stop_app_immediately(app: &NSApplication) {
     });
 }
 
+/// Tell all windows to close.
+///
+/// This will synchronously trigger `WindowEvent::Destroyed` within
+/// `windowWillClose:`, giving the application one last chance to handle
+/// those events. It doesn't matter if the user also ends up closing the
+/// windows in `Window`'s `Drop` impl, once a window has been closed once, it
+/// stays closed.
+///
+/// This ensures that no windows linger on after the event loop has exited,
+/// see <https://github.com/rust-windowing/winit/issues/4135>.
+pub(super) fn notify_windows_of_exit(app: &NSApplication) {
+    for window in app.windows() {
+        window.close();
+    }
+}
+
 /// Catches panics that happen inside `f` and when a panic
 /// happens, stops the `sharedApplication`
 #[inline]