macOS: Close windows automatically when exiting

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

@@ -54,3 +54,4 @@ changelog entry.
 - On Wayland, fixed a crash when consequently calling `set_cursor_grab` without pointer focus.
 - On Wayland, ensure that external event loop is woken-up when using pump_events and integrating via `FD`.
 - On Wayland, apply fractional scaling to custom cursors.
+- On macOS, fixed `run_app_on_demand` returning without closing open windows.

src/platform_impl/macos/app_state.rs

@@ -11,7 +11,7 @@ use objc2_app_kit::{
 use objc2_foundation::{MainThreadMarker, NSNotification, NSObject, NSObjectProtocol};
 
 use super::event_handler::EventHandler;
-use super::event_loop::{stop_app_immediately, ActiveEventLoop, PanicInfo};
+use super::event_loop::{notify_windows_of_exit, stop_app_immediately, ActiveEventLoop, PanicInfo};
 use super::observer::{EventLoopWaker, RunLoop};
 use super::{menu, WindowId, DEVICE_ID};
 use crate::event::{DeviceEvent, Event, StartCause, WindowEvent};
@@ -165,7 +165,9 @@ impl ApplicationDelegate {
 
     fn will_terminate(&self, _notification: &NSNotification) {
         trace_scope!("applicationWillTerminate:");
-        // TODO: Notify every window that it will be destroyed, like done in iOS?
+        let mtm = MainThreadMarker::from(self);
+        let app = NSApplication::sharedApplication(mtm);
+        notify_windows_of_exit(&app);
         self.internal_exit();
     }
 
@@ -392,6 +394,7 @@ impl ApplicationDelegate {
         if self.exiting() {
             let app = NSApplication::sharedApplication(mtm);
             stop_app_immediately(&app);
+            notify_windows_of_exit(&app);
         }
 
         if self.ivars().stop_before_wait.get() {

src/platform_impl/macos/event_loop.rs

@@ -421,6 +421,22 @@ 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]