macos: Fire a ModifiersChanged event on window_did_resign_key

From debugging, I determined that macOS' emission of a flagsChanged
around window switching is inconsistent.  It is fair to assume, I think,
that when the user switches windows, they do not expect their former
modifiers state to remain effective; so I think it's best to clear that
state by sending a ModifiersChanged(ModifiersState::empty()).

Signed-off-by: Kristofer Rye <kristofer.rye@gmail.com>

src/platform_impl/macos/view.rs

@@ -762,6 +762,8 @@ extern "C" fn flags_changed(this: &Object, _sel: Sel, event: id) {
             }));
         }
 
+        trace!("Queueing event with modifiers {:?}", state.modifiers);
+
         AppState::queue_event(EventWrapper::StaticEvent(Event::WindowEvent {
             // TODO Maybe memoize get_window_id if it's safe to reuse?
             window_id: WindowId(get_window_id(state.ns_window)),

src/platform_impl/macos/window_delegate.rs

@@ -16,7 +16,7 @@ use objc::{
 
 use crate::{
     dpi::LogicalSize,
-    event::{Event, WindowEvent},
+    event::{Event, ModifiersState, WindowEvent},
     platform_impl::platform::{
         app_state::AppState,
         event::{EventProxy, EventWrapper},
@@ -319,6 +319,7 @@ extern "C" fn window_did_become_key(this: &Object, _: Sel, _: id) {
 extern "C" fn window_did_resign_key(this: &Object, _: Sel, _: id) {
     trace!("Triggered `windowDidResignKey:`");
     with_state(this, |state| {
+        state.emit_event(WindowEvent::ModifiersChanged(ModifiersState::empty()));
         state.emit_event(WindowEvent::Focused(false));
     });
     trace!("Completed `windowDidResignKey:`");