Fix issue where window would not focus on startup.

Fix issue where key window would lose all mouse events once mouse left
that window.

Make sure that only window under mouse receives mouse scroll wheel
events.

src/platform/macos/events_loop.rs

@@ -224,16 +224,13 @@ impl EventsLoop {
         }
 
         let event_type = ns_event.eventType();
-        let window_id = super::window::get_window_id(ns_event.window());
+        let ns_window = ns_event.window();
+        let window_id = super::window::get_window_id(ns_window);
         let windows = self.windows.lock().unwrap();
         let maybe_window = windows.iter().find(|window| window_id == window.id());
 
-        let window = match maybe_window {
+        // FIXME: Document this. Why do we do this? Seems like it passes on events to window/app.
-            Some(window) => window,
+        // If we don't do this, window does not become main for some reason.
-            None => return None,
-        };
-
-        // FIXME: Document this. Why do we do this?
         match event_type {
             appkit::NSKeyDown => (),
             _ => appkit::NSApp().sendEvent_(ns_event),
@@ -244,6 +241,12 @@ impl EventsLoop {
             event: window_event,
         };
 
+        // Returns `Some` window if one of our windows is the key window.
+        let maybe_key_window = || windows.iter().find(|window| {
+            let is_key_window: cocoa::base::BOOL = msg_send![*window.window, isKeyWindow];
+            is_key_window == cocoa::base::YES
+        });
+
         match event_type {
 
             appkit::NSKeyDown => {
@@ -347,14 +350,23 @@ impl EventsLoop {
             appkit::NSRightMouseUp => { Some(into_event(WindowEvent::MouseInput(ElementState::Released, MouseButton::Right))) },
             appkit::NSOtherMouseDown => { Some(into_event(WindowEvent::MouseInput(ElementState::Pressed, MouseButton::Middle))) },
             appkit::NSOtherMouseUp => { Some(into_event(WindowEvent::MouseInput(ElementState::Released, MouseButton::Middle))) },
+
             appkit::NSMouseEntered => { Some(into_event(WindowEvent::MouseEntered)) },
             appkit::NSMouseExited => { Some(into_event(WindowEvent::MouseLeft)) },
+
             appkit::NSMouseMoved |
             appkit::NSLeftMouseDragged |
             appkit::NSOtherMouseDragged |
             appkit::NSRightMouseDragged => {
+                // If the mouse movement was on one of our windows, use it.
+                // Otherwise, if one of our windows is the key window (receiving input), use it.
+                // Otherwise, return `None`.
+                let window = match maybe_window.or_else(maybe_key_window) {
+                    Some(window) => window,
+                    None => return None,
+                };
+
                 let window_point = ns_event.locationInWindow();
-                let ns_window: cocoa::base::id = msg_send![ns_event, window];
                 let view_point = if ns_window == cocoa::base::nil {
                     let ns_size = foundation::NSSize::new(0.0, 0.0);
                     let ns_rect = foundation::NSRect::new(window_point, ns_size);
@@ -369,10 +381,17 @@ impl EventsLoop {
                 let x = (scale_factor * view_point.x as f32) as i32;
                 let y = (scale_factor * (view_rect.size.height - view_point.y) as f32) as i32;
                 let window_event = WindowEvent::MouseMoved(x, y);
-                Some(into_event(window_event))
+                let event = Event::WindowEvent { window_id: ::WindowId(window.id()), event: window_event };
+                Some(event)
             },
 
             appkit::NSScrollWheel => {
+                // If none of the windows received the scroll, return `None`.
+                let window = match maybe_window {
+                    Some(window) => window,
+                    None => return None,
+                };
+
                 use events::MouseScrollDelta::{LineDelta, PixelDelta};
                 let scale_factor = window.hidpi_factor();
                 let delta = if ns_event.hasPreciseScrollingDeltas() == cocoa::base::YES {