Discard mouse down after Cocoa window resize (rust-windowing#466)

* Discard mouse down after Cocoa window resize

We are sending the mouse down event after the window resize has
completed, because Cocoa uses a modal event loop to implement window
resize. This leads to a mouse down without a matching mouse up.

* Also handle event discard in poll_events

Add some explanatory comments and a changelog entry.

CHANGELOG.md

@@ -9,6 +9,7 @@
 - Implemented `Refresh` event on Windows.
 - Properly calculate the minimum and maximum window size on Windows, including window decorations.
 - Map more `MouseCursor` variants to cursor icons on Windows.
+- Discard the stray mouse down event being delivered after window resize on macOS.
 
 # Version 0.12.0 (2018-04-06)
 

src/platform/macos/events_loop.rs

@@ -6,6 +6,7 @@ use std::collections::VecDeque;
 use std::sync::{Arc, Mutex, Weak};
 use super::window::Window2;
 use std;
+use std::cell::RefCell;
 use super::DeviceId;
 
 
@@ -18,6 +19,7 @@ pub struct EventsLoop {
 pub struct Shared {
     pub windows: Mutex<Vec<Weak<Window2>>>,
     pub pending_events: Mutex<VecDeque<Event>>,
+    pub discard_event: RefCell<bool>,
     // The user event callback given via either of the `poll_events` or `run_forever` methods.
     //
     // We store the user's callback here so that it may be accessed by each of the window delegate
@@ -55,6 +57,7 @@ impl Shared {
         Shared {
             windows: Mutex::new(Vec::new()),
             pending_events: Mutex::new(VecDeque::new()),
+            discard_event: RefCell::new(false),
             user_callback: UserCallback { mutex: Mutex::new(None) },
         }
     }
@@ -99,6 +102,19 @@ impl Shared {
         }
     }
 
+    // Instructs the `EventsLoop` to discard the next input event.
+    //
+    // This is called when the window is resized, to avoid a delayed mouse down event being posted
+    // after the resize completes.
+    pub fn discard_next_event(&self) {
+        *self.discard_event.borrow_mut() = true;
+    }
+
+    fn should_discard_next_event(&self) -> bool {
+        let result = *self.discard_event.borrow();
+        *self.discard_event.borrow_mut() = false;
+        result
+    }
 }
 
 
@@ -201,7 +217,11 @@ impl EventsLoop {
 
                 match event {
                     // Call the user's callback.
-                    Some(event) => self.shared.user_callback.call_with_event(event),
+                    Some(event) => {
+                        if !self.shared.should_discard_next_event() {
+                            self.shared.user_callback.call_with_event(event);
+                        }
+                    },
                     None => break,
                 }
             }
@@ -254,7 +274,9 @@ impl EventsLoop {
                 let _: () = msg_send![pool, release];
 
                 if let Some(event) = maybe_event {
-                    self.shared.user_callback.call_with_event(event);
+                    if !self.shared.should_discard_next_event() {
+                        self.shared.user_callback.call_with_event(event);
+                    }
                     if let ControlFlow::Break = control_flow.get() {
                         break;
                     }

src/platform/macos/window.rs

@@ -87,6 +87,11 @@ impl WindowDelegate {
                 let state: *mut c_void = *this.get_ivar("winitState");
                 let state = &mut *(state as *mut DelegateState);
                 emit_resize_event(state);
+
+                // discard the pending mouse down event
+                if let Some(shared) = state.shared.upgrade() {
+                    shared.discard_next_event();
+                }
             }
         }