On Windows, fix bug causing mouse capture to not be released.

CHANGELOG.md

@@ -1,5 +1,6 @@
 # Unreleased
 
+- On Windows, fix bug causing mouse capture to not be released.
 - On Windows, fix fullscreen not preserving minimized/maximized state.
 
 # 0.24.0 (2020-12-09)

src/platform_impl/windows/event_loop.rs

@@ -619,15 +619,17 @@ fn subclass_event_target_window<T>(
 /// Capture mouse input, allowing `window` to receive mouse events when the cursor is outside of
 /// the window.
 unsafe fn capture_mouse(window: HWND, window_state: &mut WindowState) {
-    window_state.mouse.buttons_down += 1;
+    window_state.mouse.capture_count += 1;
     winuser::SetCapture(window);
 }
 
 /// Release mouse input, stopping windows on this thread from receiving mouse input when the cursor
 /// is outside the window.
-unsafe fn release_mouse(window_state: &mut WindowState) {
-    window_state.mouse.buttons_down = window_state.mouse.buttons_down.saturating_sub(1);
-    if window_state.mouse.buttons_down == 0 {
+unsafe fn release_mouse(mut window_state: parking_lot::MutexGuard<'_, WindowState>) {
+    window_state.mouse.capture_count = window_state.mouse.capture_count.saturating_sub(1);
+    if window_state.mouse.capture_count == 0 {
+        // ReleaseCapture() causes a WM_CAPTURECHANGED where we lock the window_state.
+        drop(window_state);
         winuser::ReleaseCapture();
     }
 }
@@ -1192,7 +1194,7 @@ unsafe extern "system" fn public_window_callback<T: 'static>(
                 ElementState::Released, MouseButton::Left, WindowEvent::MouseInput,
             };
 
-            release_mouse(&mut *subclass_input.window_state.lock());
+            release_mouse(subclass_input.window_state.lock());
 
             update_modifiers(window, subclass_input);
 
@@ -1234,7 +1236,7 @@ unsafe extern "system" fn public_window_callback<T: 'static>(
                 ElementState::Released, MouseButton::Right, WindowEvent::MouseInput,
             };
 
-            release_mouse(&mut *subclass_input.window_state.lock());
+            release_mouse(subclass_input.window_state.lock());
 
             update_modifiers(window, subclass_input);
 
@@ -1276,7 +1278,7 @@ unsafe extern "system" fn public_window_callback<T: 'static>(
                 ElementState::Released, MouseButton::Middle, WindowEvent::MouseInput,
             };
 
-            release_mouse(&mut *subclass_input.window_state.lock());
+            release_mouse(subclass_input.window_state.lock());
 
             update_modifiers(window, subclass_input);
 
@@ -1320,7 +1322,7 @@ unsafe extern "system" fn public_window_callback<T: 'static>(
             };
             let xbutton = winuser::GET_XBUTTON_WPARAM(wparam);
 
-            release_mouse(&mut *subclass_input.window_state.lock());
+            release_mouse(subclass_input.window_state.lock());
 
             update_modifiers(window, subclass_input);
 
@@ -1336,6 +1338,12 @@ unsafe extern "system" fn public_window_callback<T: 'static>(
             0
         }
 
+        winuser::WM_CAPTURECHANGED => {
+            // window lost mouse capture
+            subclass_input.window_state.lock().mouse.capture_count = 0;
+            0
+        }
+
         winuser::WM_TOUCH => {
             let pcount = LOWORD(wparam as DWORD) as usize;
             let mut inputs = Vec::with_capacity(pcount);

src/platform_impl/windows/window_state.rs

@@ -45,7 +45,7 @@ pub struct SavedWindow {
 #[derive(Clone)]
 pub struct MouseProperties {
     pub cursor: CursorIcon,
-    pub buttons_down: u32,
+    pub capture_count: u32,
     cursor_flags: CursorFlags,
     pub last_position: Option<PhysicalPosition<f64>>,
 }
@@ -102,7 +102,7 @@ impl WindowState {
         WindowState {
             mouse: MouseProperties {
                 cursor: CursorIcon::default(),
-                buttons_down: 0,
+                capture_count: 0,
                 cursor_flags: CursorFlags::empty(),
                 last_position: None,
             },