Fix hovering the mouse over the active window creating an endless stream of CursorMoved events (#1170)

* Fix hovering the mouse over the active window creating an endless stream of CursorMoved events

* Format

CHANGELOG.md

@@ -12,6 +12,7 @@
 - On Windows, fix handling of surrogate pairs when dispatching `ReceivedCharacter`.
 - On macOS 10.15, fix freeze upon exiting exclusive fullscreen mode.
 - On iOS, fix null window on initial `HiDpiFactorChanged` event.
+- On Windows, fix hovering the mouse over the active window creating an endless stream of CursorMoved events.
 
 # 0.20.0 Alpha 3 (2019-08-14)
 

src/platform_impl/windows/util.rs

@@ -141,6 +141,16 @@ pub fn set_cursor_hidden(hidden: bool) {
     }
 }
 
+pub fn get_cursor_clip() -> Result<RECT, io::Error> {
+    unsafe {
+        let mut rect: RECT = mem::zeroed();
+        win_to_err(|| winuser::GetClipCursor(&mut rect)).map(|_| rect)
+    }
+}
+
+/// Sets the cursor's clip rect.
+///
+/// Note that calling this will automatically dispatch a `WM_MOUSEMOVE` event.
 pub fn set_cursor_clip(rect: Option<RECT>) -> Result<(), io::Error> {
     unsafe {
         let rect_ptr = rect
@@ -151,6 +161,19 @@ pub fn set_cursor_clip(rect: Option<RECT>) -> Result<(), io::Error> {
     }
 }
 
+pub fn get_desktop_rect() -> RECT {
+    unsafe {
+        let left = winuser::GetSystemMetrics(winuser::SM_XVIRTUALSCREEN);
+        let top = winuser::GetSystemMetrics(winuser::SM_YVIRTUALSCREEN);
+        RECT {
+            left,
+            top,
+            right: left + winuser::GetSystemMetrics(winuser::SM_CXVIRTUALSCREEN),
+            bottom: top + winuser::GetSystemMetrics(winuser::SM_CYVIRTUALSCREEN),
+        }
+    }
+}
+
 pub fn is_focused(window: HWND) -> bool {
     window == unsafe { winuser::GetActiveWindow() }
 }

src/platform_impl/windows/window_state.rs

@@ -307,10 +307,25 @@ impl CursorFlags {
         let client_rect = util::get_client_rect(window)?;
 
         if util::is_focused(window) {
-            if self.contains(CursorFlags::GRABBED) {
-                util::set_cursor_clip(Some(client_rect))?;
-            } else {
-                util::set_cursor_clip(None)?;
+            let cursor_clip = match self.contains(CursorFlags::GRABBED) {
+                true => Some(client_rect),
+                false => None,
+            };
+
+            let rect_to_tuple = |rect: RECT| (rect.left, rect.top, rect.right, rect.bottom);
+            let active_cursor_clip = rect_to_tuple(util::get_cursor_clip()?);
+            let desktop_rect = rect_to_tuple(util::get_desktop_rect());
+
+            let active_cursor_clip = match desktop_rect == active_cursor_clip {
+                true => None,
+                false => Some(active_cursor_clip),
+            };
+
+            // We do this check because calling `set_cursor_clip` incessantly will flood the event
+            // loop with `WM_MOUSEMOVE` events, and `refresh_os_cursor` is called by `set_cursor_flags`
+            // which at times gets called once every iteration of the eventloop.
+            if active_cursor_clip != cursor_clip.map(rect_to_tuple) {
+                util::set_cursor_clip(cursor_clip)?;
             }
         }