windows: ignore spurious mouse move messages (#1435)

Fixes https://github.com/rust-windowing/winit/issues/1428

CHANGELOG.md

@@ -9,6 +9,7 @@
 - On Web, add the ability to query "Light" or "Dark" system theme send `ThemeChanged` on change.
 - Fix `Event::to_static` returning `None` for user events.
 - On Wayland, Hide CSD for fullscreen windows.
+- On Windows, ignore spurious mouse move messages.
 
 # 0.21.0 (2020-02-04)
 

src/platform_impl/windows/event_loop.rs

@@ -856,15 +856,25 @@ unsafe extern "system" fn public_window_callback<T: 'static>(
             let x = windowsx::GET_X_LPARAM(lparam) as f64;
             let y = windowsx::GET_Y_LPARAM(lparam) as f64;
             let position = PhysicalPosition::new(x, y);
-
-            subclass_input.send_event(Event::WindowEvent {
-                window_id: RootWindowId(WindowId(window)),
-                event: CursorMoved {
-                    device_id: DEVICE_ID,
-                    position,
-                    modifiers: event::get_key_mods(),
-                },
-            });
+            let cursor_moved;
+            {
+                // handle spurious WM_MOUSEMOVE messages
+                // see https://devblogs.microsoft.com/oldnewthing/20031001-00/?p=42343
+                // and http://debugandconquer.blogspot.com/2015/08/the-cause-of-spurious-mouse-move.html
+                let mut w = subclass_input.window_state.lock();
+                cursor_moved = w.mouse.last_position != Some(position);
+                w.mouse.last_position = Some(position);
+            }
+            if cursor_moved {
+                subclass_input.send_event(Event::WindowEvent {
+                    window_id: RootWindowId(WindowId(window)),
+                    event: CursorMoved {
+                        device_id: DEVICE_ID,
+                        position,
+                        modifiers: event::get_key_mods(),
+                    },
+                });
+            }
 
             0
         }

src/platform_impl/windows/window_state.rs

@@ -1,5 +1,5 @@
 use crate::{
-    dpi::Size,
+    dpi::{PhysicalPosition, Size},
     platform_impl::platform::{event_loop, icon::WinIcon, util},
     window::{CursorIcon, Fullscreen, WindowAttributes},
 };
@@ -48,6 +48,7 @@ pub struct MouseProperties {
     pub cursor: CursorIcon,
     pub buttons_down: u32,
     cursor_flags: CursorFlags,
+    pub last_position: Option<PhysicalPosition<f64>>,
 }
 
 bitflags! {
@@ -106,6 +107,7 @@ impl WindowState {
                 cursor: CursorIcon::default(),
                 buttons_down: 0,
                 cursor_flags: CursorFlags::empty(),
+                last_position: None,
             },
 
             min_size: attributes.min_inner_size,