On Windows, fix MT safety when starting drag

CHANGELOG.md

@@ -11,6 +11,8 @@ Unreleased` header.
 
 # Unreleased
 
+- On Windows, fix so `drag_window` and `drag_resize_window` can be called from another thread.
+
 # 0.29.3
 
 - On Wayland, apply correct scale to `PhysicalSize` passed in `WindowBuilder::with_inner_size` when possible.

src/platform_impl/windows/window.rs

@@ -472,6 +472,19 @@ impl Window {
     }
 
     unsafe fn handle_os_dragging(&self, wparam: WPARAM) {
+        let window = self.window.clone();
+        let window_state = self.window_state.clone();
+
+        self.thread_executor.execute_in_thread(move || {
+            {
+                let mut guard = window_state.lock().unwrap();
+                if !guard.dragging {
+                    guard.dragging = true;
+                } else {
+                    return;
+                }
+            }
+
             let points = {
                 let mut pos = unsafe { mem::zeroed() };
                 unsafe { GetCursorPos(&mut pos) };
@@ -481,18 +494,19 @@ impl Window {
                 x: points.x as i16,
                 y: points.y as i16,
             };
-        unsafe { ReleaseCapture() };
 
-        self.window_state_lock().dragging = true;
+            // ReleaseCapture needs to execute on the main thread
+            unsafe { ReleaseCapture() };
 
             unsafe {
                 PostMessageW(
-                self.hwnd(),
+                    window.0,
                     WM_NCLBUTTONDOWN,
                     wparam,
                     &points as *const _ as LPARAM,
                 )
             };
+        });
     }
 
     #[inline]