On Windows, fix RedrawRequested delivery

When calling `Window::request_redraw` from the `RedrawRequested`
handler the `RedrawWindow` won't result in `WM_PAINT` being delivered
due since user callback is run before `DefWindowProcW` is called.

Track whether the user called `Window::request_redraw` and ask for
`RedrawWindow` after running the said function during `WM_PAINT`
handling.

Fixes #3150.

CHANGELOG.md

@@ -41,6 +41,7 @@ And please only add new entries to the top of this list, right below the `# Unre
 - On macOS, fix assertion when pressing `Globe` key.
 - On Windows, updated `WM_MOUSEMOVE` to detect when cursor Enter or Leave window client area while captured and send the corresponding events. (#3153)
 - On macOS, fix crash when accessing tabbing APIs.
+- On Windows, fix `RedrawRequested` not being delivered when calling `Window::request_redraw` from `RedrawRequested`.
 
 # 0.29.1-beta
 

src/platform_impl/windows/event_loop.rs

@@ -1168,19 +1168,28 @@ unsafe fn public_window_callback_inner<T: 'static>(
         }
 
         WM_PAINT => {
-            if userdata.event_loop_runner.should_buffer() {
-                // this branch can happen in response to `UpdateWindow`, if win32 decides to
-                // redraw the window outside the normal flow of the event loop.
-                unsafe { RedrawWindow(window, ptr::null(), 0, RDW_INTERNALPAINT) };
-            } else {
+            userdata.window_state_lock().redraw_requested =
+                userdata.event_loop_runner.should_buffer();
+
+            // We'll buffer only in response to `UpdateWindow`, if win32 decides to redraw the
+            // window outside the normal flow of the event loop. This way mark event as handled
+            // and request a normal redraw with `RedrawWindow`.
+            if !userdata.event_loop_runner.should_buffer() {
                 userdata.send_event(Event::WindowEvent {
                     window_id: RootWindowId(WindowId(window)),
                     event: WindowEvent::RedrawRequested,
                 });
             }
-            result = ProcResult::DefWindowProc(wparam);
-        }
 
+            // NOTE: calling `RedrawWindow` during `WM_PAINT` does nothing, since to mark
+            // `WM_PAINT` as handled we should call the `DefWindowProcW`. Call it and check whether
+            // user asked for redraw during `RedrawRequested` event handling and request it again
+            // after marking `WM_PAINT` as handled.
+            result = ProcResult::Value(unsafe { DefWindowProcW(window, msg, wparam, lparam) });
+            if std::mem::take(&mut userdata.window_state_lock().redraw_requested) {
+                unsafe { RedrawWindow(window, ptr::null(), 0, RDW_INTERNALPAINT) };
+            }
+        }
         WM_WINDOWPOSCHANGING => {
             let mut window_state = userdata.window_state_lock();
             if let Some(ref mut fullscreen) = window_state.fullscreen {

src/platform_impl/windows/window.rs

@@ -149,6 +149,8 @@ impl Window {
 
     #[inline]
     pub fn request_redraw(&self) {
+        // NOTE: mark that we requested a redraw to handle requests during `WM_PAINT` handling.
+        self.window_state.lock().unwrap().redraw_requested = true;
         unsafe {
             RedrawWindow(self.hwnd(), ptr::null(), 0, RDW_INTERNALPAINT);
         }

src/platform_impl/windows/window_state.rs

@@ -52,6 +52,9 @@ pub(crate) struct WindowState {
     pub is_active: bool,
     pub is_focused: bool,
 
+    // Flag whether redraw was requested.
+    pub redraw_requested: bool,
+
     pub dragging: bool,
 
     pub skip_taskbar: bool,
@@ -166,6 +169,7 @@ impl WindowState {
 
             is_active: false,
             is_focused: false,
+            redraw_requested: false,
 
             dragging: false,
 

src/window.rs

@@ -585,8 +585,7 @@ impl Window {
     /// ## Platform-specific
     ///
     /// - **Windows** This API uses `RedrawWindow` to request a `WM_PAINT` message and `RedrawRequested`
-    ///   is emitted in sync with any `WM_PAINT` messages. **Calling this method from `RedrawRequested`
-    ///   event handler won't produce a `RedrawRequested` event**.
+    ///   is emitted in sync with any `WM_PAINT` messages.
     /// - **iOS:** Can only be called on the main thread.
     /// - **Wayland:** The events are aligned with the frame callbacks when [`Window::pre_present_notify`]
     ///                is used.