windows: Coalesce wake-up events

winit-core/src/event_loop/mod.rs

@@ -140,12 +140,6 @@ impl EventLoopProxy {
     ///
     /// [`proxy_wake_up`]: crate::application::ApplicationHandler::proxy_wake_up
     /// [`ApplicationHandler::proxy_wake_up()`]: crate::application::ApplicationHandler::proxy_wake_up
-    ///
-    /// # Platform-specific
-    ///
-    /// - **Windows**: The wake-up may be ignored under high contention, see [#3687].
-    ///
-    /// [#3687]: https://github.com/rust-windowing/winit/pull/3687
     pub fn wake_up(&self) {
         self.proxy.wake_up();
     }

winit-win32/src/event_loop.rs

@@ -416,7 +416,10 @@ impl ActiveEventLoop {
 
 impl RootActiveEventLoop for ActiveEventLoop {
     fn create_proxy(&self) -> RootEventLoopProxy {
-        let event_loop_proxy = EventLoopProxy { target_window: self.0.thread_msg_target };
+        let event_loop_proxy = EventLoopProxy {
+            has_sent_wakeup_msg: self.0.has_sent_wakeup_msg.clone(),
+            target_window: self.0.thread_msg_target,
+        };
         RootEventLoopProxy::new(Arc::new(event_loop_proxy))
     }
 
@@ -766,6 +769,7 @@ type ThreadExecFn = Box<Box<dyn FnMut()>>;
 
 #[derive(Debug)]
 pub struct EventLoopProxy {
+    has_sent_wakeup_msg: Arc<AtomicBool>,
     target_window: HWND,
 }
 
@@ -774,7 +778,18 @@ unsafe impl Sync for EventLoopProxy {}
 
 impl EventLoopProxyProvider for EventLoopProxy {
     fn wake_up(&self) {
-        unsafe { PostMessageW(self.target_window, USER_EVENT_MSG_ID.get(), 0, 0) };
+        if self.has_sent_wakeup_msg.swap(true, Ordering::AcqRel) {
+            // Do not send a wakeup event if one has already been sent, but hasn't been processed
+            // yet. This prevents errors when the internal message queue fills up, and effectively
+            // coalesces wakeups.
+            tracing::trace!("avoided sending wake up, previous wake-up has yet to be processed");
+            return;
+        }
+        if unsafe { PostMessageW(self.target_window, PROXY_WAKEUP_MSG_ID.get(), 0, 0) } == 0 {
+            // _can_ technically fail, but realistically won't, since we've prevented the most
+            // common case (queue full) above.
+            tracing::error!("failed waking event loop: {}", std::io::Error::last_os_error());
+        }
     }
 }
 
@@ -830,7 +845,7 @@ impl LazyMessageId {
 
 // Message sent by the `EventLoopProxy` when we want to wake up the thread.
 // WPARAM and LPARAM are unused.
-static USER_EVENT_MSG_ID: LazyMessageId = LazyMessageId::new("Winit::WakeupMsg\0");
+static PROXY_WAKEUP_MSG_ID: LazyMessageId = LazyMessageId::new("Winit::WakeupMsg\0");
 // Message sent when we want to execute a closure in the thread.
 // WPARAM contains a Box<Box<dyn FnMut()>> that must be retrieved with `Box::from_raw`,
 // and LPARAM is unused.
@@ -2471,12 +2486,9 @@ unsafe extern "system" fn thread_event_target_callback(
             unsafe { DefWindowProcW(window, msg, wparam, lparam) }
         },
 
-        _ if msg == USER_EVENT_MSG_ID.get() => {
-            // synthesis a placeholder UserEvent, so that if the callback is
-            // re-entered it can be buffered for later delivery. the real
-            // user event is still in the mpsc channel and will be pulled
-            // once the placeholder event is delivered to the wrapper
-            // `event_handler`
+        _ if msg == PROXY_WAKEUP_MSG_ID.get() => {
+            // Reset the sent state, allowing new `PROXY_WAKEUP_MSG_ID` messages to be sent.
+            userdata.event_loop_runner.has_sent_wakeup_msg.store(false, Ordering::Release);
             userdata.send_wakeup();
             0
         },

winit-win32/src/event_loop/runner.rs

@@ -2,6 +2,7 @@ use std::any::Any;
 use std::cell::{Cell, RefCell};
 use std::collections::VecDeque;
 use std::rc::Rc;
+use std::sync::atomic::{AtomicBool, Ordering};
 use std::sync::{Arc, Mutex};
 use std::time::Instant;
 use std::{fmt, mem, panic};
@@ -30,6 +31,8 @@ pub(crate) struct EventLoopRunner {
     // can't stall an external loop beyond a frame
     pub(super) interrupt_msg_dispatch: Cell<bool>,
 
+    pub(super) has_sent_wakeup_msg: Arc<AtomicBool>,
+
     control_flow: Cell<ControlFlow>,
     exit: Cell<Option<i32>>,
     runner_state: Cell<RunnerState>,
@@ -69,8 +72,6 @@ pub(crate) enum Event {
     Device { device_id: DeviceId, event: DeviceEvent },
     Window { window_id: WindowId, event: WindowEvent },
     BufferedScaleFactorChanged(HWND, f64, PhysicalSize<u32>),
-    // FIXME(madsmtm): Coalesce these into a flag (or similar) instead of handling them as events.
-    // https://github.com/rust-windowing/winit/pull/3687
     WakeUp,
 }
 
@@ -80,6 +81,7 @@ impl EventLoopRunner {
             thread_id,
             thread_msg_target,
             interrupt_msg_dispatch: Cell::new(false),
+            has_sent_wakeup_msg: Arc::new(AtomicBool::new(false)),
             runner_state: Cell::new(RunnerState::Uninitialized),
             control_flow: Cell::new(ControlFlow::default()),
             exit: Cell::new(None),
@@ -131,6 +133,7 @@ impl EventLoopRunner {
             thread_id: _,
             thread_msg_target: _,
             interrupt_msg_dispatch,
+            has_sent_wakeup_msg,
             runner_state,
             panic_error,
             control_flow: _,
@@ -140,6 +143,7 @@ impl EventLoopRunner {
             event_buffer: _,
         } = self;
         interrupt_msg_dispatch.set(false);
+        has_sent_wakeup_msg.store(false, Ordering::Release);
         runner_state.set(RunnerState::Uninitialized);
         panic_error.set(None);
         exit.set(None);

winit/src/changelog/unreleased.md

@@ -260,3 +260,4 @@ changelog entry.
 - On Windows, `Window::theme` will return the correct theme after setting it through `Window::set_theme`.
 - On Windows, `Window::set_theme` will change the title bar color immediately now.
 - On Windows 11, prevent incorrect shifting when dragging window onto a monitor with different DPI.
+- On Windows, coalesce wake-up events to avoid filling the message queue.