Windows: Implement EventLoopExtPumpEvents and EventLoopExtRunOnDemand

A surprising amount of work was required to enable these extensions
on Windows.

I had originally assumed that pump_events was going to be very similar
to run except would use PeekMessageW instead of GetMessageW to avoid
blocking the external loop but I found the Windows backend broke
several assumptions I had.

Overall I think these changes can hopefully be considered a quite a
significant simplification (I think it's a net deletion of a fair amount
of code) and I think it also helps bring it into slightly closer alignment
with other backends too

Key changes:
- I have removed the `wait_thread` that was a fairly fiddly way of handling
  `ControlFlow::WaitUntil` timeouts in favor of using `SetTimer` which works
  with the same messages picked up by `GetMessage` and `PeekMessage`.
- I have removed the ordering guarantees between `MainEventsCleared`,
  `RedrawRequested` and `RedrawEventsCleared` events due to the complexity in
  maintaining this artificial ordering, which is already not supported
  consistently across backends anyway (in particular this ordering already
  isn't compatible with how MacOS / iOS work).
- `RedrawRequested` events are now directly dispatched via `WM_PAINT` messages
  - comparable to how `RedrawRequested` is dispatched via `drawRect` in the
  MacOS backend.
- I have re-worked how `NewEvents`, `MainEventsCleared`, and `RedrawEventsCleared`
  get dispatched to be more in line with the MacOS backend and also more in line
  with how we have recently discussed defining them for all platforms.

  `NewEvents` is conceptually delivered when the event loop "wakes up" and
  `MainEventsCleared` gets dispatched when the event loop is about to ask the
  OS to wait for new events.

  This is a more portable model, and is already how these events work in the
  MacOS backend.

  `RedrawEventsCleared` are just delivered after `MainEventsCleared` but this
  event no longer has a useful meaning.

Probably the most controversial thing here is that this "breaks" the ordering
rules for redraw event handling, but since my changes interacted with how the
order is maintained I was very reluctant to figure out how to continue
maintaining something that we have recently been discussing changing:

https://github.com/rust-windowing/winit/issues/2640.

Additionally, since the MacOS backend already doesn't strictly maintain this
order it's somewhat academic to see this as a breakage if Winit applications
can't really rely on it already.

This updates the documentation for `request_redraw()` to reflect that we
no longer guarantee that `RedrawRequested` events must be dispatched
after `MainEventsCleared`.

src/platform/mod.rs

@@ -11,8 +11,8 @@
 //!
 //! And the following platform-specific modules:
 //!
-//! - `run_ondemand` (available on `android`)
-//! - `pump_events` (available on `android`)
+//! - `run_ondemand` (available on `windows`, `android`)
+//! - `pump_events` (available on `windows`, `android`)
 //! - `run_return` (available on `windows`, `unix`, `macos`, and `android`)
 //!
 //! However only the module corresponding to the platform you're compiling to will be available.
@@ -36,10 +36,10 @@ pub mod windows;
 #[cfg(x11_platform)]
 pub mod x11;
 
-#[cfg(any(android_platform))]
+#[cfg(any(windows_platform, android_platform))]
 pub mod run_ondemand;
 
-#[cfg(any(android_platform,))]
+#[cfg(any(windows_platform, android_platform,))]
 pub mod pump_events;
 
 #[cfg(any(

src/platform_impl/windows/event_loop.rs

@@ -14,7 +14,6 @@ use std::{
         mpsc::{self, Receiver, Sender},
         Arc, Mutex, MutexGuard,
     },
-    thread,
     time::{Duration, Instant},
 };
 
@@ -23,13 +22,11 @@ use raw_window_handle::{RawDisplayHandle, WindowsDisplayHandle};
 
 use windows_sys::Win32::{
     Devices::HumanInterfaceDevice::MOUSE_MOVE_RELATIVE,
-    Foundation::{BOOL, HANDLE, HWND, LPARAM, LRESULT, POINT, RECT, WAIT_TIMEOUT, WPARAM},
+    Foundation::{BOOL, HANDLE, HWND, LPARAM, LRESULT, POINT, RECT, WPARAM},
     Graphics::Gdi::{
-        GetMonitorInfoW, GetUpdateRect, MonitorFromRect, MonitorFromWindow, RedrawWindow,
-        ScreenToClient, ValidateRect, MONITORINFO, MONITOR_DEFAULTTONULL, RDW_INTERNALPAINT,
-        SC_SCREENSAVE,
+        GetMonitorInfoW, MonitorFromRect, MonitorFromWindow, RedrawWindow, ScreenToClient,
+        ValidateRect, MONITORINFO, MONITOR_DEFAULTTONULL, RDW_INTERNALPAINT, SC_SCREENSAVE,
     },
-    Media::{timeBeginPeriod, timeEndPeriod, timeGetDevCaps, TIMECAPS, TIMERR_NOERROR},
     System::{
         Ole::RevokeDragDrop,
         Threading::{GetCurrentThreadId, INFINITE},
@@ -54,34 +51,35 @@ use windows_sys::Win32::{
         },
         WindowsAndMessaging::{
             CreateWindowExW, DefWindowProcW, DestroyWindow, DispatchMessageW, GetCursorPos,
-            GetMenu, GetMessageW, LoadCursorW, MsgWaitForMultipleObjectsEx, PeekMessageW,
-            PostMessageW, PostThreadMessageW, RegisterClassExW, RegisterWindowMessageA, SetCursor,
-            SetWindowPos, TranslateMessage, CREATESTRUCTW, GIDC_ARRIVAL, GIDC_REMOVAL, GWL_STYLE,
-            GWL_USERDATA, HTCAPTION, HTCLIENT, MINMAXINFO, MNC_CLOSE, MSG, MWMO_INPUTAVAILABLE,
-            NCCALCSIZE_PARAMS, PM_NOREMOVE, PM_QS_PAINT, PM_REMOVE, PT_PEN, PT_TOUCH, QS_ALLEVENTS,
-            RI_KEY_E0, RI_KEY_E1, RI_MOUSE_WHEEL, SC_MINIMIZE, SC_RESTORE, SIZE_MAXIMIZED,
-            SWP_NOACTIVATE, SWP_NOMOVE, SWP_NOSIZE, SWP_NOZORDER, WHEEL_DELTA, WINDOWPOS,
-            WM_CAPTURECHANGED, WM_CLOSE, WM_CREATE, WM_DESTROY, WM_DPICHANGED, WM_ENTERSIZEMOVE,
-            WM_EXITSIZEMOVE, WM_GETMINMAXINFO, WM_IME_COMPOSITION, WM_IME_ENDCOMPOSITION,
-            WM_IME_SETCONTEXT, WM_IME_STARTCOMPOSITION, WM_INPUT, WM_INPUT_DEVICE_CHANGE,
-            WM_KEYDOWN, WM_KEYUP, WM_KILLFOCUS, WM_LBUTTONDOWN, WM_LBUTTONUP, WM_MBUTTONDOWN,
-            WM_MBUTTONUP, WM_MENUCHAR, WM_MOUSEHWHEEL, WM_MOUSEMOVE, WM_MOUSEWHEEL, WM_NCACTIVATE,
-            WM_NCCALCSIZE, WM_NCCREATE, WM_NCDESTROY, WM_NCLBUTTONDOWN, WM_PAINT, WM_POINTERDOWN,
-            WM_POINTERUP, WM_POINTERUPDATE, WM_RBUTTONDOWN, WM_RBUTTONUP, WM_SETCURSOR,
-            WM_SETFOCUS, WM_SETTINGCHANGE, WM_SIZE, WM_SYSCOMMAND, WM_SYSKEYDOWN, WM_SYSKEYUP,
-            WM_TOUCH, WM_WINDOWPOSCHANGED, WM_WINDOWPOSCHANGING, WM_XBUTTONDOWN, WM_XBUTTONUP,
-            WNDCLASSEXW, WS_EX_LAYERED, WS_EX_NOACTIVATE, WS_EX_TOOLWINDOW, WS_EX_TRANSPARENT,
-            WS_OVERLAPPED, WS_POPUP, WS_VISIBLE,
+            GetMenu, GetMessageW, KillTimer, LoadCursorW, PeekMessageW, PostMessageW,
+            RegisterClassExW, RegisterWindowMessageA, SetCursor, SetTimer, SetWindowPos,
+            TranslateMessage, CREATESTRUCTW, GIDC_ARRIVAL, GIDC_REMOVAL, GWL_STYLE, GWL_USERDATA,
+            HTCAPTION, HTCLIENT, MINMAXINFO, MNC_CLOSE, MSG, NCCALCSIZE_PARAMS, PM_REMOVE, PT_PEN,
+            PT_TOUCH, RI_KEY_E0, RI_KEY_E1, RI_MOUSE_WHEEL, SC_MINIMIZE, SC_RESTORE,
+            SIZE_MAXIMIZED, SWP_NOACTIVATE, SWP_NOMOVE, SWP_NOSIZE, SWP_NOZORDER, WHEEL_DELTA,
+            WINDOWPOS, WM_CAPTURECHANGED, WM_CLOSE, WM_CREATE, WM_DESTROY, WM_DPICHANGED,
+            WM_ENTERSIZEMOVE, WM_EXITSIZEMOVE, WM_GETMINMAXINFO, WM_IME_COMPOSITION,
+            WM_IME_ENDCOMPOSITION, WM_IME_SETCONTEXT, WM_IME_STARTCOMPOSITION, WM_INPUT,
+            WM_INPUT_DEVICE_CHANGE, WM_KEYDOWN, WM_KEYUP, WM_KILLFOCUS, WM_LBUTTONDOWN,
+            WM_LBUTTONUP, WM_MBUTTONDOWN, WM_MBUTTONUP, WM_MENUCHAR, WM_MOUSEHWHEEL, WM_MOUSEMOVE,
+            WM_MOUSEWHEEL, WM_NCACTIVATE, WM_NCCALCSIZE, WM_NCCREATE, WM_NCDESTROY,
+            WM_NCLBUTTONDOWN, WM_PAINT, WM_POINTERDOWN, WM_POINTERUP, WM_POINTERUPDATE,
+            WM_RBUTTONDOWN, WM_RBUTTONUP, WM_SETCURSOR, WM_SETFOCUS, WM_SETTINGCHANGE, WM_SIZE,
+            WM_SYSCOMMAND, WM_SYSKEYDOWN, WM_SYSKEYUP, WM_TOUCH, WM_WINDOWPOSCHANGED,
+            WM_WINDOWPOSCHANGING, WM_XBUTTONDOWN, WM_XBUTTONUP, WNDCLASSEXW, WS_EX_LAYERED,
+            WS_EX_NOACTIVATE, WS_EX_TOOLWINDOW, WS_EX_TRANSPARENT, WS_OVERLAPPED, WS_POPUP,
+            WS_VISIBLE,
         },
     },
 };
 
 use crate::{
     dpi::{PhysicalPosition, PhysicalSize},
+    error::RunLoopError,
     event::{DeviceEvent, Event, Force, Ime, RawKeyEvent, Touch, TouchPhase, WindowEvent},
     event_loop::{ControlFlow, DeviceEvents, EventLoopClosed, EventLoopWindowTarget as RootELW},
     keyboard::{KeyCode, ModifiersState},
-    platform::scancode::KeyCodeExtScancode,
+    platform::{pump_events::PumpStatus, scancode::KeyCodeExtScancode},
     platform_impl::platform::{
         dark_mode::try_theme,
         dpi::{become_dpi_aware, dpi_to_scale_factor},
@@ -99,6 +97,8 @@ use crate::{
 };
 use runner::{EventLoopRunner, EventLoopRunnerShared};
 
+use self::runner::RunnerState;
+
 use super::window::set_skip_taskbar;
 
 type GetPointerFrameInfoHistory = unsafe extern "system" fn(
@@ -216,13 +216,7 @@ impl<T: 'static> EventLoop<T> {
 
         let thread_msg_target = create_event_target_window::<T>();
 
-        thread::Builder::new()
-            .name("winit wait thread".to_string())
-            .spawn(move || wait_thread(thread_id, thread_msg_target))
-            .expect("Failed to spawn winit wait thread");
-        let wait_thread_id = get_wait_thread_id();
-
-        let runner_shared = Rc::new(EventLoopRunner::new(thread_msg_target, wait_thread_id));
+        let runner_shared = Rc::new(EventLoopRunner::new(thread_msg_target));
 
         let thread_msg_sender =
             insert_event_target_window_data::<T>(thread_msg_target, runner_shared.clone());
@@ -253,34 +247,246 @@ impl<T: 'static> EventLoop<T> {
     where
         F: 'static + FnMut(Event<'_, T>, &RootELW<T>, &mut ControlFlow),
     {
-        let exit_code = self.run_return(event_handler);
+        let exit_code = match self.run_ondemand(event_handler) {
+            Err(RunLoopError::ExitFailure(code)) => code,
+            Err(_err) => 1,
+            Ok(_) => 0,
+        };
         ::std::process::exit(exit_code);
     }
 
-    pub fn run_return<F>(&mut self, mut event_handler: F) -> i32
+    pub fn run_return<F>(&mut self, event_handler: F) -> i32
     where
         F: FnMut(Event<'_, T>, &RootELW<T>, &mut ControlFlow),
     {
-        let event_loop_windows_ref = &self.window_target;
+        match self.run_ondemand(event_handler) {
+            Err(RunLoopError::ExitFailure(code)) => code,
+            Err(_err) => 1,
+            Ok(_) => 0,
+        }
+    }
 
-        unsafe {
-            self.window_target
-                .p
-                .runner_shared
-                .set_event_handler(move |event, control_flow| {
+    pub fn run_ondemand<F>(&mut self, mut event_handler: F) -> Result<(), RunLoopError>
+    where
+        F: FnMut(Event<'_, T>, &RootELW<T>, &mut ControlFlow),
+    {
+        {
+            let runner = &self.window_target.p.runner_shared;
+            if runner.state() != RunnerState::Uninitialized {
+                return Err(RunLoopError::AlreadyRunning);
+            }
+
+            let event_loop_windows_ref = &self.window_target;
+            // # Safety
+            // We make sure to call runner.clear_event_handler() before
+            // returning
+            unsafe {
+                runner.set_event_handler(move |event, control_flow| {
                     event_handler(event, event_loop_windows_ref, control_flow)
                 });
+            }
         }
 
+        let exit_code = loop {
+            if let ControlFlow::ExitWithCode(code) = self.wait_and_dispatch_message() {
+                break code;
+            }
+
+            if let ControlFlow::ExitWithCode(code) = self.dispatch_peeked_messages() {
+                break code;
+            }
+        };
+
+        let runner = &self.window_target.p.runner_shared;
+        runner.loop_destroyed();
+
+        // # Safety
+        // We assume that this will effectively call `runner.clear_event_handler()`
+        // to meet the safety requirements for calling `runner.set_event_handler()` above.
+        runner.reset_runner();
+
+        if exit_code == 0 {
+            Ok(())
+        } else {
+            Err(RunLoopError::ExitFailure(exit_code))
+        }
+    }
+
+    pub fn pump_events<F>(&mut self, mut event_handler: F) -> PumpStatus
+    where
+        F: FnMut(Event<'_, T>, &RootELW<T>, &mut ControlFlow),
+    {
+        {
+            let runner = &self.window_target.p.runner_shared;
+            let event_loop_windows_ref = &self.window_target;
+
+            // # Safety
+            // We make sure to call runner.clear_event_handler() before
+            // returning
+            //
+            // Note: we're currently assuming nothing can panic and unwind
+            // to leave the runner in an unsound state with an associated
+            // event handler.
+            unsafe {
+                runner.set_event_handler(move |event, control_flow| {
+                    event_handler(event, event_loop_windows_ref, control_flow)
+                });
+                runner.wakeup();
+            }
+        }
+
+        self.dispatch_peeked_messages();
+
         let runner = &self.window_target.p.runner_shared;
 
-        let exit_code = unsafe {
-            let mut msg = mem::zeroed();
+        let status = if let ControlFlow::ExitWithCode(code) = runner.control_flow() {
+            runner.loop_destroyed();
 
-            runner.poll();
-            'main: loop {
-                if GetMessageW(&mut msg, 0, 0, 0) == false.into() {
-                    break 'main 0;
+            // Immediately reset the internal state for the loop to allow
+            // the loop to be run more than once.
+            runner.reset_runner();
+            PumpStatus::Exit(code)
+        } else {
+            runner.prepare_wait();
+            PumpStatus::Continue
+        };
+
+        // We wait until we've checked for an exit status before clearing the
+        // application callback, in case we need to dispatch a LoopDestroyed event
+        //
+        // # Safety
+        // This pairs up with our call to `runner.set_event_handler` and ensures
+        // the application's callback can't be held beyond its lifetime.
+        runner.clear_event_handler();
+
+        status
+    }
+
+    /// Wait for one message and dispatch it, optionally with a timeout if control_flow is `WaitUntil`
+    fn wait_and_dispatch_message(&mut self) -> ControlFlow {
+        let start = Instant::now();
+
+        let runner = &self.window_target.p.runner_shared;
+
+        let timeout = match runner.control_flow() {
+            ControlFlow::Wait => None,
+            ControlFlow::Poll => Some(Duration::ZERO),
+            ControlFlow::WaitUntil(wait_deadline) => {
+                Some(wait_deadline.saturating_duration_since(start))
+            }
+            ControlFlow::ExitWithCode(_code) => unreachable!(),
+        };
+
+        fn get_msg_with_timeout(msg: &mut MSG, timeout: Option<Duration>) -> PumpStatus {
+            unsafe {
+                // A timeout of None means wait indefinitely (so we don't need to call SetTimer)
+                let timer_id = timeout.map(|timeout| SetTimer(0, 0, dur2timeout(timeout), None));
+                let get_status = GetMessageW(msg, 0, 0, 0);
+                if let Some(timer_id) = timer_id {
+                    KillTimer(0, timer_id);
+                }
+                // A return value of 0 implies `WM_QUIT`
+                if get_status == 0 {
+                    PumpStatus::Exit(0)
+                } else {
+                    PumpStatus::Continue
+                }
+            }
+        }
+
+        /// Fetch the next MSG either via PeekMessage or GetMessage depending on whether the
+        /// requested timeout is `ZERO` (and so we don't want to block)
+        ///
+        /// Returns `None` if if no MSG was read, else a `Continue` or `Exit` status
+        fn wait_for_msg(msg: &mut MSG, timeout: Option<Duration>) -> Option<PumpStatus> {
+            if timeout == Some(Duration::ZERO) {
+                unsafe {
+                    if PeekMessageW(msg, 0, 0, 0, PM_REMOVE) != 0 {
+                        Some(PumpStatus::Continue)
+                    } else {
+                        None
+                    }
+                }
+            } else {
+                Some(get_msg_with_timeout(msg, timeout))
+            }
+        }
+
+        // We aim to be consistent with the MacOS backend which has a RunLoop
+        // observer that will dispatch MainEventsCleared when about to wait for
+        // events, and NewEvents after the RunLoop wakes up.
+        //
+        // We emulate similar behaviour by treating `GetMessage` as our wait
+        // point and wake up point (when it returns) and we drain all other
+        // pending messages via `PeekMessage` until we come back to "wait" via
+        // `GetMessage`
+        //
+        runner.prepare_wait();
+
+        // # Safety
+        // The Windows API has no documented requirement for bitwise
+        // initializing a `MSG` struct (it can be uninitialized memory for the C
+        // API) and there's no API to construct or initialize a `MSG`. This
+        // is the simplest way avoid unitialized memory in Rust
+        let mut msg = unsafe { mem::zeroed() };
+        let msg_status = wait_for_msg(&mut msg, timeout);
+
+        // Before we potentially exit, make sure to consistently emit an event for the wake up
+        runner.wakeup();
+
+        match msg_status {
+            None => {} // No MSG to dispatch
+            Some(PumpStatus::Exit(code)) => {
+                runner.set_exit_control_flow(code);
+                return runner.control_flow();
+            }
+            Some(PumpStatus::Continue) => {
+                unsafe {
+                    let handled = if let Some(callback) = self.msg_hook.as_deref_mut() {
+                        callback(&mut msg as *mut _ as *mut _)
+                    } else {
+                        false
+                    };
+                    if !handled {
+                        TranslateMessage(&msg);
+                        DispatchMessageW(&msg);
+                    }
+                }
+
+                if let Err(payload) = runner.take_panic_error() {
+                    runner.reset_runner();
+                    panic::resume_unwind(payload);
+                }
+            }
+        }
+
+        runner.control_flow()
+    }
+
+    /// Dispatch all queued messages via `PeekMessageW`
+    fn dispatch_peeked_messages(&mut self) -> ControlFlow {
+        let runner = &self.window_target.p.runner_shared;
+
+        // We generally want to continue dispatching all pending messages
+        // but we also allow dispatching to be interrupted as a means to
+        // ensure the `pump_events` won't indefinitely block an external
+        // event loop if there are too many pending events. This interrupt
+        // flag will be set after dispatching `RedrawRequested` events.
+        runner.interrupt_msg_dispatch.set(false);
+
+        // # Safety
+        // The Windows API has no documented requirement for bitwise
+        // initializing a `MSG` struct (it can be uninitialized memory for the C
+        // API) and there's no API to construct or initialize a `MSG`. This
+        // is the simplest way avoid unitialized memory in Rust
+        let mut msg = unsafe { mem::zeroed() };
+
+        let mut control_flow = runner.control_flow();
+
+        loop {
+            unsafe {
+                if PeekMessageW(&mut msg, 0, 0, 0, PM_REMOVE) == false.into() {
+                    break;
                 }
 
                 let handled = if let Some(callback) = self.msg_hook.as_deref_mut() {
@@ -292,26 +498,24 @@ impl<T: 'static> EventLoop<T> {
                     TranslateMessage(&msg);
                     DispatchMessageW(&msg);
                 }
-
-                if let Err(payload) = runner.take_panic_error() {
-                    runner.reset_runner();
-                    panic::resume_unwind(payload);
-                }
-
-                if let ControlFlow::ExitWithCode(code) = runner.control_flow() {
-                    if !runner.handling_events() {
-                        break 'main code;
-                    }
-                }
             }
-        };
 
-        unsafe {
-            runner.loop_destroyed();
+            if let Err(payload) = runner.take_panic_error() {
+                runner.reset_runner();
+                panic::resume_unwind(payload);
+            }
+
+            control_flow = runner.control_flow();
+            if let ControlFlow::ExitWithCode(_code) = control_flow {
+                break;
+            }
+
+            if runner.interrupt_msg_dispatch.get() {
+                break;
+            }
         }
 
-        runner.reset_runner();
-        exit_code
+        control_flow
     }
 
     pub fn create_proxy(&self) -> EventLoopProxy<T> {
@@ -391,109 +595,6 @@ fn main_thread_id() -> u32 {
     unsafe { MAIN_THREAD_ID }
 }
 
-fn get_wait_thread_id() -> u32 {
-    unsafe {
-        let mut msg = mem::zeroed();
-        let result = GetMessageW(
-            &mut msg,
-            -1,
-            SEND_WAIT_THREAD_ID_MSG_ID.get(),
-            SEND_WAIT_THREAD_ID_MSG_ID.get(),
-        );
-        assert_eq!(
-            msg.message,
-            SEND_WAIT_THREAD_ID_MSG_ID.get(),
-            "this shouldn't be possible. please open an issue with Winit. error code: {result}"
-        );
-        msg.lParam as u32
-    }
-}
-
-static WAIT_PERIOD_MIN: Lazy<Option<u32>> = Lazy::new(|| unsafe {
-    let mut caps = TIMECAPS {
-        wPeriodMin: 0,
-        wPeriodMax: 0,
-    };
-    if timeGetDevCaps(&mut caps, mem::size_of::<TIMECAPS>() as u32) == TIMERR_NOERROR {
-        Some(caps.wPeriodMin)
-    } else {
-        None
-    }
-});
-
-fn wait_thread(parent_thread_id: u32, msg_window_id: HWND) {
-    unsafe {
-        let mut msg: MSG;
-
-        let cur_thread_id = GetCurrentThreadId();
-        PostThreadMessageW(
-            parent_thread_id,
-            SEND_WAIT_THREAD_ID_MSG_ID.get(),
-            0,
-            cur_thread_id as LPARAM,
-        );
-
-        let mut wait_until_opt = None;
-        'main: loop {
-            // Zeroing out the message ensures that the `WaitUntilInstantBox` doesn't get
-            // double-freed if `MsgWaitForMultipleObjectsEx` returns early and there aren't
-            // additional messages to process.
-            msg = mem::zeroed();
-
-            if wait_until_opt.is_some() {
-                if PeekMessageW(&mut msg, 0, 0, 0, PM_REMOVE) != false.into() {
-                    TranslateMessage(&msg);
-                    DispatchMessageW(&msg);
-                }
-            } else if GetMessageW(&mut msg, 0, 0, 0) == false.into() {
-                break 'main;
-            } else {
-                TranslateMessage(&msg);
-                DispatchMessageW(&msg);
-            }
-
-            if msg.message == WAIT_UNTIL_MSG_ID.get() {
-                wait_until_opt = Some(*WaitUntilInstantBox::from_raw(msg.lParam as *mut _));
-            } else if msg.message == CANCEL_WAIT_UNTIL_MSG_ID.get() {
-                wait_until_opt = None;
-            }
-
-            if let Some(wait_until) = wait_until_opt {
-                let now = Instant::now();
-                if now < wait_until {
-                    // Windows' scheduler has a default accuracy of several ms. This isn't good enough for
-                    // `WaitUntil`, so we request the Windows scheduler to use a higher accuracy if possible.
-                    // If we couldn't query the timer capabilities, then we use the default resolution.
-                    if let Some(period) = *WAIT_PERIOD_MIN {
-                        timeBeginPeriod(period);
-                    }
-                    // `MsgWaitForMultipleObjects` is bound by the granularity of the scheduler period.
-                    // Because of this, we try to reduce the requested time just enough to undershoot `wait_until`
-                    // by the smallest amount possible, and then we busy loop for the remaining time inside the
-                    // NewEvents message handler.
-                    let resume_reason = MsgWaitForMultipleObjectsEx(
-                        0,
-                        ptr::null(),
-                        dur2timeout(wait_until - now).saturating_sub(WAIT_PERIOD_MIN.unwrap_or(1)),
-                        QS_ALLEVENTS,
-                        MWMO_INPUTAVAILABLE,
-                    );
-                    if let Some(period) = *WAIT_PERIOD_MIN {
-                        timeEndPeriod(period);
-                    }
-                    if resume_reason == WAIT_TIMEOUT {
-                        PostMessageW(msg_window_id, PROCESS_NEW_EVENTS_MSG_ID.get(), 0, 0);
-                        wait_until_opt = None;
-                    }
-                } else {
-                    PostMessageW(msg_window_id, PROCESS_NEW_EVENTS_MSG_ID.get(), 0, 0);
-                    wait_until_opt = None;
-                }
-            }
-        }
-    }
-}
-
 // Implementation taken from https://github.com/rust-lang/rust/blob/db5476571d9b27c862b95c1e64764b0ac8980e23/src/libstd/sys/windows/mod.rs
 fn dur2timeout(dur: Duration) -> u32 {
     // Note that a duration is a (u64, u32) (seconds, nanoseconds) pair, and the
@@ -612,8 +713,6 @@ impl<T: 'static> EventLoopProxy<T> {
     }
 }
 
-type WaitUntilInstantBox = Box<Instant>;
-
 /// A lazily-initialized window message ID.
 pub struct LazyMessageId {
     /// The ID.
@@ -673,13 +772,6 @@ static USER_EVENT_MSG_ID: LazyMessageId = LazyMessageId::new("Winit::WakeupMsg\0
 // WPARAM contains a Box<Box<dyn FnMut()>> that must be retrieved with `Box::from_raw`,
 // and LPARAM is unused.
 static EXEC_MSG_ID: LazyMessageId = LazyMessageId::new("Winit::ExecMsg\0");
-static PROCESS_NEW_EVENTS_MSG_ID: LazyMessageId = LazyMessageId::new("Winit::ProcessNewEvents\0");
-/// lparam is the wait thread's message id.
-static SEND_WAIT_THREAD_ID_MSG_ID: LazyMessageId = LazyMessageId::new("Winit::SendWaitThreadId\0");
-/// lparam points to a `Box<Instant>` signifying the time `PROCESS_NEW_EVENTS_MSG_ID` should
-/// be sent.
-static WAIT_UNTIL_MSG_ID: LazyMessageId = LazyMessageId::new("Winit::WaitUntil\0");
-static CANCEL_WAIT_UNTIL_MSG_ID: LazyMessageId = LazyMessageId::new("Winit::CancelWaitUntil\0");
 // Message sent by a `Window` when it wants to be destroyed by the main thread.
 // WPARAM and LPARAM are unused.
 pub static DESTROY_MSG_ID: LazyMessageId = LazyMessageId::new("Winit::DestroyMsg\0");
@@ -795,74 +887,6 @@ fn normalize_pointer_pressure(pressure: u32) -> Option<Force> {
     }
 }
 
-/// Flush redraw events for Winit's windows.
-///
-/// Winit's API guarantees that all redraw events will be clustered together and dispatched all at
-/// once, but the standard Windows message loop doesn't always exhibit that behavior. If multiple
-/// windows have had redraws scheduled, but an input event is pushed to the message queue between
-/// the `WM_PAINT` call for the first window and the `WM_PAINT` call for the second window, Windows
-/// will dispatch the input event immediately instead of flushing all the redraw events. This
-/// function explicitly pulls all of Winit's redraw events out of the event queue so that they
-/// always all get processed in one fell swoop.
-///
-/// Returns `true` if this invocation flushed all the redraw events. If this function is re-entrant,
-/// it won't flush the redraw events and will return `false`.
-#[must_use]
-unsafe fn flush_paint_messages<T: 'static>(
-    except: Option<HWND>,
-    runner: &EventLoopRunner<T>,
-) -> bool {
-    if !runner.redrawing() {
-        runner.main_events_cleared();
-        let mut msg = mem::zeroed();
-        runner.owned_windows(|redraw_window| {
-            if Some(redraw_window) == except {
-                return;
-            }
-
-            if PeekMessageW(
-                &mut msg,
-                redraw_window,
-                WM_PAINT,
-                WM_PAINT,
-                PM_REMOVE | PM_QS_PAINT,
-            ) == false.into()
-            {
-                return;
-            }
-
-            TranslateMessage(&msg);
-            DispatchMessageW(&msg);
-        });
-        true
-    } else {
-        false
-    }
-}
-
-unsafe fn process_control_flow<T: 'static>(runner: &EventLoopRunner<T>) {
-    match runner.control_flow() {
-        ControlFlow::Poll => {
-            PostMessageW(
-                runner.thread_msg_target(),
-                PROCESS_NEW_EVENTS_MSG_ID.get(),
-                0,
-                0,
-            );
-        }
-        ControlFlow::Wait => (),
-        ControlFlow::WaitUntil(until) => {
-            PostThreadMessageW(
-                runner.wait_thread_id(),
-                WAIT_UNTIL_MSG_ID.get(),
-                0,
-                Box::into_raw(WaitUntilInstantBox::new(until)) as isize,
-            );
-        }
-        ControlFlow::ExitWithCode(_) => (),
-    }
-}
-
 /// Emit a `ModifiersChanged` event whenever modifiers have changed.
 /// Returns the current modifier state
 fn update_modifiers<T>(window: HWND, userdata: &WindowData<T>) {
@@ -988,13 +1012,6 @@ unsafe fn public_window_callback_inner<T: 'static>(
     lparam: LPARAM,
     userdata: &WindowData<T>,
 ) -> LRESULT {
-    RedrawWindow(
-        userdata.event_loop_runner.thread_msg_target(),
-        ptr::null(),
-        0,
-        RDW_INTERNALPAINT,
-    );
-
     let mut result = ProcResult::DefWindowProc(wparam);
 
     // Send new modifiers before sending key events.
@@ -1116,7 +1133,6 @@ unsafe fn public_window_callback_inner<T: 'static>(
                 window_id: RootWindowId(WindowId(window)),
                 event: Destroyed,
             });
-            userdata.event_loop_runner.remove_window(window);
             result = ProcResult::Value(0);
         }
 
@@ -1132,13 +1148,7 @@ unsafe fn public_window_callback_inner<T: 'static>(
                 // redraw the window outside the normal flow of the event loop.
                 RedrawWindow(window, ptr::null(), 0, RDW_INTERNALPAINT);
             } else {
-                let managing_redraw =
-                    flush_paint_messages(Some(window), &userdata.event_loop_runner);
                 userdata.send_event(Event::RedrawRequested(RootWindowId(WindowId(window))));
-                if managing_redraw {
-                    userdata.event_loop_runner.redraw_events_cleared();
-                    process_control_flow(&userdata.event_loop_runner);
-                }
             }
             result = ProcResult::DefWindowProc(wparam);
         }
@@ -2270,27 +2280,8 @@ unsafe extern "system" fn thread_event_target_callback<T: 'static>(
             userdata_removed = true;
             0
         }
-        // Because WM_PAINT comes after all other messages, we use it during modal loops to detect
-        // when the event queue has been emptied. See `process_event` for more details.
         WM_PAINT => {
             ValidateRect(window, ptr::null());
-            // If the WM_PAINT handler in `public_window_callback` has already flushed the redraw
-            // events, `handling_events` will return false and we won't emit a second
-            // `RedrawEventsCleared` event.
-            if userdata.event_loop_runner.handling_events() {
-                if userdata.event_loop_runner.should_buffer() {
-                    // This branch can be triggered when a nested win32 event loop is triggered
-                    // inside of the `event_handler` callback.
-                    RedrawWindow(window, ptr::null(), 0, RDW_INTERNALPAINT);
-                } else {
-                    // This WM_PAINT handler will never be re-entrant because `flush_paint_messages`
-                    // doesn't call WM_PAINT for the thread event target (i.e. this window).
-                    assert!(flush_paint_messages(None, &userdata.event_loop_runner));
-                    userdata.event_loop_runner.redraw_events_cleared();
-                    process_control_flow(&userdata.event_loop_runner);
-                }
-            }
-
             // Default WM_PAINT behaviour. This makes sure modals and popups are shown immediatly when opening them.
             DefWindowProcW(window, msg, wparam, lparam)
         }
@@ -2329,40 +2320,6 @@ unsafe extern "system" fn thread_event_target_callback<T: 'static>(
             function();
             0
         }
-        _ if msg == PROCESS_NEW_EVENTS_MSG_ID.get() => {
-            PostThreadMessageW(
-                userdata.event_loop_runner.wait_thread_id(),
-                CANCEL_WAIT_UNTIL_MSG_ID.get(),
-                0,
-                0,
-            );
-
-            // if the control_flow is WaitUntil, make sure the given moment has actually passed
-            // before emitting NewEvents
-            if let ControlFlow::WaitUntil(wait_until) = userdata.event_loop_runner.control_flow() {
-                let mut msg = mem::zeroed();
-                while Instant::now() < wait_until {
-                    if PeekMessageW(&mut msg, 0, 0, 0, PM_NOREMOVE) != false.into() {
-                        // This works around a "feature" in PeekMessageW. If the message PeekMessageW
-                        // gets is a WM_PAINT message that had RDW_INTERNALPAINT set (i.e. doesn't
-                        // have an update region), PeekMessageW will remove that window from the
-                        // redraw queue even though we told it not to remove messages from the
-                        // queue. We fix it by re-dispatching an internal paint message to that
-                        // window.
-                        if msg.message == WM_PAINT {
-                            let mut rect = mem::zeroed();
-                            if GetUpdateRect(msg.hwnd, &mut rect, false.into()) == false.into() {
-                                RedrawWindow(msg.hwnd, ptr::null(), 0, RDW_INTERNALPAINT);
-                            }
-                        }
-
-                        break;
-                    }
-                }
-            }
-            userdata.event_loop_runner.poll();
-            0
-        }
         _ => DefWindowProcW(window, msg, wparam, lparam),
     };
 

src/platform_impl/windows/event_loop/runner.rs

@@ -1,16 +1,13 @@
 use std::{
     any::Any,
     cell::{Cell, RefCell},
-    collections::{HashSet, VecDeque},
-    mem, panic, ptr,
+    collections::VecDeque,
+    mem, panic,
     rc::Rc,
     time::Instant,
 };
 
-use windows_sys::Win32::{
-    Foundation::HWND,
-    Graphics::Gdi::{RedrawWindow, RDW_INTERNALPAINT},
-};
+use windows_sys::Win32::Foundation::HWND;
 
 use crate::{
     dpi::PhysicalSize,
@@ -30,7 +27,11 @@ type EventHandler<T> = Cell<Option<Box<dyn FnMut(Event<'_, T>, &mut ControlFlow)
 pub(crate) struct EventLoopRunner<T: 'static> {
     // The event loop's win32 handles
     pub(super) thread_msg_target: HWND,
-    wait_thread_id: u32,
+
+    // Setting this will ensure pump_events will return to the external
+    // loop asap. E.g. set after each RedrawRequested to ensure pump_events
+    // can't stall an external loop beyond a frame
+    pub(super) interrupt_msg_dispatch: Cell<bool>,
 
     control_flow: Cell<ControlFlow>,
     runner_state: Cell<RunnerState>,
@@ -38,8 +39,6 @@ pub(crate) struct EventLoopRunner<T: 'static> {
     event_handler: EventHandler<T>,
     event_buffer: RefCell<VecDeque<BufferedEvent<T>>>,
 
-    owned_windows: Cell<HashSet<HWND>>,
-
     panic_error: Cell<Option<PanicError>>,
 }
 
@@ -47,7 +46,7 @@ pub type PanicError = Box<dyn Any + Send + 'static>;
 
 /// See `move_state_to` function for details on how the state loop works.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
-enum RunnerState {
+pub(crate) enum RunnerState {
     /// The event loop has just been created, and an `Init` event must be sent.
     Uninitialized,
     /// The event loop is idling.
@@ -55,9 +54,6 @@ enum RunnerState {
     /// The event loop is handling the OS's events and sending them to the user's callback.
     /// `NewEvents` has been sent, and `MainEventsCleared` hasn't.
     HandlingMainEvents,
-    /// The event loop is handling the redraw events and sending them to the user's callback.
-    /// `MainEventsCleared` has been sent, and `RedrawEventsCleared` hasn't.
-    HandlingRedrawEvents,
     /// The event loop has been destroyed. No other events will be emitted.
     Destroyed,
 }
@@ -68,20 +64,30 @@ enum BufferedEvent<T: 'static> {
 }
 
 impl<T> EventLoopRunner<T> {
-    pub(crate) fn new(thread_msg_target: HWND, wait_thread_id: u32) -> EventLoopRunner<T> {
+    pub(crate) fn new(thread_msg_target: HWND) -> EventLoopRunner<T> {
         EventLoopRunner {
             thread_msg_target,
-            wait_thread_id,
+            interrupt_msg_dispatch: Cell::new(false),
             runner_state: Cell::new(RunnerState::Uninitialized),
             control_flow: Cell::new(ControlFlow::Poll),
             panic_error: Cell::new(None),
             last_events_cleared: Cell::new(Instant::now()),
             event_handler: Cell::new(None),
             event_buffer: RefCell::new(VecDeque::new()),
-            owned_windows: Cell::new(HashSet::new()),
         }
     }
 
+    /// Associate the application's event handler with the runner
+    ///
+    /// # Safety
+    /// This is ignoring the lifetime of the application handler (which may not
+    /// outlive the EventLoopRunner) and can lead to undefined behaviour if
+    /// the handler is not cleared before the end of real lifetime.
+    ///
+    /// All public APIs that take an event handler (`run`, `run_ondemand`,
+    /// `pump_events`) _must_ pair a call to `set_event_handler` with
+    /// a call to `clear_event_handler` before returning to avoid
+    /// undefined behaviour.
     pub(crate) unsafe fn set_event_handler<F>(&self, f: F)
     where
         F: FnMut(Event<'_, T>, &mut ControlFlow),
@@ -93,18 +99,22 @@ impl<T> EventLoopRunner<T> {
         assert!(old_event_handler.is_none());
     }
 
+    pub(crate) fn clear_event_handler(&self) {
+        self.event_handler.set(None);
+    }
+
     pub(crate) fn reset_runner(&self) {
         let EventLoopRunner {
             thread_msg_target: _,
-            wait_thread_id: _,
+            interrupt_msg_dispatch,
             runner_state,
             panic_error,
             control_flow,
             last_events_cleared: _,
             event_handler,
             event_buffer: _,
-            owned_windows: _,
         } = self;
+        interrupt_msg_dispatch.set(false);
         runner_state.set(RunnerState::Uninitialized);
         panic_error.set(None);
         control_flow.set(ControlFlow::Poll);
@@ -114,18 +124,11 @@ impl<T> EventLoopRunner<T> {
 
 /// State retrieval functions.
 impl<T> EventLoopRunner<T> {
+    #[allow(unused)]
     pub fn thread_msg_target(&self) -> HWND {
         self.thread_msg_target
     }
 
-    pub fn wait_thread_id(&self) -> u32 {
-        self.wait_thread_id
-    }
-
-    pub fn redrawing(&self) -> bool {
-        self.runner_state.get() == RunnerState::HandlingRedrawEvents
-    }
-
     pub fn take_panic_error(&self) -> Result<(), PanicError> {
         match self.panic_error.take() {
             Some(err) => Err(err),
@@ -133,12 +136,16 @@ impl<T> EventLoopRunner<T> {
         }
     }
 
-    pub fn control_flow(&self) -> ControlFlow {
-        self.control_flow.get()
+    pub fn state(&self) -> RunnerState {
+        self.runner_state.get()
     }
 
-    pub fn handling_events(&self) -> bool {
-        self.runner_state.get() != RunnerState::Idle
+    pub fn set_exit_control_flow(&self, code: i32) {
+        self.control_flow.set(ControlFlow::ExitWithCode(code))
+    }
+
+    pub fn control_flow(&self) -> ControlFlow {
+        self.control_flow.get()
     }
 
     pub fn should_buffer(&self) -> bool {
@@ -177,42 +184,25 @@ impl<T> EventLoopRunner<T> {
             None
         }
     }
-    pub fn register_window(&self, window: HWND) {
-        let mut owned_windows = self.owned_windows.take();
-        owned_windows.insert(window);
-        self.owned_windows.set(owned_windows);
-    }
-
-    pub fn remove_window(&self, window: HWND) {
-        let mut owned_windows = self.owned_windows.take();
-        owned_windows.remove(&window);
-        self.owned_windows.set(owned_windows);
-    }
-
-    pub fn owned_windows(&self, mut f: impl FnMut(HWND)) {
-        let mut owned_windows = self.owned_windows.take();
-        for hwnd in &owned_windows {
-            f(*hwnd);
-        }
-        let new_owned_windows = self.owned_windows.take();
-        owned_windows.extend(&new_owned_windows);
-        self.owned_windows.set(owned_windows);
-    }
 }
 
 /// Event dispatch functions.
 impl<T> EventLoopRunner<T> {
-    pub(crate) unsafe fn poll(&self) {
+    pub(crate) fn prepare_wait(&self) {
+        self.move_state_to(RunnerState::Idle);
+    }
+
+    pub(crate) fn wakeup(&self) {
         self.move_state_to(RunnerState::HandlingMainEvents);
     }
 
-    pub(crate) unsafe fn send_event(&self, event: Event<'_, T>) {
+    pub(crate) fn send_event(&self, event: Event<'_, T>) {
         if let Event::RedrawRequested(_) = event {
-            if self.runner_state.get() != RunnerState::HandlingRedrawEvents {
-                warn!("RedrawRequested dispatched without explicit MainEventsCleared");
-                self.move_state_to(RunnerState::HandlingRedrawEvents);
-            }
             self.call_event_handler(event);
+            // As a rule, to ensure that `pump_events` can't block an external event loop
+            // for too long, we always guarantee that `pump_events` will return control to
+            // the external loop asap after a `RedrawRequested` event is dispatched.
+            self.interrupt_msg_dispatch.set(true);
         } else if self.should_buffer() {
             // If the runner is already borrowed, we're in the middle of an event loop invocation. Add
             // the event to a buffer to be processed later.
@@ -220,25 +210,16 @@ impl<T> EventLoopRunner<T> {
                 .borrow_mut()
                 .push_back(BufferedEvent::from_event(event))
         } else {
-            self.move_state_to(RunnerState::HandlingMainEvents);
             self.call_event_handler(event);
             self.dispatch_buffered_events();
         }
     }
 
-    pub(crate) unsafe fn main_events_cleared(&self) {
-        self.move_state_to(RunnerState::HandlingRedrawEvents);
-    }
-
-    pub(crate) unsafe fn redraw_events_cleared(&self) {
-        self.move_state_to(RunnerState::Idle);
-    }
-
-    pub(crate) unsafe fn loop_destroyed(&self) {
+    pub(crate) fn loop_destroyed(&self) {
         self.move_state_to(RunnerState::Destroyed);
     }
 
-    unsafe fn call_event_handler(&self, event: Event<'_, T>) {
+    fn call_event_handler(&self, event: Event<'_, T>) {
         self.catch_unwind(|| {
             let mut control_flow = self.control_flow.take();
             let mut event_handler = self.event_handler.take()
@@ -255,7 +236,7 @@ impl<T> EventLoopRunner<T> {
         });
     }
 
-    unsafe fn dispatch_buffered_events(&self) {
+    fn dispatch_buffered_events(&self) {
         loop {
             // We do this instead of using a `while let` loop because if we use a `while let`
             // loop the reference returned `borrow_mut()` doesn't get dropped until the end
@@ -278,24 +259,22 @@ impl<T> EventLoopRunner<T> {
     ///    Uninitialized
     ///          |
     ///          V
-    ///  HandlingMainEvents
-    ///   ^            |
-    ///   |            V
-    /// Idle <--- HandlingRedrawEvents
-    ///   |
-    ///   V
-    /// Destroyed
+    ///        Idle
+    ///       ^    |
+    ///       |    V
+    /// HandlingMainEvents
+    ///         |
+    ///         V
+    ///     Destroyed
     /// ```
     ///
     /// Attempting to transition back to `Uninitialized` will result in a panic. Attempting to
-    /// transition *from* `Destroyed` will also reuslt in a panic. Transitioning to the current
+    /// transition *from* `Destroyed` will also result in a panic. Transitioning to the current
     /// state is a no-op. Even if the `new_runner_state` isn't the immediate next state in the
     /// runner state machine (e.g. `self.runner_state == HandlingMainEvents` and
     /// `new_runner_state == Idle`), the intermediate state transitions will still be executed.
-    unsafe fn move_state_to(&self, new_runner_state: RunnerState) {
-        use RunnerState::{
-            Destroyed, HandlingMainEvents, HandlingRedrawEvents, Idle, Uninitialized,
-        };
+    fn move_state_to(&self, new_runner_state: RunnerState) {
+        use RunnerState::{Destroyed, HandlingMainEvents, Idle, Uninitialized};
 
         match (
             self.runner_state.replace(new_runner_state),
@@ -304,17 +283,12 @@ impl<T> EventLoopRunner<T> {
             (Uninitialized, Uninitialized)
             | (Idle, Idle)
             | (HandlingMainEvents, HandlingMainEvents)
-            | (HandlingRedrawEvents, HandlingRedrawEvents)
             | (Destroyed, Destroyed) => (),
 
             // State transitions that initialize the event loop.
             (Uninitialized, HandlingMainEvents) => {
                 self.call_new_events(true);
             }
-            (Uninitialized, HandlingRedrawEvents) => {
-                self.call_new_events(true);
-                self.call_event_handler(Event::MainEventsCleared);
-            }
             (Uninitialized, Idle) => {
                 self.call_new_events(true);
                 self.call_event_handler(Event::MainEventsCleared);
@@ -332,19 +306,11 @@ impl<T> EventLoopRunner<T> {
             (Idle, HandlingMainEvents) => {
                 self.call_new_events(false);
             }
-            (Idle, HandlingRedrawEvents) => {
-                self.call_new_events(false);
-                self.call_event_handler(Event::MainEventsCleared);
-            }
             (Idle, Destroyed) => {
                 self.call_event_handler(Event::LoopDestroyed);
             }
 
-            (HandlingMainEvents, HandlingRedrawEvents) => {
-                self.call_event_handler(Event::MainEventsCleared);
-            }
             (HandlingMainEvents, Idle) => {
-                warn!("RedrawEventsCleared emitted without explicit MainEventsCleared");
                 self.call_event_handler(Event::MainEventsCleared);
                 self.call_redraw_events_cleared();
             }
@@ -354,24 +320,11 @@ impl<T> EventLoopRunner<T> {
                 self.call_event_handler(Event::LoopDestroyed);
             }
 
-            (HandlingRedrawEvents, Idle) => {
-                self.call_redraw_events_cleared();
-            }
-            (HandlingRedrawEvents, HandlingMainEvents) => {
-                warn!("NewEvents emitted without explicit RedrawEventsCleared");
-                self.call_redraw_events_cleared();
-                self.call_new_events(false);
-            }
-            (HandlingRedrawEvents, Destroyed) => {
-                self.call_redraw_events_cleared();
-                self.call_event_handler(Event::LoopDestroyed);
-            }
-
             (Destroyed, _) => panic!("cannot move state from Destroyed"),
         }
     }
 
-    unsafe fn call_new_events(&self, init: bool) {
+    fn call_new_events(&self, init: bool) {
         let start_cause = match (init, self.control_flow()) {
             (true, _) => StartCause::Init,
             (false, ControlFlow::Poll) => StartCause::Poll,
@@ -402,10 +355,9 @@ impl<T> EventLoopRunner<T> {
             self.call_event_handler(Event::Resumed);
         }
         self.dispatch_buffered_events();
-        RedrawWindow(self.thread_msg_target, ptr::null(), 0, RDW_INTERNALPAINT);
     }
 
-    unsafe fn call_redraw_events_cleared(&self) {
+    fn call_redraw_events_cleared(&self) {
         self.call_event_handler(Event::RedrawEventsCleared);
         self.last_events_cleared.set(Instant::now());
     }

src/platform_impl/windows/window.rs

@@ -987,8 +987,6 @@ impl<'a, T: 'static> InitData<'a, T> {
             None
         };
 
-        self.event_loop.runner_shared.register_window(win.window.0);
-
         event_loop::WindowData {
             window_state: win.window_state.clone(),
             event_loop_runner: self.event_loop.runner_shared.clone(),

src/window.rs

@@ -524,25 +524,25 @@ impl Window {
         self.window.scale_factor()
     }
 
-    /// Emits a [`Event::RedrawRequested`] event in the associated event loop after all OS
-    /// events have been processed by the event loop.
+    /// Requests a future [`Event::RedrawRequested`] event to be emitted in a way that is
+    /// synchronized and / or throttled by the windowing system.
     ///
     /// This is the **strongly encouraged** method of redrawing windows, as it can integrate with
     /// OS-requested redraws (e.g. when a window gets resized).
     ///
-    /// This function can cause `RedrawRequested` events to be emitted after [`Event::MainEventsCleared`]
-    /// but before `Event::NewEvents` if called in the following circumstances:
-    /// * While processing `MainEventsCleared`.
-    /// * While processing a `RedrawRequested` event that was sent during `MainEventsCleared` or any
-    ///   directly subsequent `RedrawRequested` event.
+    /// Applications should always aim to redraw whenever they receive a `RedrawRequested` event.
+    ///
+    /// There are no strong guarantees about when exactly a `RedrawRequest` event will be emitted
+    /// with respect to other events, since the requirements can vary significantly between
+    /// windowing systems.
     ///
     /// ## Platform-specific
     ///
+    /// - **Windows** This API uses `RedrawWindow` to request a `WM_PAINT` message and `RedrawRequested`
+    ///   is emitted in sync with any `WM_PAINT` messages
     /// - **iOS:** Can only be called on the main thread.
-    /// - **Android:** Subsequent calls after `MainEventsCleared` are not handled.
     ///
     /// [`Event::RedrawRequested`]: crate::event::Event::RedrawRequested
-    /// [`Event::MainEventsCleared`]: crate::event::Event::MainEventsCleared
     #[inline]
     pub fn request_redraw(&self) {
         self.window.request_redraw()