Prevent event loop pause while a caption button is pressed on Windows (eventloop-2.0)

src/platform_impl/windows/event_loop.rs

@@ -833,17 +833,64 @@ unsafe extern "system" fn public_window_callback<T>(
         }
 
         winuser::WM_NCLBUTTONDOWN => {
-            // jumpstart the modal loop
-            winuser::RedrawWindow(
-                window,
-                ptr::null(),
-                ptr::null_mut(),
-                winuser::RDW_INTERNALPAINT,
-            );
-            if wparam == winuser::HTCAPTION as _ {
-                winuser::PostMessageW(window, winuser::WM_MOUSEMOVE, 0, 0);
+            match wparam as isize {
+                winuser::HTMAXBUTTON | winuser::HTMINBUTTON | winuser::HTCLOSE => {
+                    // Discard the non-client button-down event and defer the processing until
+                    // the non-client button-up event. This prevents a pause in the message loop
+                    // while the user has the mouse button pressed on one of these buttons.
+                    let mut window_state = subclass_input.window_state.lock();
+                    window_state.deferred_nclbuttondown_wparam = Some(wparam);
+                    0
+                }
+                _ => {
+                    // jumpstart the modal loop
+                    winuser::RedrawWindow(
+                        window,
+                        ptr::null(),
+                        ptr::null_mut(),
+                        winuser::RDW_INTERNALPAINT,
+                    );
+                    if wparam == winuser::HTCAPTION as _ {
+                        winuser::PostMessageW(window, winuser::WM_MOUSEMOVE, 0, 0);
+                    }
+                    commctrl::DefSubclassProc(window, msg, wparam, lparam)
+                }
+            }
+        }
+
+        winuser::WM_NCLBUTTONUP => {
+            let deferred_wparam = subclass_input
+                .window_state
+                .lock()
+                .deferred_nclbuttondown_wparam
+                .filter(|deferred_wparam| *deferred_wparam == wparam)
+                .map(|wparam| wparam as isize)
+                .take();
+            // Manually apply min, max, restore, and close since we discarded the button down event,
+            // but only in the event that the user did not cancel the operation. If the user canelled
+            // the operation, WM_LBUTTONUP will fire and we'll clear the deferred state there as well.
+            match deferred_wparam {
+                Some(winuser::HTMAXBUTTON) => {
+                    let window_state = subclass_input.window_state.lock();
+                    let window_flags = window_state.window_flags();
+                    drop(window_state);
+                    if window_flags.contains(WindowFlags::MAXIMIZED) {
+                        winuser::ShowWindow(window, winuser::SW_RESTORE);
+                    } else {
+                        winuser::ShowWindow(window, winuser::SW_MAXIMIZE);
+                    }
+                    0
+                }
+                Some(winuser::HTMINBUTTON) => {
+                    winuser::ShowWindow(window, winuser::SW_MINIMIZE);
+                    0
+                }
+                Some(winuser::HTCLOSE) => {
+                    winuser::PostMessageW(window, winuser::WM_CLOSE, 0, 0);
+                    0
+                }
+                _ => commctrl::DefSubclassProc(window, msg, wparam, lparam),
             }
-            commctrl::DefSubclassProc(window, msg, wparam, lparam)
         }
 
         winuser::WM_CLOSE => {
@@ -1149,6 +1196,12 @@ unsafe extern "system" fn public_window_callback<T>(
                 ElementState::Released, MouseButton::Left, WindowEvent::MouseInput,
             };
 
+            subclass_input
+                .window_state
+                .lock()
+                .deferred_nclbuttondown_wparam
+                .take();
+
             release_mouse(&mut *subclass_input.window_state.lock());
 
             subclass_input.send_event(Event::WindowEvent {

src/platform_impl/windows/window_state.rs

@@ -8,7 +8,7 @@ use parking_lot::MutexGuard;
 use std::{io, ptr};
 use winapi::{
     shared::{
-        minwindef::DWORD,
+        minwindef::{DWORD, WPARAM},
         windef::{HWND, RECT},
     },
     um::winuser,
@@ -30,6 +30,7 @@ pub struct WindowState {
     pub dpi_factor: f64,
 
     pub fullscreen: Option<MonitorHandle>,
+    pub deferred_nclbuttondown_wparam: Option<WPARAM>,
     window_flags: WindowFlags,
 }
 
@@ -111,6 +112,7 @@ impl WindowState {
 
             fullscreen: None,
             window_flags: WindowFlags::empty(),
+            deferred_nclbuttondown_wparam: None,
         }
     }