On Windows, fix window size for maximized, undecorated windows (#2584)

Co-authored-by: Amr Bashir <amr.bashir2015@gmail.com>

CHANGELOG.md

@@ -8,6 +8,7 @@ And please only add new entries to the top of this list, right below the `# Unre
 
 # Unreleased
 
+- On Windows, fix window size for maximized, undecorated windows.
 - On Windows and macOS, add `WindowBuilder::with_active`.
 - Add `Window::is_minimized`.
 - On X11, fix errors handled during `register_xlib_error_hook` invocation bleeding into winit.

src/platform_impl/windows/event_loop.rs

@@ -977,6 +977,20 @@ unsafe fn public_window_callback_inner<T: 'static>(
                 return DefWindowProcW(window, msg, wparam, lparam);
             }
 
+            let params = &mut *(lparam as *mut NCCALCSIZE_PARAMS);
+
+            if util::is_maximized(window) {
+                // Limit the window size when maximized to the current monitor.
+                // Otherwise it would include the non-existent decorations.
+                //
+                // Use `MonitorFromRect` instead of `MonitorFromWindow` to select
+                // the correct monitor here.
+                // See https://github.com/MicrosoftEdge/WebView2Feedback/issues/2549
+                let monitor = MonitorFromRect(&params.rgrc[0], MONITOR_DEFAULTTONULL);
+                if let Ok(monitor_info) = monitor::get_monitor_info(monitor) {
+                    params.rgrc[0] = monitor_info.monitorInfo.rcWork;
+                }
+            } else if window_flags.contains(WindowFlags::MARKER_UNDECORATED_SHADOW) {
                 // Extend the client area to cover the whole non-client area.
                 // https://docs.microsoft.com/en-us/windows/win32/winmsg/wm-nccalcsize#remarks
                 //
@@ -988,8 +1002,6 @@ unsafe fn public_window_callback_inner<T: 'static>(
                 // Unfortunately this results in janky resize behavior, where the compositor is
                 // ahead of the window surface. Currently, there seems no option to achieve this
                 // with the Windows API.
-            if window_flags.contains(WindowFlags::MARKER_UNDECORATED_SHADOW) {
-                let params = &mut *(lparam as *mut NCCALCSIZE_PARAMS);
                 params.rgrc[0].top += 1;
                 params.rgrc[0].bottom += 1;
             }

src/platform_impl/windows/util.rs

@@ -23,11 +23,11 @@ use windows_sys::{
             HiDpi::{DPI_AWARENESS_CONTEXT, MONITOR_DPI_TYPE, PROCESS_DPI_AWARENESS},
             Input::KeyboardAndMouse::GetActiveWindow,
             WindowsAndMessaging::{
-                ClipCursor, GetClientRect, GetClipCursor, GetSystemMetrics, GetWindowRect,
+                ClipCursor, GetClientRect, GetClipCursor, GetSystemMetrics, GetWindowPlacement,
-                IsIconic, ShowCursor, IDC_APPSTARTING, IDC_ARROW, IDC_CROSS, IDC_HAND, IDC_HELP,
+                GetWindowRect, IsIconic, ShowCursor, IDC_APPSTARTING, IDC_ARROW, IDC_CROSS,
-                IDC_IBEAM, IDC_NO, IDC_SIZEALL, IDC_SIZENESW, IDC_SIZENS, IDC_SIZENWSE, IDC_SIZEWE,
+                IDC_HAND, IDC_HELP, IDC_IBEAM, IDC_NO, IDC_SIZEALL, IDC_SIZENESW, IDC_SIZENS,
-                IDC_WAIT, SM_CXVIRTUALSCREEN, SM_CYVIRTUALSCREEN, SM_XVIRTUALSCREEN,
+                IDC_SIZENWSE, IDC_SIZEWE, IDC_WAIT, SM_CXVIRTUALSCREEN, SM_CYVIRTUALSCREEN,
-                SM_YVIRTUALSCREEN,
+                SM_XVIRTUALSCREEN, SM_YVIRTUALSCREEN, SW_MAXIMIZE, WINDOWPLACEMENT,
             },
         },
     },
@@ -91,6 +91,15 @@ impl WindowArea {
     }
 }
 
+pub fn is_maximized(window: HWND) -> bool {
+    unsafe {
+        let mut placement: WINDOWPLACEMENT = mem::zeroed();
+        placement.length = mem::size_of::<WINDOWPLACEMENT>() as u32;
+        GetWindowPlacement(window, &mut placement);
+        placement.showCmd == SW_MAXIMIZE
+    }
+}
+
 pub fn set_cursor_hidden(hidden: bool) {
     static HIDDEN: AtomicBool = AtomicBool::new(false);
     let changed = HIDDEN.swap(hidden, Ordering::SeqCst) ^ hidden;