On Windows, fix window size for maximized, undecorated windows

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 macOS, run most actions on the main thread, which is strictly more correct, but might make multithreaded applications block slightly more.
 - On macOS, fix panic when getting current monitor without any monitor attached.
 - On Windows and MacOS, add API to enable/disable window buttons (close, minimize, ...etc).

src/platform_impl/windows/event_loop.rs

@@ -978,19 +978,27 @@ unsafe fn public_window_callback_inner<T: 'static>(
                 return DefWindowProcW(window, msg, wparam, lparam);
             }
 
-            // Extend the client area to cover the whole non-client area.
-            // https://docs.microsoft.com/en-us/windows/win32/winmsg/wm-nccalcsize#remarks
-            //
-            // HACK(msiglreith): To add the drop shadow we slightly tweak the non-client area.
-            // This leads to a small black 1px border on the top. Adding a margin manually
-            // on all 4 borders would result in the caption getting drawn by the DWM.
-            //
-            // Another option would be to allow the DWM to paint inside the client area.
-            // 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);
+            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.
+                let monitor = monitor::current_monitor(window);
+                if let Ok(monitor_info) = monitor::get_monitor_info(monitor.hmonitor()) {
+                    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
+                //
+                // HACK(msiglreith): To add the drop shadow we slightly tweak the non-client area.
+                // This leads to a small black 1px border on the top. Adding a margin manually
+                // on all 4 borders would result in the caption getting drawn by the DWM.
+                //
+                // Another option would be to allow the DWM to paint inside the client area.
+                // 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.
                 params.rgrc[0].top += 1;
                 params.rgrc[0].bottom += 1;
             }

src/platform_impl/windows/util.rs

@@ -23,10 +23,11 @@ use windows_sys::{
             HiDpi::{DPI_AWARENESS_CONTEXT, MONITOR_DPI_TYPE, PROCESS_DPI_AWARENESS},
             Input::KeyboardAndMouse::GetActiveWindow,
             WindowsAndMessaging::{
-                ClipCursor, GetClientRect, GetClipCursor, GetSystemMetrics, GetWindowRect,
-                ShowCursor, IDC_APPSTARTING, IDC_ARROW, IDC_CROSS, IDC_HAND, IDC_HELP, IDC_IBEAM,
-                IDC_NO, IDC_SIZEALL, IDC_SIZENESW, IDC_SIZENS, IDC_SIZENWSE, IDC_SIZEWE, IDC_WAIT,
-                SM_CXVIRTUALSCREEN, SM_CYVIRTUALSCREEN, SM_XVIRTUALSCREEN, SM_YVIRTUALSCREEN,
+                ClipCursor, GetClientRect, GetClipCursor, GetSystemMetrics, GetWindowPlacement,
+                GetWindowRect, ShowCursor, IDC_APPSTARTING, IDC_ARROW, IDC_CROSS, IDC_HAND,
+                IDC_HELP, IDC_IBEAM, IDC_NO, IDC_SIZEALL, IDC_SIZENESW, IDC_SIZENS, IDC_SIZENWSE,
+                IDC_SIZEWE, IDC_WAIT, SM_CXVIRTUALSCREEN, SM_CYVIRTUALSCREEN, SM_XVIRTUALSCREEN,
+                SM_YVIRTUALSCREEN, SW_MAXIMIZE, WINDOWPLACEMENT,
             },
         },
     },
@@ -90,6 +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;