Merge pull request unoplatform#21758 from ramezgerges/win32_maximized_initial_frame

fix(win32): generate an initial frame with the correct size when starting up as maximized

Author: ramezgerges

src/Uno.UI.Runtime.Skia.Win32.Support/NativeMethods.txt

@@ -78,6 +78,7 @@ WM_NCHITTEST
 WM_NCMOUSEMOVE
 WM_NCLBUTTONDOWN
 WM_NCLBUTTONUP
+WM_NCPAINT
 WM_NCPOINTERUPDATE
 WM_NCPOINTERDOWN
 WM_NCPOINTERUP

src/Uno.UI.Runtime.Skia.Win32/UI/Xaml/Window/Win32WindowWrapper.cs

@@ -6,6 +6,7 @@
 using System.IO;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
+using System.Threading;
 using Microsoft.UI.Input;
 using Microsoft.UI.Windowing;
 using Microsoft.UI.Xaml;
@@ -29,6 +30,7 @@
 using Windows.Win32.Graphics.Gdi;
 using Windows.Win32.UI.HiDpi;
 using Windows.Win32.UI.WindowsAndMessaging;
+using Uno.UI.Dispatching;
 using Point = System.Drawing.Point;
 
 namespace Uno.UI.Runtime.Skia.Win32;
@@ -208,6 +210,15 @@ private unsafe LRESULT WndProcInner(HWND hwnd, uint msg, WPARAM wParam, LPARAM l
 
 		switch (msg)
 		{
+			case PInvoke.WM_NCPAINT:
+				// see the comment in the WM_ERASEBKGND handler
+				if (WasShown && _beforeFirstEraseBkgnd && (_pendingState is OverlappedPresenterState.Maximized || Window?.AppWindow.Presenter is FullScreenPresenter))
+				{
+					OnWindowSizeOrLocationChanged(); // In case the window size has changed but WM_SIZE is not fired yet. This happens specifically if the window is starting maximized using _pendingState
+					XamlRoot!.VisualTree.RootElement.UpdateLayout(); // relayout in response to the new window size
+					(XamlRoot?.Content?.Visual.CompositionTarget as CompositionTarget)?.OnRenderFrameOpportunity(); // force an early render
+				}
+				break;
 			case PInvoke.WM_ACTIVATE:
 				OnWmActivate(wParam);
 				return new LRESULT(0);
@@ -253,11 +264,15 @@ private unsafe LRESULT WndProcInner(HWND hwnd, uint msg, WPARAM wParam, LPARAM l
 				if (_beforeFirstEraseBkgnd)
 				{
 					// Without drawing on the first WM_ERASEBKGND, we get an initial white frame
-					// Note that we don't call OnRenderFrameOpportunity here, but before showing
-					// the window in ShowCore. The problem is that any minor delay will cause
-					// a split-second white flash, so we're keeping the "time to blit" to a
-					// minimum by "rendering" before the window is shown and only drawing when
-					// receiving the first WM_ERASEBKGND
+					// Note that we don't call OnRenderFrameOpportunity here, but in ShowCore right before
+					// showing the window or in WM_NCPAINT which is the first message received after showing
+					// the window in ShowCore and after a possible window size change because the window was
+					// shown in a maximized/fullscreen state.
+					// The problem is that any minor delay will cause a split-second white flash, so we're keeping
+					// the "time to blit" to a minimum by "rendering" asap and only "drawing" when
+					// receiving the first WM_ERASEBKGND. Even then, there is still a race between our drawing a frame
+					// and the next screen refresh and while in most cases we are able to win the race and not get this
+					// split second of "whiteness", it's not a guarantee, especially on a slower device.
 					_beforeFirstEraseBkgnd = false;
 					// The render timer might already be running. This is fine. The CompositionTarget
 					// contract allows calling OnNativePlatformFrameRequested multiple times.
@@ -466,20 +481,14 @@ protected internal override void Activate()
 
 	protected override void ShowCore()
 	{
-		// see the comment in WndProc's WM_ERASEBKGND handling
-		if (_beforeFirstEraseBkgnd)
-		{
-			(XamlRoot?.Content?.Visual.CompositionTarget as CompositionTarget)?.OnRenderFrameOpportunity();
-		}
-
 		if (Window?.AppWindow.Presenter is FullScreenPresenter)
 		{
 			// The window takes a split second to be rerendered with the fullscreen window size but
 			// no fix has been found for this yet.
 			SetWindowStyle(WINDOW_STYLE.WS_DLGFRAME, false);
 			_ = PInvoke.ShowWindow(_hwnd, SHOW_WINDOW_CMD.SW_MAXIMIZE);
 		}
-		else if (Window?.AppWindow.Presenter is OverlappedPresenter overlappedPresenter)
+		else if (Window?.AppWindow.Presenter is OverlappedPresenter)
 		{
 			switch (_pendingState)
 			{
@@ -489,17 +498,18 @@ protected override void ShowCore()
 				case OverlappedPresenterState.Minimized:
 					_ = PInvoke.ShowWindow(_hwnd, SHOW_WINDOW_CMD.SW_MINIMIZE);
 					break;
-				case OverlappedPresenterState.Restored:
-					_ = PInvoke.ShowWindow(_hwnd, SHOW_WINDOW_CMD.SW_RESTORE);
-					break;
 				default:
-					_ = PInvoke.ShowWindow(_hwnd, SHOW_WINDOW_CMD.SW_SHOWDEFAULT);
+					// see the comment in the WM_ERASEBKGND handler
+					OnWindowSizeOrLocationChanged(); // In case the window size has changed but WM_SIZE is not fired yet. This happens specifically if the window is starting maximized using _pendingState
+					XamlRoot!.VisualTree.RootElement.UpdateLayout(); // relayout in response to the new window size
+					(XamlRoot?.Content?.Visual.CompositionTarget as CompositionTarget)?.OnRenderFrameOpportunity(); // force an early render
+					PInvoke.ShowWindow(_hwnd, SHOW_WINDOW_CMD.SW_SHOWDEFAULT);
 					break;
 			}
 		}
 		else
 		{
-			PInvoke.ShowWindow(_hwnd, SHOW_WINDOW_CMD.SW_SHOWDEFAULT);
+			throw new InvalidOperationException("Unsupported Window Presenter.");
 		}
 	}