Win32 child window not shown initially if created outside of the WinUI3 Windows constructor

[gmgallo]

Using your sample code I was able to host an OpenGL page inside a WinUI3 window. However I ran into an issue which I'm not sure if it is a Windows bug or a missing step. I'm creating the child window the same way as shown in your code.

   IntPtr hchild = CreateWindowEx(WS_EX_LAYERED , "Static", "",
       WS_VISIBLE | WS_CHILD, x, y, width, height, hwndParent, IntPtr.Zero, IntPtr.Zero, IntPtr.Zero);

 SetLayeredWindowAttributes(hchild, 0, (byte)(255 * nOpacity / 100), LWA_ALPHA);

 ShowWindow(hchild, SW_SHOWNORMAL);

If created in the parent window constructor, the child window displays precisely overlayed as expected. However, if the child window creation is delayed and created later by a by a member function of the parent class, it doesn't show until the parent window is moved or resized!, either by user interaction or programatically.

Create button already pressed, The counter on top shows OpenGL renders on the child window.

After moving the parent window by hand.

The same can be accomplished by calling MoveWindow() on the parent just one pixel in any direction. I've tried several Wind32 calls on the child to bring it to the front without success. Adding the flag WS_EX_TOPMOST to the child window makes no difference.

[castorix]

They apparently changed something in messages/painting, as I noticed a similar problem in another sample (WinUI3_DesktopDuplication) which worked fine before.
As quick workaround, I handled VisibilityChanged of main window where I call GetWindowRect then SetWindowPos twice to resize it by -1, than original size (only once with a boolean, otherwise it crashes on closing...)... It fixes the problem but it is not great.

[gmgallo]

That is exaclty what I came up with like this which is barely noticeable, even with the 100ms delay.

void ShakeParentWindow()
{
	RECT R;
	GetWindowRect(m_hWndParent, &R); 
	int width = R.right - R.left;
	int height = R.bottom - R.top;
	MoveWindow(m_hWndParent, R.left, R.top, --width, height, false);
	Task::Delay(100);
	MoveWindow(m_hWndParent, R.left, R.top, ++width, height, false);
}

The only caveat is that it won't work if the parent window is expanded to full screen.

One other issue that I ran into is that I'm creating the window as a descendent of, and laying on top of, a UserControl and make it transparent with WS_EX_TRANSPARENT, so the UserControl gets the mouse imput, but it doesn't. I guess that the OpenGL layer is catching the window messages.

[castorix]

After more tests, I found a simpler way ;
I added :

SetWindowPos(hWndContainer, IntPtr.Zero, 0, 0, 0, 0, SWP_NOSIZE | SWP_NOMOVE | SWP_SHOWWINDOW | SWP_FRAMECHANGED);

at the end of ContainerPanelInit
(I did not test with Fullscreen...)
(I updated it in ContainerPanel.xaml.cs)

[gmgallo]

Yes, that works for me too, also in full screen mode. Thanks!
But the thing is that the child screen won't show up until its content gets updated by the application. But that is not a problem in my case.

[gmgallo]

Here is some hint in case you need to capture mouse input on the Win32 window. It requires only a few quick changes.

1. Add WS_EX_TRANSPARENT to CreateWindowEx() so, it will let the mouse events pass through.
2. UserControl has the hooks but for some reason the evens don't trigger. Instead derive the control from other controls. I used Canvas but others should work as well.
3. Add a Background color to the Canvas, otherwise the mouse events will pass through it too.