SizeToContent and ExtendsContentIntoTitleBar

Describe the bug

Not really a (wpfui-) bug but setting ExtendsContentIntoTitleBar to true and setting SizeToContent to WidthAndHeight gives the strange effect of the content being misaligned in the window, leaving a black area right and bottom, as described here

Since this rates to be common using FluentWindow, maybe a fix can be included there.

Meanwhile, a fix that seems to work but gives som flicker:
In the windows ContentRendered call

private void corewnd_ContentRendered(object sender, System.EventArgs e)
  {
      var firstrender = true;

      if (firstrender)
      {
          InvalidateVisual();
          firstrender = false;
      }
}

To Reproduce

Set FluentWindow ExtendsContentIntoTitleBar=true and WidthAndHeight=SizeToContent and add content.

Expected behavior

Normal content display

Screenshots

No response

OS version

Win 11 Pro

.NET version

8

WPF-UI NuGet version

4.0.3

Additional context

No response

Start with the FluentWindow implementation and reproduce the issue on Windows 11 with .NET 8 using ExtendsContentIntoTitleBar=true, SizeToContent=WidthAndHeight, and sample content. Investigate the misalignment and black right/bottom areas, using the ContentRendered workaround only as context. Done means content displays normally without the black area or flicker.