[ WPF Gallery ] White Border Around Main Window on Windows 10 - After Commit d38f0fcc

[cubekon]

Short Description:
Since commit d38f0fcc, a white border appears around the main window in the WPF Gallery application, but only on Windows 10.

Cause:
The root cause seems to be a different rendering behavior of WindowChrome between Windows 10 and Windows 11.
Maybe there is a connection with this ticket: The WindowChrome class needs to be updated & fixed #3887

Proposed Fix:
Force NonClientFrameEdges = NonClientFrameEdges.None during initialization and updates:

WindowChrome.SetWindowChrome(
    this,
    new WindowChrome
    {
        CaptionHeight = 50,
        CornerRadius = new CornerRadius(12),
        GlassFrameThickness = new Thickness(-1),
        ResizeBorderThickness = ResizeMode == ResizeMode.NoResize ? default : new Thickness(4),
        UseAeroCaptionButtons = true,
        NonClientFrameEdges = NonClientFrameEdges.None // <-- For windows 10 only
    }
);

And in UpdateMainWindowVisuals():

var wc = WindowChrome.GetWindowChrome(this);
if (wc is not null)
{
    wc.NonClientFrameEdges = NonClientFrameEdges.None; // <-- For windows 10 only
}

Additional Suggestion:
Enable Immersive Dark Mode to prevent flashing white backgrounds during window resize:

[DllImport("dwmapi.dll")]
private static extern int DwmSetWindowAttribute(IntPtr hwnd, int attr, ref int attrValue, int attrSize);

private const int DWMWA_USE_IMMERSIVE_DARK_MODE = 20;

private void ApplyWindowDarkMode()
{
    IntPtr hwnd = new WindowInteropHelper(this).Handle;
    int darkMode = 1;
    DwmSetWindowAttribute(hwnd, DWMWA_USE_IMMERSIVE_DARK_MODE, ref darkMode, sizeof(int));
}

this.Loaded += (s, e) => ApplyWindowDarkMode();

Environment:

• Windows 10
• .NET version: 9 and 10
• WPF Gallery latest main branch

[dipeshmsft]

Thanks @cubekon for bringing this issue up. I will take a look at this and get this fixed.