Add ability to control window border color on Windows 11

[kapios010]

Is your feature request related to a problem? Please describe.

There is currently no way to change the border color on Windows 11 without using P/Invoke. It would make for a nice stylization feature (and I want to use it for an app I'm making).

Describe the solution you'd like

Introducing a StyledProperty BorderColorHint to the Window similarly to #17294 might be a good choice, but it could cause confusion with BorderBrush (a property which I've tried to unsuccessfully use to set the window border color).

Describe alternatives you've considered

While it's possible to manually call DwmSetWindowAttribute, it'd be simpler to have a dedicated property.

Additional context

Custom-colored borders as visible in windowed VS2026:

[maxkatz6]

There is currently no way to change the border color on Windows 11 without using P/Invoke

Another option is to completely remove platform window decorations and render your own border. While it's not ideal, I know, it works now. And I have a feeling that's what VS exactly does on your screenshot, at least that's what was common with windows apps before DwmSetWindowAttribute option existed.

[StefanKoell]

I'm also doing it myself in Avalonia (creating a "root" border) and it is a bit tricky to get it right. Corner radius, clipping, thickness is quite hard to match what Windows natively does. It always looks a bit "off". The upside is that you have more control over the border and that it works on all platforms this way, although I'm not sure this is desirable. So having - on Windows at least - native platform support for the border would be a nice addition.

[kapios010]

I've done it like this myself:

using Avalonia.Media.Color;

 internal partial class WindowBorder
 {

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

     private const int ATTR = 34;

     public static bool SetBorderColor(IntPtr hwnd, Color color)
     {
         if (!IsWindows11OrGreater()) return false;
         uint convColor = (uint)((color.B << 16) | (color.G << 8) | color.R);
         return DwmSetWindowAttribute(hwnd, ATTR, ref convColor, sizeof(uint)) == 0;
     }

     public static bool SetBorderDefault(IntPtr hwnd)
     {
         if (!IsWindows11OrGreater()) return false;
         uint color = 0xFFFFFFFF;
         return DwmSetWindowAttribute(hwnd, ATTR, ref color, sizeof(uint)) == 0;
     }

     public static bool SetBorderNone(IntPtr hwnd)
     {
         if (!IsWindows11OrGreater()) return false;
         uint color = 0xFFFFFFFE;
         return DwmSetWindowAttribute(hwnd, ATTR, ref color, sizeof(uint)) == 0;
     }

     private static bool IsWindows11OrGreater()
     {
         return Environment.OSVersion.Version.Major >= 10 && Environment.OSVersion.Version.Build >= 22000;
     }
}

        public MainWindow()
        {
            InitializeComponent();
            IntPtr? handle = this.TryGetPlatformHandle()?.Handle;
            if (handle != null)
                WindowBorder.SetBorderColor((IntPtr)handle, Color.Parse("#1070b0"));
        }

Which isn't that difficult really, but I think that having it as a property on Window would be a nice quality of life update.