Undocumented API for Windows 10 Acrylic style.

UndocumentedAcrylic.cs

internal enum WindowCompositionAttribute
{
    WCA_UNDEFINED = 0,
    WCA_NCRENDERING_ENABLED = 1,
    WCA_NCRENDERING_POLICY = 2,
    WCA_TRANSITIONS_FORCEDISABLED = 3,
    WCA_ALLOW_NCPAINT = 4,
    WCA_CAPTION_BUTTON_BOUNDS = 5,
    WCA_NONCLIENT_RTL_LAYOUT = 6,
    WCA_FORCE_ICONIC_REPRESENTATION = 7,
    WCA_EXTENDED_FRAME_BOUNDS = 8,
    WCA_HAS_ICONIC_BITMAP = 9,
    WCA_THEME_ATTRIBUTES = 10,
    WCA_NCRENDERING_EXILED = 11,
    WCA_NCADORNMENTINFO = 12,
    WCA_EXCLUDED_FROM_LIVEPREVIEW = 13,
    WCA_VIDEO_OVERLAY_ACTIVE = 14,
    WCA_FORCE_ACTIVEWINDOW_APPEARANCE = 15,
    WCA_DISALLOW_PEEK = 16,
    WCA_CLOAK = 17,
    WCA_CLOAKED = 18,
    WCA_ACCENT_POLICY = 19,
    WCA_FREEZE_REPRESENTATION = 20,
    WCA_EVER_UNCLOAKED = 21,
    WCA_VISUAL_OWNER = 22,
    WCA_HOLOGRAPHIC = 23,
    WCA_EXCLUDED_FROM_DDA = 24,
    WCA_PASSIVEUPDATEMODE = 25,
    WCA_LAST = 26
}

internal enum AccentState
{
    ACCENT_DISABLED = 0,
    ACCENT_ENABLE_GRADIENT = 1,
    ACCENT_ENABLE_TRANSPARENTGRADIENT = 2,
    ACCENT_ENABLE_BLURBEHIND = 3,
    ACCENT_ENABLE_ACRYLICBLURBEHIND = 4, // RS4 1803.17063
    ACCENT_ENABLE_HOSTBACKDROP = 5, // RS5 1809
    ACCENT_INVALID_STATE = 6
}

[StructLayout(LayoutKind.Sequential)]
internal struct WINCOMPATTRDATA
{
    internal WindowCompositionAttribute attribute;
    internal IntPtr pData;
    internal int cbSize;
}

[StructLayout(LayoutKind.Sequential)]
internal struct ACCENTPOLICY
{
    internal AccentState accentState;
    internal int accentFlags;
    internal uint gradientColor;
    internal int animationId;
}

[DllImport("user32.dll")]
internal static extern int SetWindowCompositionAttribute(IntPtr hwnd, ref WINCOMPATTRDATA pAttrData);

private uint ColorToAbgr(Color color)
{
    return ((uint)color.A << 24) | 
           ((uint)color.B << 16) | ((uint)color.G << 8) | color.R;
}

public int SetWindows10Acrylic(IntPtr hWnd)
{
    var result = 0;

    var accent = new ACCENTPOLICY
    {
        accentState = AccentState.ACCENT_ENABLE_ACRYLICBLURBEHIND,
        accentFlags = 0,
        gradientColor = ColorToAbgr(Color.FromArgb(5, Color.Black)),
        animationId = 0
    };

    var accentSize = Marshal.SizeOf(accent);
    var accentPtr = Marshal.AllocHGlobal(accentSize);

    try
    {
        Marshal.StructureToPtr(accent, accentPtr, false);

        var compData = new WINCOMPATTRDATA
        {
            attribute = WindowCompositionAttribute.WCA_ACCENT_POLICY,
            pData = accentPtr,
            cbSize = accentSize
        };

        result = SetWindowCompositionAttribute(hWnd, ref compData);
    }
    finally
    {
        Marshal.FreeHGlobal(accentPtr);
    }

    return result;
}