Check if the cursor is over a specified Window.

CursorHelper.cs

using System.Runtime.InteropServices;
using System.Windows;
using System.Windows.Interop;

public static class CursorHelper
{
    [DllImport("User32.dll", SetLastError = true)]
    [return: MarshalAs(UnmanagedType.Bool)]
    private static extern bool GetCursorPos(out POINT lpPoint);

    [DllImport("User32.dll")]
    private static extern nint WindowFromPoint(POINT p);

    [StructLayout(LayoutKind.Sequential)]
    internal struct POINT
    {
        public int x;
        public int y;
    }

    public static bool IsMouseOverWindow(Window window)
    {
        if (GetCursorPos(out POINT p))
        {
            nint h1 = WindowFromPoint(p);
            nint h2 = new WindowInteropHelper(window).EnsureHandle();
            return (h1 == h2);
        }
        return false;
    }
}