Get the direction of avilable space in the monitor where a specified window locates.

WindowHelper.cs

    using System;
    using System.Runtime.InteropServices;

    public enum Direction { None, TopLeft, TopRight, BottomRight, BottomLeft }

    public static class WindowHelper
    {
        public static Direction GetAvailableDirection(IntPtr windowHandle)
        {
            if (!GetWindowRect(windowHandle, out RECT buffer))
                return Direction.None;

            System.Drawing.Rectangle windowRect = buffer;

            IntPtr monitorHandle = MonitorFromWindow(windowHandle, MONITOR_DEFAULTTO.MONITOR_DEFAULTTONULL);
            if (monitorHandle == IntPtr.Zero)
                return Direction.None;

            MONITORINFO info = new() { cbSize = (uint)Marshal.SizeOf<MONITORINFO>() };
            if (!GetMonitorInfo(monitorHandle, ref info))
                return Direction.None;

            System.Drawing.Rectangle workingAreaRect = info.rcWork;

            bool isWindowAlignedTop = (windowRect.Top - workingAreaRect.Top) < (workingAreaRect.Bottom - windowRect.Bottom);
            bool isWindowAlignedLeft = (windowRect.Left - workingAreaRect.Left) < (workingAreaRect.Right - windowRect.Right);

            return (isWindowAlignedTop, isWindowAlignedLeft) switch
            {
                (true, true) => Direction.BottomRight,
                (true, false) => Direction.BottomLeft,
                (false, true) => Direction.TopRight,
                (false, false) => Direction.TopLeft
            };
        }

        [DllImport("User32.dll", SetLastError = true)]
        [return: MarshalAs(UnmanagedType.Bool)]
        private static extern bool GetWindowRect(
            IntPtr hWnd,
            out RECT lpRect);

        [DllImport("User32.dll")]
        private static extern IntPtr MonitorFromWindow(
            IntPtr hwnd,
            MONITOR_DEFAULTTO dwFlags);

        private enum MONITOR_DEFAULTTO : uint
        {
            MONITOR_DEFAULTTONULL = 0x00000000,
            MONITOR_DEFAULTTOPRIMARY = 0x00000001,
            MONITOR_DEFAULTTONEAREST = 0x00000002,
        }

        [DllImport("User32.dll", CharSet = CharSet.Unicode)]
        [return: MarshalAs(UnmanagedType.Bool)]
        private static extern bool GetMonitorInfo(
            IntPtr hMonitor,
            ref MONITORINFO lpmi);

        [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
        private struct MONITORINFO
        {
            public uint cbSize;
            public RECT rcMonitor;
            public RECT rcWork;
            public uint dwFlags;
        }

        [StructLayout(LayoutKind.Sequential)]
        private struct RECT
        {
            public int left;
            public int top;
            public int right;
            public int bottom;

            public static implicit operator System.Drawing.Rectangle(RECT rect)
            {
                return new System.Drawing.Rectangle(
                    rect.left,
                    rect.top,
                    rect.right - rect.left,
                    rect.bottom - rect.top);
            }
        }
    }
}