TransparentWindow.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using System;
using System.Runtime.InteropServices;
public class TransparentWindow : MonoBehaviour
{
    [DllImport("user32.dll")]
    public static extern int MessageBox(IntPtr hWnd, string text, string caption, uint type);

    [DllImport("user32.dll")]
    private static extern IntPtr GetActiveWindow();

    [DllImport("user32.dll")]
    private static extern int SetWindowLong(IntPtr hWnd, int nIndex, uint dwNewLong);

    [DllImport("user32.dll")]
    static extern int SetLayeredWindowAttributes(IntPtr hwnd, uint crKey, byte bAlpha, uint dwFlags);

    [DllImport("user32.dll")]
    static extern bool SetWindowPos(IntPtr hWnd, IntPtr hWndInsertAfter, int X, int Y, int cx, int cy, uint uFlags);

    private struct MARGINS
    {
        public int cxLeftWidth;
        public int cdRightWidth;
        public int cyTopHeight;
        public int cyBottomHeight;
    }

    [DllImport("Dwmapi.dll")]
    private static extern uint DwmExtendFrameIntoClientArea(IntPtr hWnd, ref MARGINS margins);

    const int GWL_EXSTYLE = -20;
    const uint WS_EX_LAYERED = 0x00080000;
    const uint WS_EX_TRANSPARENT = 0x00000020;

    static readonly IntPtr HWND_TOPMOST = new IntPtr(-1);

    const uint LWA_COLORKEY = 0x00000001;

    public void Start()
#if !UNITY_EDITOR_
    { 
        //MessageBox(new IntPtr(0), "Hello World!", "Hello Dialog", 0);
        IntPtr hWnd = GetActiveWindow();

        MARGINS margins = new MARGINS { cxLeftWidth = -1};
        DwmExtendFrameIntoClientArea(hWnd, ref margins);

        //unlock the window to interact with the os
        SetWindowLong(hWnd, GWL_EXSTYLE, WS_EX_LAYERED);//| WS_EX_TRANSPARENT);
        SetLayeredWindowAttributes(hWnd, 0, 0, LWA_COLORKEY);

        //hWnd window handle
        SetWindowPos(hWnd, HWND_TOPMOST, 0, 0, 0, 0, 0);
#endif

        Application.runInBackground = true;
    }


}