What makes this click through?

Question

I'm trying to figure out how this code works but I just can not figure out what makes it click through.

Yes, this code is not mine since i'm trying to learn/understand it.

Assume I want the tranparancy but not the click through what needs to be changed and why?

I have been over the Windows styles pages over and over and still can not get my head around the click through part.

using System;
using System.Runtime.InteropServices;
using UnityEngine;

public class TransparentWindow : MonoBehaviour
{
    [SerializeField]
    private Material m_Material;

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

    [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 bool ShowWindowAsync(IntPtr hWnd, int nCmdShow);

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

    [DllImport("user32.dll", EntryPoint = "SetWindowPos")]
    private static extern int SetWindowPos(IntPtr hwnd, int hwndInsertAfter, int x, int y, int cx, int cy, int uFlags);

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

    const int GWL_STYLE = -16;
    const uint WS_POPUP = 0x80000000;
    const uint WS_VISIBLE = 0x10000000;
    const int HWND_TOPMOST = -1;

    void Start()
    {
        #if !UNITY_EDITOR // You really don't want to enable this in the editor..

        int fWidth = Screen.width;
        int fHeight = Screen.height;
        var margins = new MARGINS() { cxLeftWidth = -1 };
        var hwnd = GetActiveWindow();

        SetWindowLong(hwnd, GWL_STYLE, WS_POPUP | WS_VISIBLE);

        // Transparent windows with click through
        SetWindowLong(hwnd, -20, 524288 | 32);//GWL_EXSTYLE=-20; WS_EX_LAYERED=524288=&h80000, WS_EX_TRANSPARENT=32=0x00000020L        
        SetLayeredWindowAttributes(hwnd, 0, 255, 2);// Transparency=51=20%, LWA_ALPHA=2
        SetWindowPos(hwnd, HWND_TOPMOST, 0, 0, fWidth, fHeight, 32 | 64); //SWP_FRAMECHANGED = 0x0020 (32); //SWP_SHOWWINDOW = 0x0040 (64)
        DwmExtendFrameIntoClientArea(hwnd, ref margins);

        #endif
    }

    void OnRenderImage(RenderTexture from, RenderTexture to)
    {
        Graphics.Blit(from, to, m_Material);
    }
}

Answer 1

This function:

SetWindowLong(hwnd, -20, 524288 | 32);

does the trick. Windows implements the rule that Mircosoft made which is that a window that is transparent to the user must be transparent to the mouse. With the transparency bit set WS_EX_TRANSPARENT the window becomes transparent to mouse too and click passes to the painted layer behind the transparent window.

You need not understand but make use of this 'OS feature' which was probably implemented to cover for something else.

Read this article about the subject and this answer that explains the parameters