Question

Is it possible somehow to get the handle of the window below the current active window retrieved by GetForegroundWindow, z-order wise? In other words, the window that is beneath the current one, no matter its size and position.

Answer 1

You may try this pinvoke calls:

[DllImport("User32")] extern IntPtr GetTopWindow(IntPtr hWnd); 
[DllImport("User32")] extern IntPtr GetNextWindow(IntPtr hWnd, uint wCmd); 

and use this as parameter

uint GW_HWNDNEXT = 2; 

So first get the top window (or yours). After that call GetNextWindow and for the result handle again and again,… so you will get all windows