Detect overlapping window

Hi all,
Im wondering if anyone can help me..

Ive been trying to modify the window.cpp code so that when a window is moved it can detect whether there is another window under it. I have tried looking into the window dragging code in window.cpp to try and capture when a window has moved on top of another one. I have also tried working with a list of windows that i retrived from Desktop.cpp but when i use that list in the window.cpp file the virtual machine gives me an error. Am I looking at the right areas or is there another section of code i should be looking at?

any help would be appreciated..

thanks