How can I check if a custom window was created on the desktop or not?

Interesting. Appearantly there isn't a property under $window for it. When you create the window, set the window name into a variable ...

You could check whether $window(windowname).y and $window(windowname).dy are the same and that $window(windowname).x and $window(windowname).dx are the same. If they are then it's a desktop window.

$window(name).mdi -returns: $true for normal; $false for desktop windows

Thx

*curious*

What does mdi stand for?

You're welcome

Multiple Document Interface

To try and explain better,
If a program has a main window, and other child windows, it is MDI.
e.g. mIRC is MDI, Notepad isn't
The use of .mdi here is a little wrong (I think) but means "is the child window a <normal> child" (confined within mIRC's main window) or not.
Hope it helps a little, I'm not good at explaining.