Oh, that's how I did it... I used pixel_for_bmap to get the pixel of the button that the mouse was over, then used pixel_to_vec to see if it was over a transparent part. I copied and pasted that bit into other projects, so it seemed to me like it worked that way by default, LOL!

I'll give you an example, so you can see it. It's easy...

But this is straight out the manual:

*Free-form buttons can be used when the button function retrieves the pixel at the mouse position through pixel_for_bmap(), and uses the pixel color or alpha value to determine whether the button was hit or not.

I'll show you code asap. I'm kinda busy atm... But there's still no need to "chop up" your buttons, or use windows, etc.