I'm trying to make a jump thru behavior with medium success: Sometimes the platforms work, other times, they just won't.
First I thought it was because I was using a tiled background instead of a sprite, and when I changed a platform to sprite it worked, but now I copy-pasted it and it's not working!
I made the platforms visible: dl.dropbox.com/u/41192999/problem.capx
I emulated the behavior using solids and checking for the Y of the player :3
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Hmm, looks like a bug - moving to bugs forum.
This should be fixed in r79.4 now, thanks for the report.