Comment by duskwuff

The QuickDraw source code refers to the contents of the Region structure as an "unpacked array of sorted inversion points". It's a little short on details, but you can sort of get a sense of how it works by looking at the implementation of PtInRgn(Point, RegionHandle):

https://github.com/historicalsource/supermario/blob/9dd3c4be...

As far as I can tell, it's a bounding box (in typical L/T/R/B format), followed by a sequence of the X/Y coordinates of every "corner" inside the region. It's fairly compact for most region shapes which arise from overlapping rectangular windows, and very fast to perform hit tests on.