[NWoodsman]

source?

(I only ask because you sound knowledgeable and your answer implies some hefty interesting legacy)

A better way to put it is OP is a "reverse engineer" whereas you sound like a "forward engineer"

[BearOso]

For the first part, look up Stacking Window Manager on Wikipedia.

As for Windows XP's transparent corners, I used to make msstyle themes back in that era, so I happened to know how the theme engine works.