Sorry to bother you, but I read this comment (https://news.ycombinator.com/item?id=31101384) and I was wondering myself: how did XP managed to have the transparent rounded corners? AFAIK, it doesn't use any compositing, yet the system is able to tell what should be "behind" the window to whatever that was in the transparent regions of the corners. The same thing applies (I guess) to the shadows of the right-click menus.
The transparency is a 1-pixel mask bitmap. There's no partial blending required. Shadows of right click menus are different -- Windows actually had a secret, weird compositor ever since Windows 2000 called "layered windows", but the app has to be rewritten to use them.
