http://bugs.winehq.org/show_bug.cgi?id=23841
--- Comment #4 from Vincent Povirk madewokherd@gmail.com 2010-08-24 13:42:49 --- This is probably a window manager bug. Maybe I'll write a simple testcase later.
The X shape extension is used to create non-rectangular windows. Windows has a similar feature. In both cases, the program supplies a region for the window's shape. Both drawing and interaction are restricted to the intersection of that region and the window's "natural" region (the rectangle it would normally occupy).
I happen to know that the new Steam GUI sets a very large region for its windows, basically (0,0)-(INT_MAX,INT_MAX), which should be the same as not setting a custom shape at all. This triggered a bug in Wine that made the region 0 size, which is why the Steam windows used to be invisible. Wine now correctly sets that large region.
My guess is that these compositing window managers are not intersecting the X shape region with the window's rectangle when they draw it.