Tonight I wrote some little Python application to play around with Cairo (PyCairo that is), and XShape. The result? Something similar to MacSlow's Cairo-Clock, although much more basic.
Just start it in some terminal. Clicking on the clock allows you to move it, clicking it while pressing a shift keys starts resizing mode. When running on top of a terminal/browser/whatever with scrollbars, try to move your pointer close to the clock's circle, inside the area that's normally covered by the clock's window/frame, then use your scrollwheel. As you should notice, the scroll events are not sent to the clock window, but to the underlying window: it's really a circle, not a transparant rectangular window which still catches all events anyway.
Have fun with it!
Edit: there's a little screencast online. Warning: colors look bad, this is caused by the recording software.
pycairo, composite, the whole works, but disguised as a system-monitor/clock.
But I only use xshape when a compositing manager is not available, otherwise I just use a xinput-mask
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