By the way, a recent posting to the mailing list from Mark Waddingham says that hit testing is done "front to back" (which is what I'd expect) and implies top layers are tested first, followed by lower layers. Klaus' test seems to show otherwise, so there may be a difference between hit testing (clicks) and script references. That doesn't sound right though.
Rendering requires a linear back to front scan of each control, painting each one which is within the visible rect of the window the card sits in.
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Hit testing is similar, although the process is from front to back, and stops as soon as a control says it is the target of the mouse / user interaction event.