This bug appears when the Windows task bar is configured to remain visible at the bottom of the screen. In this case, when we display a panel containing an element that makes it expand to the full screen height, the panel overlaps with the task bar instead of being limited to the usable desktop area. The result is that the lower portion of the panel is hidden under the task bar. I've observed this on Windows XP while testing the new Downloads panel.

While this issue isn't blocking for bug 564934 if we use level="top", we should still understand if, in general, windows that are auto-sized to the screen width or height should be limited by the physical or logical desktop size. From MSDN: "To get the coordinates of the portion of the screen not obscured by the system taskbar or by application desktop toolbars, call the SystemParametersInfo function with the SPI_GETWORKAREA value." This issue is probably related, or a duplicate, of bug 203621 if I understand correctly.

We d(In reply to comment #1) > While this issue isn't blocking for bug 564934 if we use level="top", we > should > still understand if, in general, windows that are auto-sized to the screen > width or height should be limited by the physical or logical desktop size. We do, in fact, already determine whether a menu or panel should be constrained to the full screen size, or to the size excluding system provided bars and menus. The result however, is platform specific. There doesn't seem to be any precedent for restricting panels on Windows to the latter size.

Yay duplicates that allow us to fix other bugs! :-) (found through bug 1002486 and the relevant code and comments)