The bug has been opened on GNOME Panel menus close unexpectedly on specific mouse clicks

"Binary package hint: gnome-panel

When traversing down a Panel menu, the mouse selects the item under the mouse. When traversing up the menu, there is a sort of dead area in which the item under the mouse is NOT selected. For launchers, this is merely a cosmetic error, of one pixel: for folders on the menu, the gap has an additional pixel if the mouse is moved perfectly vertical: if the mouse is quickly clicked on that second, higher, pixel, the menu will disappear! If the mouse remains on the second pixel for a few moments without clicking, or if it is moved horizontally, the selection seems to catch up to the mouse, and there is no problem if you then click.

I found this error in regular use of the menus, however, so it's certainly not arcane! It was rather disconcerting to have my menus close on me for no apparent reason!
..."