GNOME menu bar moved suddenly

[Tommy Trussell]

I'm running Dapper on this system, with all the current updates.

I was inspecting some pdf files (that I had created from scans) and
while I was scrolling in evince I noticed the desktop changed -- it
turns out the menu bar (the one with the applications menu and the
update icon, etc.) suddenly jumped from the top to the bottom of the
screen.

A few weeks ago I had all the menus disappear from the menu bar so I
just logged out, went to a command line ad wiped out one of the .gnome
directories (can't remember now which one) and logged in again to get
the defaults back.

What might I be doing to make strange things happen in GNOME? Could my
cheapo mouse be causing this? (I only ask because just before the menu
bar moved I was twirling the scroll wheel and pressing various mouse
buttons over the pdf document to explore any scroll/zoom options I
might find in evince.)