KDE 4.3.1
Debian unstable
panel autohide or not
compositing *off*, whether with the "Suspend Compositing" button, or by unchecking "Enable desktop effects".

I'm talking about the tooltips/boxes that open up over the panel when the mouse points to some application icon on the panel or in the task selector on the panel.

If I slide the mouse pointer from side to side over the panel, the popups open and close correctly, following the mouse.
But, if I:
1 open a window (other than iceweasel) covering the area of the desktop above the panel (not covering it)
2 move the mouse pointer to an icon on the panel until the popup opens over that window
3 leave the panel *upward*, onto the desktop until the popup closes
4 move the mouse pointer over the desktop sideways
5 move the mouse *downward* back onto some *other* icon on the panel

Then:
The new popup appears, and also a hole through to the desktop appears where that *previous* popup was in step 2. This happens every time.

This is a bug in Plasma. Please report it at bugs.kde.org with complete steps to reproduce, if it hasn't been reported at all. Thanks in advance.