Using gnome-panel with metacity, the bug DOES NOT occur.
Using xfce4-panel with metacity, the bug DOES NOT occur.
Using gnome-panel with xfwm4, the bug DOES occur.
Using xfce4-panel with xfwm4, the bug DOES occur.
So, that would seem to point to the window manager rather than the panel.
More data:
Using gnome-panel with metacity, the bug DOES NOT occur.
Using xfce4-panel with metacity, the bug DOES NOT occur.
Using gnome-panel with xfwm4, the bug DOES occur.
Using xfce4-panel with xfwm4, the bug DOES occur.
So, that would seem to point to the window manager rather than the panel.