Recent versions of the gnome-panel have introduced a catastrophic performance
bug that arrises when the panel is hidden and used in combination with icewm.
I have to use a hidden gnome-panel with icewm, because it seems to be the only
way to talk to network-manager.

Anyway, if I have a maximised window in icewm, and the cursor strays over the
gnome-panel, it causes the window to re-render many times at different sizes
before returning to the initial state.  If the window is something complicated
like a browser, the process can take about 30 seconds, and locks icewm while
it is occurring.