Resolvido! Eu adicionei
'style "xfce-panel-window-style"
{
# Time in miliseconds before the panel will unhide on an enter event
XfcePanelWindow::popup-delay = 225
# Time in miliseconds before the panel will hide on a leave event
XfcePanelWindow::popdown-delay = 350
}
class "XfcePanelWindow" style "xfce-panel-window-style"
para o ~ / .gtkrc-2.0. Isso afeta todos os painéis (não posso segmentar apenas um), mas ... ok.