After a long overdue update last week (since Novemberish), the Gnome
panels no longer appear when my laptop is connected to an external
monitor and the resolution is higher than the native laptop display
resolution. Gnome-panel is running but does not seem to be doing
anything. Killing it has no effect. Occasionally it will show up for the
first few minutes after the resolution is changed or turning off the
laptop's display in Monitor Preferences, but it will then disappear.