Comment 5 for bug 1397095

Only external panel plugins placed left of the window buttons and close to the left screen seem to be affected by this glitch. Hovering over these panel items with the cursor restores the panel appearance.

xfce4-panel 4.10.0git-d296ca7 (Xfce 4.10)
Xubuntu 14.04 LTS Alpha 2

Related bug report (xfce4-whiskermenu-plugin):
https://github.com/gottcode/xfce4-whiskermenu-plugin/issues/37