Created attachment 2319 
Mixer plugin icon on panel when hovered over

I use Xfce 4.6.1 in Arch Linux. There is no volume slider visible next to the icon of the mixer plugin. I attached a screenshot which shows that when I hover over the icon of the mixer plugin on the panel, the right demarkation is missing, indicating that I don't get to see everything that is supposed to be displayed here. I suppose there should be a volume slider there. When I hover over other icons on the panel, I get to see a border on the right, too.