Comment 6 for bug 1430289

I believed this option is enabled by default.

I use Fedora + xfce for years by now, and didn't touch that part of configuration for long time, so I'm not sure if that is enabled by default, well in Fedora.
But I installed completely new xfce desktop environment in Ubuntu two days ago, and remember clearly that I didn't configure that part, then I install cairo-dock and enable the default-panel theme, and the problem came out.