xfce4-settings-manager-->Window Manager Tweaks-->Compositor -->"show shadows under dock windows" is enabled by default, it is totally OK, but if I install another dock like cairo-dock (other docks are not tested, I normally only keep a very tiny hidden xfce panel just for save), it will cause shadows at the top and the bottom of the screen, at first, I thought this is the problem of cairo-dock theme, because they only exist in the theme called default-panel or default-single, using another theme or kill the cairo-dock will totally be fine, so I post a bug at the [cairo-dock bug site](https://bugs.launchpad.net/cairo-dock-core/+bug/1430289), but the developer there solved this by pointing it out that this may be caused by "show shadows under dock windows", and it turns out yes. (You can jump into that site to check more info and screenshot.)

And this option seems to be enabled by default, so that developer suggests that to add a whitelist (add shadows only for XFCE-Panel) or blacklist (do not add shadows for Cairo-Dock).

FYI:
cairo-dock v3.4.0
Fedora 21 32-bit (or Ubuntu 14.04 32-bit)
Xfce v4.12