Created attachment 5983 xfwm4 bug Screenshot xfwm4 version 4.11.3 caused massive strips running over the whole display whenever docky or plank is started, unchecking "Show shadows under dock windows" in Settings Manager >> Window Manager Tweaks >> Compositor, or downgrading back to xfwm 4.11.2 make the strip disappear This strange xfwm4 behaviour was noticed on Xubuntu 14.04 and reproduced by a moderator of the xfce forum
This is perfectly normal and expected. What you see is the shadow. These windows are ARGB windows, means that they manage their transparency areas by themselves. But their window itself is square and xfwm4 cannot tell (and does not care about) their actual shape, so it draws the shadows out of the actual window boundaries, which unfortunately is way off the actual size of what is drawn inside the app, but xfwm4 cannot know this. It simple means that drawing shadows under dock windows is *not* compatible with those dock windows, just disable shadow under docks, that's the only solution.