I am running Ubuntu MATE 17.04 on a Dell DL-7370 laptop connected via USB-C Display Link HDMI (using a Plugable docking station) to an external 4K display. Generally this works fine. I am able to arrange the monitors the way I want using the MATE display settings. I like to have the external display placed above the laptop display (rather than to the right of the laptop display which seems to be the default arrangement). The problem is that windows opened or dragged onto the external display cannot be placed in the center of the display. If I try to drag them into the center they jump suddenly from the left side to right (or vice versa). It is as if there is a vertical band in the center of the display where no windows can be positioned. Outside this zone, windows move very smoothly when dragged. It is almost as if the left and right sides of the external display are being treated as two separate displays with a “dead zone” in the middle. When windows are dragged from one side to the other they jump across this central zone as if there was nothing there.
Any ideas what might be going on here?