Cannot place windows in center of external display

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?

Sounds like a case of Window Snapping to me.

System, Preferences, Look and Feel, Mate Tweak, the Windows tab, see if enable Window snapping is checked under Window Behavior.

I have already disabled window snapping.

The external display is very wide (3840 x 2160). The problem occurs even if the window is not close to the display edges.

The only other idea that I can come up with involves the relative placement of your screens, one above the other. If you place them side to side in the default position, does this window dragging behavior persist?

You may have hit upon a bug…

Yes, it may be a bug. With the default placement, before changing the display arrangement, the problem did not occur.

Furthermore, in other distros (Antergos, OpenSuse, Kaos, Kubuntu) running on the same hardware with the same display arrangement (one above the other), the problem never occurred.

So it’s a bug in Ubuntu Mate. Perhaps someone else can provide the procedure to report this bug. I’ve forgotten. Would someone please help DCB on this matter?

I know that bugs can be reported here:

https://bugs.launchpad.net/ubuntu-mate

However, I am not sure about any procedure.

Maybe submitting a short video alongside with your monitor specifications and graphics card details would help further?

Thanks. I will file a bug and post a video.

Graphics specs:
Card: Intel HD Graphics 515
External monitor: Dell P2715Q
Interface for external monitor: Plugable USB-C Triple Display Docking Station

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