I have a Dell XPS 15 7590 laptop with a 4K display. The desktop environment is rendered through an Intel GPU (nVidia GPU is inactive). When I connect it to a FullHD external monitor through HDMI (though it seems to think it's DisplayPort) and set the dual monitor setup to join displays, the result looks unexpected.
There is only one background. The rest is filled with whatever leftovers what was in those pixels before with a looping short animation that can cause nausea. It's somewhat similar to the windows of frozen programs on early Windows. Here is an example how it can look. Changing background to cause it to reload makes it even worse.
Any idea how may I fix the problem? I could not find anything on google and I'd prefer to keep using Gnome Shell.
EDIT: I have found a somewhat working solution. If I set the background image to expanding (via command line, there is no such option in the menu), plug the monitor and restart gnome-shell, the background scales to fill both screens. However, the image is larger than it should and it works only once. When I disconnect it and connect it again, it sets HiDPI scaling (which I don't want) and somehow restricts the external display's possible resolutions, removing FullHD from the list. I could add the actual resolution using xrandr, but it wasn't able to set FullHD neither in the GUI menu nor through xrandr. Restarting the computer helped, but it happened again when I disconnected it and connected it back.
EDIT #2: Seems related to be related to this unsolved issue: Ubuntu 19.10 laptop with external monitor - display issues (worked fine with 19.04)
EDIT #3: this happens also on Wayland, Gnome Classic or Budgie