Unify outputs does not work Pressing the buttons in the system setting kscreen window causes the two screens to be shown as overlapped, yet when one presses apply, the screens remain configured to be non-overlapping, as testified by querying xrandr on the command line. This is not an issue of the hardware or the graphics drivers. Using xrandr to force one display to be the `--same-as` the other effectively unifies the outputs.
Could you please test this with 5.11? We have fixed a large number of bugs that could cause this behavior since Plasma 5.5.
Sorry, I have just realized that I indicated the wrong version. I am currently on 5.10.5. Will test with 5.11 as soon as it is shipped in the dedicated PPA for the up to date plasma for kubuntu. I cannot be 100% sure, but I would say that what I am observing is a relatively recent regression, though.
https://phabricator.kde.org/D8391 and https://phabricator.kde.org/D8390 should fix this.
Both diffs from comment #3 are committed, please reopen or add a comment if these do not fix the issue.