In full-screen games such as doom legacy (http://doomlegacy.sourceforge.net/), when setting a resolution other than the one set in KDE Display and Monitor settings, the resolution is not restored after closing the game, e.g. if I set my desktop resolution to be 1920x1080, run a game in 1024x768 and then close it, the resolution becomes set to 1024x768 – and, for example, desktop widgets and icons are rearranged automatically to fit this resolution, which is very inconvenient. The bug doesn’t happen in other desktop environments.
Are you able to reproduce using a more recent version of KDE Plasma than 5.8.6? You're using a Long-Term Support version of Plasma, but it's possible or even likely that this is already fixed in a newer version.
Seems it's fixed in later versions in the sense that the screen resolution is restored. However, desktop widgets still get rearranged (i.e. after closing a fullscreen game that had a lower resolution, they get placed somewhere else in the desktop rather than returning to where they were before).
Thanks for the info!
*** This bug has been marked as a duplicate of bug 360478 ***
Small update: in plasma 5.12.0 the screen resolution is again not restored the first time a fullscreen game changes it. From the second time onwards it is restored tough.