Bug 459185

SUMMARY
There are a couple of different problems here. I have really only noticed this with games but any application with resolution options like games have will suffer similar problems.

- X11 Games will always show the maximum resolution of any display regardless of which monitor is chosen for the game. The game will know which monitor it is opened on, but it will always list the "maximum" resolution out of all of my monitors, even if that specific monitor doesn't have the resolution (see second point for what I mean by "maximum"). e.g. on my 1080p displays it still shows running at 1440p to be available. The game knows it is on a 1080p display as it shows a different selected monitor, but it will still let me run the game at 1440p.
- X11 Games on a scaled display list a higher resolution than they did on Plasma 5.25, but they still do not list the native resolution as the maximum. On my 4k display with 150% scaling, previously 1440p was the maximum shown resolution. However now oddly the maximum is "2880x1620". I have verified that the display should in fact be 3840x2160, Display settings even list this.

I group these two issues together as they are both related to X11 application resolutions on multi-monitor mixed-DPI displays. I hope this is alright. 

STEPS TO REPRODUCE
1. Open a game or any application that lets you view/select resolutions and monitors to run the application on. A game is probably the most straightforward.
2. Flick through the game's resolution options, it will show higher than what is currently available on the current display
3. Game will also not show the native resolution of the current display, though it will likely get *fairly* close. In my case with a 4k@150% scaling display, it shows 2880x1620.

OBSERVED RESULT
Games show higher resolution than what is available on the display it is currently on. It shows the maximum seen resolution out of all available displays.

Games also do not show a display's native resolution, though it gets closer than 5.25 did which simply capped the resolution at the display's scaled resolution.


EXPECTED RESULT
Games should only show the maximum resolution available on the current display it is on. It should also show the true native resolution of the current display that it is on, such as allowing a game to run at native 4k on a scaled 4k display.


SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: 

ADDITIONAL INFORMATION
Some tools such as Gamescope could get around the issues posed, but Gamescope has a few limitations including a significant performance overhead when you have several displays. It also isn't perfect in other respects.

There may be other ways to set/force an application's resolution, but Plasma should be able to take care of this automatically for the best user experience.