This is a KWin script that uses the new built-in tiling support for the upcoming 5.27 release (it won't work with prior Plasma versions)
More information about the new built-in tiles: https://notmart.org/blog/2022/10/kwin-and-tiling

This script is targeted mainly at large ultra wide monitors, where some gamescan look a bit strange when made fullscreen and so wide. The idea is to take a portion of the ultra wide monitor with more "normal" proportions and treat it as it was an independent screen. The script will arrange a selected subset of applications when they go fullscreen in this subset of the screen, defined by a KWin tile (it automatically takes the largest tile defined by the user) panels are covered as well.

The effect can be configured with a whitelist of applications that will do it, as the main target is games, by default only gamescope is allowed. Many games won't work correctly when they are set fullscreen but their area doesn't actually cover the entirety of the screen, so to be ran directly they have to be tried one by one. When launched inside Gamescope, it will guarantee that the game renders and handles input correctly under any pseudo screen geometry.

More info about Gamescope
https://github.com/Plagman/gamescope