I happen to have a non-hidpi screen. When switching to 2X in the KScreen KCM, the screen resolution goes crazy because there is no support. Now your screen elements are bigger than the physical space and you can't interact with the elements in the same way. 2 suggestions to mitigate this problem: Below the 1x, 2x menu add a warning: "Your screen does not support HDPI. It may not look right if you change this setting" If the user proceeds and you didn't realize it was going to look bad: Create a popup with a "Revert" button to go back to the original resolution with an Enter or Esc
*** Bug 420536 has been marked as a duplicate of this bug. ***
There is a reversion timer feature now at least.
...aaaand that's going to be the right way to support recovering from manually setting the wrong display scaling. https://invent.kde.org/plasma/kscreen/-/merge_requests/152 will also help by automatically setting a better scale out of the box, reducing the need for manually setting a scale.