You're probably aware of the root issue, but let me do a recap: The default font size in plasma is 10pt, which theoreticaly translates to roughly 3.5mm on screen, regardless of the physical size of the screen. This is obviously not something anyone want for a sane default font size. 10pt looks good on most X11 machines because of the bug in xorg that fails to properly detect the physical size of a screen and returns a dpi of 96 as a result, does exist on most of X11 machines (apparently, considering how the bug was recently fixed and then immediately unfixed because people were complaing). On those machines, if they have a ppi of 144 like most screens nowadays, 10pt is 2.4mm, much better. The problem I want to report is about the "force font DPI" option in the systems settings. Its not enabled by default, and the tooltip says to enable it if "the real DPI of the hardware is not detected properly". If you're going with a default 10pt font size, you should force the 96dpi by default too, and say "uncheck this if your xorg is properly configured and adjust your font size". Or choose a saner default font size like 7 or 8, and let "force font DPI" unckecked and the tooltip text as it is. But then I assume lots of people would be complaining their fonts are too big (they don't want to be bothered configuring their xorg properly if it were "working" for them before...) I don't want to tell you what to do or what to choose, I just want to point out the inconsistency of what plasma actually does and the "force font DPI" checkbox+tooltip.
Xorg defaulting to 96 DPI isn't it being broken; this has been its standard behavior since forever. And it's not just KDE code that relies on this; tons of other things do too. Everything that uses Qt, for example. The only reason to force the font DPI to 96 on the KDE side is as an insurance against the Xorg issue being "fixed" again. I assume they're not going to make that mistake again soon! The time to refine this behavior and set a better default was 20 years ago, not today when Xorg is practically abandonware and everyone's working on Wayland support. However everything actually works better than expected even if they do it again. You can disable Qt scaling and only use DPI-based scaling. Of course then all the icons are the wrong size. But this is a Qt issue that we are simply inheriting.
I understand what you're saying, but then again, what I wanted to report is the fact that the tooltip says: "It may be useful when the real DPI of the harware is not detected properly [...]" Which is not true, in fact, if the real DPI of hardware is not detected properly, xorg defaults to 96 as you said and thus checking this box is not useful. It may be useful if xorg returns anything else than 96 is what it should really say... For many years I've been carefully setting the real dpi in the xorg of all my machines first thing after an install, and I've always wondered why the defaults never worked for me. That little bit of info would probably have helped me understand a little.
Gotcha. Changing the tooltip text to reflect the reality of the situation is reasonable.
A possibly relevant merge request was started @ https://invent.kde.org/plasma/plasma-workspace/-/merge_requests/1217
Git commit 1b379f8ed33db240df1bdd71f5a91a182f828916 by Nate Graham. Committed on 06/12/2021 at 16:24. Pushed by ngraham into branch 'master'. kcms/fonts: make Force Fonts DPI tooltip text more accurate and clearer This UI control currently has an extremely long tooltip that talks about a bunch of technical jargon the user is likely to know nothing about, and also says inaccurate things by mentioning hardware where the DPI is not automatically detected, when in fact Qt software explicitly avoids scaling things by DPI in the first place. FIXED-IN: 5.24 M +1 -1 kcms/fonts/package/contents/ui/main.qml https://invent.kde.org/plasma/plasma-workspace/commit/1b379f8ed33db240df1bdd71f5a91a182f828916