Please try to read this honestly I promise I'm not screwing with you or misunderstanding.
Outside of this conversation mixed DPI for about 20 years has meant having more than one output of substantially differing DPI most commonly 4K + 1080p.
X11
Under X11 the idea of low DPI applications doesn't exist because broadly speaking every toolkit under the sun supporting integer scaling to deal with this common case about 15 years ago. Individual apps further cannot not support viewport scaling either because it happens at a layer out of scope of the application.
To test I did a search for gtk2 and loaded up fsv a gtk2 port of file system visualizer copyright 1999. It and the QT6 apps released last month I have all appear correctly scaled across high and low and high dpi monitors. The bugaboo of "low dpi apps" just flat out doesn't exist. Read that again if it doesn't make sense.
Wayland
Now leaving the Kindom of X the specter of low DPI apps appears. Suddenly instead of a unified identically scaled virtual space we have little isolated X kingdoms sprinkled inside of wayland space like pimples on a dogs ass. Because we are no longer playing at scaling in them down outside the apps purview the app must now explicitly support scaling and behold the only low DPI apps on the Linux ecosystem hath appeared!
ONLY WAYLAND HAS LOW DPI APPS
Worse its users mostly gnome users smugly assume that X users suffer from problems only they have but worse.
Remember GTK2 app from 1999 on 4K monitor, QT6 app from 2024 on 1080p monitor no manual adjustment perfect scaling. Put them side by side on the 4K perfect scaling. No farking about with anything ever.
I said Mixed DPI. You don't have a low DPI program running next to a high DPI program on your desktop right now, do you?
See, that's why I said you have been feigning ignorance.
And I don't blame you, ignorance is bliss! :D