I'm literally sitting at a station with 3 monitors 2 high DPI and 1 low DPI wherein UI elements of a window moved from one monitor to the other are identically sized and this was the case when all 3 monitors were different sizes and DPI as well.
In this case the UI is scaled up by an integer factor on all screens so as to look nice on the highest DPI screen. It is scaled down from the higher resolution by a decimal factor that could be but needn't be an integer. If the factor is proportional to the difference in DPI the result is UI elements being sized precisely the same size across different size and dpi monitors.
All monitors share a singular scaling factor and DPI. Apps thus need to support high DPI but needn't do anything smart to support scaling because it happens outside of the apps remit.
This can be achieved again with xrandr --scale OR in the nvidia-settings GUI by setting viewport in to a higher resolution than viewport out. No the result isn't blurry.
First thing first, the X11 protocol is not aware of DPI. Period. And that has implications. You can apply all sorts of hackish solutions on top of it but (1) the solutions will most likely be out-of-band (not pure X11) and involve DBus/envvars/Xft.dpi, and (2) per-output.
Out-of-band solutions are effectively bandaids and they unnecessarily increase development difficulty of GUI programs on Linux, since developers now have to be aware of the various side channels (fragmentation!) that communicate the DPI scale.
This is why SDL2 for the longest period did not support HiDPI on X11, but does on Wayland, MacOS, and Windows. A COSMIC dev just recently made a complaint about XSettings too! [0] You can't just ignore those problems, "Linux is hard to develop for, blah blah fragmentation blah blah" I am sure you have heard of.
Another thing. Per-output HiDPI is fine when all your programs support high DPI, but it's unworkable if you want to mix LoDPI and HiDPI applications in a single screen, i.e. if an application has better user experience if it is upscaled and blurry (!), you are SOL unless you want to apply scaling to your entire desktop.
You also lose the opportunity to implement some neat features like temporarily scaling up a window if it is being magnified or screenshot. (The idea's been floating about in the KDE community)
Finally, we can argue for days, but the HiDPI page on Arch wiki already says a lot when a good 90% of the article is about acheiving good DPI scaling on X11 [1]. Even the Wayland sections have an XWayland subsection in them...
Scaling is a built in feature of X for over 20 years intended for this use case no bandaids of any sort or awareness by the applications of differing DPI.
From the perspective of apps there is only one DPI with all scaling down handled by X at a layer below the apps level. There really aren't any lodpi apps on a modern X system only Wayland has issues scaling X apps correctly. Gtk Apps not updated this decade can handle scaling up by integer factors and because X scales the lower dpi monitor down to the appropriate size the app doesn't need to handle anything else.
Its very odd for folks to argue that my mixed DPI system using only basic boring old xorg.conf somehow doesn't exist. I mean would you like to come to my apartment and open a few different apps?
Its in a way ironic that between pure X, pure Wayland, and Wayland + xwayland only the last can't handle mixed DPI.
> There really aren't any lodpi apps on a modern X system only Wayland has issues scaling X apps correctly.
The problem starts at XWayland, not Wayland. And XWayland is just your typical X server with all its X intricacies.
Only if we can somehow communicate our intended scale to the X clients through XWayland... oh, we can't!
> Its very odd for folks to argue that my mixed DPI system using only basic boring old xorg.conf somehow doesn't exist.
You keep on confusing per-window and per-output DPI. I can only assume you are feigning ignorance at this point. Good luck with attitude.
I still have a couple GTK2 apps installed that doesn't support HiDPI, those are best used upscaled but... they can't. Those apps are unable to communicate to the server they are rendered at 1x (rather, the server doesn't have the notion of per-window scales at all).
The desirable quantity is the result not the mechanics specifically that one can simply open windows on any display and move them between monitors without worrying about scaling. Ideally an element on one ought to be the same number of mm should you pull out a ruler.
This is easily achievable on X11 and its easily done with Wayland.
You mentioned not being able to do scaling per window. Well only with xwayland do you ever need to do so. You need it to be smarter than it is and so scaling on xwayland sucks in a way that isn't a factor in X or Wayland.
Since Wayland is dependant on xwayland for unsupported apps and mixed DPI + xwayland sucks effectively only Wayland sucks at mixed DPI.
> The desirable quantity is the result not the mechanics specifically that one can simply open windows on any display and move them between monitors without worrying about scaling.
Then next time don't bring up compatibility when you talk about the benefits of staying on X11. :) You are breaking the legacy programs anyway by forcing LoDPI programs to remain un-upscaled and therefore unusable on HiDPI displays.
> Well only with xwayland do you ever need to do so.
I am sorry, but the problem I mentioned exists as well on a pure X11 setup.
And the "solution" for this problem, on this setup, is to scale up the entire desktop (blurriness everywhere) which is decidedly worse than XWayland where only a single window is affected, assuming your other HiDPI-aware programs run natively on Wayland (which, from my experience, is pretty much true -- after all, HiDPI programs most likely are still actively developed).
The only other viable choice on a pure-X11 setup is a non-solution: just live with the tiny UI whenever you are using HiDPI-unaware programs.
Either you bring up documentation showing people that X11 has something akin to Wayland's buffer_scale event, or consider this discussion finished. It is tiring to talk to a brick wall.
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.
