With X, all monitors must share the same pix density, and with Wayland, X apps (running via XWayland) are blurry, like VS Code for instance (scaled up on hidpi displays).
To get proper fractional scaling on X11 with monitors of different pixel densities, you'll want to use either:
1. GNOME Shell with a patched version of Mutter, which is included by default in Ubuntu[1] and its derivatives (such as Pop!_OS and elementary OS). Manjaro also offers the patched version of Mutter as the mutter-x11-scaling package.[2] You'll need to enable the x11-randr-fractional-scaling experimental feature for any of these distros.[3]
2. Cinnamon, included by default in Linux Mint
To get proper fractional scaling on Wayland, you also have two options:
1. GNOME Shell (no patch needed), with the scale-monitor-framebuffer experimental feature enabled[3]
2. KDE Plasma, which is still a little bit buggy on Wayland
[1] https://salsa.debian.org/gnome-team/mutter/-/blob/ubuntu/mas...
[2] https://github.com/puxplaying/mutter-x11-scaling
[3] https://www.omgubuntu.co.uk/2019/06/enable-fractional-scalin...
[edit] Looks like this: https://imgur.com/a/Zcc031e
Relevant excerpt from my install log
> Setting global scale to 200-250% in KDE 5 and upping a few fonts seems to fix virtually all HiDPI-issues, except for SDDM and the console framebuffer. You may also want to resize the start bar size, but that is personal preference.
> For SDDM:
> Put in /etc/sddm.conf.d/kde_settings.conf
[X11]
ServerArguments=-dpi 240
EnableHiDPI=trueI tried x11, I tried wayland, I tried scaling, I tried fractional scaling, I tried changing default font size. I always ended up with text too small, text too blurry or inconsistencies between applications.
For correct text rendering, you must set the screen to its native resolution and then set a correct value for the DPI so that a font of 12 points (or of 10 points, whichever you prefer) should have the right size that you like in a default font.
If you use Gnome or KDE, I do not know where they can set the exact value for the screen DPI, but they must have such a setting, otherwise it is indeed not possible to use properly a HiDPI display.
EDIT: At least KDE has a "Force Font DPI" setting, while the corresponding setting in Gnome seems harder to find.
It appears that the reason while I never had any problems with bad text rendering on HiDPI in Linux is that I never attempted to use the so-called HiDPI support from Gnome and KDE, which is just an extremely ugly and inefficient workaround for GUI applications that have been written incompetently, so they lack the options for changing the sizes of fonts and of other GUI elements.
In my experience, most Linux applications have the required options, so they do not need the "HiDPI support" workaround, which is guaranteed to render badly any text.
It's usually not about x11 or wayland, it's about the window manager support.
Laptop screen: 17 inch 3840x2160, 200% scaling
Dual external screens: 27 inch 3840x2160, 150% scaling
No issues with the above. I only have 4K screens, so I haven't tested this with different resolutions, but at the very least different scaling does work.
Wayland is probably easier for developers to work with - the X codebase by now is ancient. But it will lack some functionality that X has, probably forever.
`xrandr` allows different per-output scaling.
For example if you have a 4k laptop and and 4k monitor that is 50% bigger than the laptop display, X will pretend that the monitor is 5760x3240 and scale down to the actual pixels.
Having said all that, I dislike scaled anything, so I choose a pixel density between the highest and lowest densities of my physical devices, and manually pick the font size of my terminals.
Also integer scale is a happy place - wayland still has no real non-integer scale protocol. (See: https://gitlab.freedesktop.org/wayland/wayland-protocols/-/i...)
xrandr --dpi 192 --output eDP-1 --auto --output HDMI-1 --left-of e-DP1 --auto --scale 2
I don't have my laptop with me, so I am not 100% sure. You can also go the other way, but the result is more blurry.
Otherwise, as said in another post, apply tips from ArchLinux wiki. I have also put my experience here: https://vincent.bernat.ch/en/blog/2018-4k-hidpi-dual-screen-... (notably if you want something a bit dynamic with a laptop).
You can do it all with xrandr IIRC, but not with Arandr. Arandr just helps with positioning if you have say, three monitors of different sizes which don't line up in any typical way.
$ cat .Xresources
Xft.dpi: 144
URxvt.font: xft:Anonymice Powerline:style=Regular:size=18
The only application I use which is not wayland native is IntelliJ IDEA.
Pure HiDPI works okay-ish by modifying density settings (blurriness is AFAIK solved in current Electron builds?); mixed setups are still hopeless in my experience.
On the laptop I am writing from right now I multi-boot a couple of Linux distros and Windows 11. My preferred scaling level is 125% on Windows and it generally works great, but I stick to 100% on Linux because fractional scaling is still imperfect (yes, sadly even on Wayland).
For most other applications, finding a resolution which you can run HIDPI at 2x (fractional scaling didn't look good)
https://wiki.archlinux.org/title/HiDPI
So in case you consider XFCE, also GTK and other frameworks need to be configured. With closed-source Nvidia drivers the configuration options with Xrandr are limited though IIRC.
I use monitors with different sizes and I am not aware of a way to specify in Linux a scale factor for each monitor, so if I move a window from a larger monitor to a smaller monitor, then the window, including its text, will become smaller (or possibly greater, if the second monitor has a much lower resolution), the ratio in sizes depending on both the ratio between the physical sizes and the ratio between the resolutions.
If the monitors would have very different resolutions and/or sizes, then there is no doubt that the setup would become difficult to use, unless you use some applications that you always open on a certain monitor and then you do not move them to other monitors, so you can select in the application preferences a font size and a window size appropriate for that monitor.
What I do not understand, is your reference to a blurry rendering, because the application window should be rendered in the same way on any monitor, which would lead to different text sizes, but not to differences in blurriness.
I have been using HiDPI displays in Linux for 6 or 7 years, without any problems and I also do not understand what is meant when people talk about "scaling", this might be something specific to Gnome or KDE, which I do not use. In any case "scaling" (as in Windows) sounds like something that should be strictly avoided. The fonts must be rendered for the right number of dots per font size, they must not be scaled from renderings for other DPIs (which would indeed cause blurriness).
In XFCE, you have one global setting for fonts "Custom DPI setting", which, together with the settings for anti-aliasing and hinting, determines how all the typefaces will be rendered.
After setting an appropriate high value for DPI and choosing in your applications adequate font sizes, there are no blurriness problems whatsoever, if you take care to use decent typefaces. The default typefaces in most Linux distributions are seldom good, but there are many excellent typefaces, both free and commercial.
The only programs with which I ever had problems on HiDPI in Linux are those written in Java, which very frequently lack any options for changing the font used by the program, which I consider to be a capital sin for any GUI program. Moreover, for many Java programs, the installers that use a GUI crash and die on monitors with more than 8 bit per pixel. In my experience Java is always the farthest from its claim of "write once, run everywhere".
I am guessing that if you are not getting what the problem is here, you must be pretty young and have good eyesight. Perhaps you also mainly use single-screen setups.
(By comparison, I am 53, myopic, and normally wear varilux spectacles to give me different focal lengths which my eyes' lenses can no longer provide.)
As an example: I have 3 screens on my work setup. A 24" screen in portrait (80dpi), a 22" in landscape (87dpi) and the laptop's built-in 12½" (110dpi).
The difference between the 2 external screens is not really noticeable, but the internal screen's density is significantly higher. When I put windows on that display, their contents become much smaller, to the extent that they are hard to read.
That is the problem here. On my Macs, when I attach an external screen, the OS adjusts the scaling factor so that windows, text, controls and so on are the same size on all displays. You can even position a window over the split between monitors and the OS adjusts the contents on the fly.
X.org can't do that. You get one global scaling factor for all displays.
I would like my internal screen to show its contents at 1¼× the size of the external screens, but X.org can't do that.
Wayland can, but I don't like most of the desktops that run on Wayland. I can't stand GNOME and dislike KDE. I prefer Xfce or Ubuntu Unity -- but they don't run on Wayland. (Yet, in the case of Xfce.)
This leaves 2 less-than-ideal choices: run LCDs at non-native resolutions in order to roughly match on-screen sizes, or put up with things changing size and possibly being unreadably small.
These options are tolerable on standard-definition screens, but they aren't on HiDPI screens. Worse still, a mixture of HiDPI and SD screens can be totally unusable.
Regolith-linux was the easiest to set up for hdpi and best solution for me. Fedora used to be decent but the fractional scaling just doesn't work.
I would suggest running under Wayland, and making all apps use Wayland natively.
--enable-features=UseOzonePlatform --ozone-platform=wayland
[1] https://wiki.archlinux.org/title/Visual_Studio_Code#Running_...
It shouldn't when running natively; it should render everything at the correct size rather than scaling them. X apps get blurry because they're rendered large and downscaled; that shouldn't happen for native Wayland apps.