Does it work out of the box or do you need to do something specific. I have a custom built desktop with a 4k and a 1080p monitor. I can get one of them to look good. :( I’m on mint.
Works out of the box for me since moving to Fedora running Wayland and Gnome.
I just set one monitor to 100% and the other (4k) to 200%. In fact I think that choice was originally made for me but you can change it.
When I first started using it a couple of years ago you'd get old artifacts when moving a window from one monitor to another. e.g. the mouse pointer could be the wrong size.
Over the years, like many things on Linux, it's got more polished.
Linux supports this, but applications on Linux may not (or not gracefully). I've found Chrome on Ubuntu (GNOME with Wayland) just doesn't adjust its scale when being dragged across displays with differing UI scaling settings. I don't have this problem on Windows or Mac OS.
- Gnome actually does the scaling in the compositor, not in the X server itself.
- Most desktop environments should be doing all that for you. At the time I wrote that (over 2 years ago), in Gnome you had to go in to an advanced settings menu and enable "experimental fractional scaling"; I'm not sure if that's still true today.
- Some things pick up the DPI from the XSETTINGS protocol, not from the XRDB. Specifically, parts of Java's AWT/Swing do this. But other parts use the XRDB. And they conflict with eachother. When I last looked at Swing (April-ish 2020) it was impossible to get it to do the right thing on X11 (I was working on a patch, but then I had some life disruptions and never came back to it).
Linux definitely does support this.
I use this exact feature with Gnome and Wayland daily to combine a 4K XPS13 with a 27” 1440p monitor.