Mixed scale DPI screens are not a problem if you use Wayland, which is basically the default now.
Font rendering though? YMMV.
I have a laptop running Ubuntu and connected to my 4k monitor in front of me. Has an Intel gpu. I do not even want to use multiple displays, I’m gonna go with the external only.
Wayland can do fractional scaling but then VsCode, Firefox, Discord and whatever apps I tried became blurry shit. Firefox has an environment variable to fix it. VsCode has an experimental build and command line flags. I have no idea about Discord. I don’t care if its the app developers fault. It is just plain bad.
X11 can do fractional scaling but fonts are a little blurry for some reason and I have the worst screen tearing I’ve ever seen in my life. It is unusable.
macOS and Windows do this perfectly. It is not something you think about. Windows has some weird looking apps here and there but the shittyneas is not even close.
For context, I have (currently) a precision 5520 laptop;
I run arch with sway, I have a big USB-C 4k at work and 2*16:10 FHD Dell USB-C monitors at home.
The only issues I have with linux is that I don't have Microsoft Office, and Zoom+Wayland is buggy as hell.
Use the closest integer scaling and adjust default font sizes (and if needed other sizes like window title height or panel height).
It's also best if you just don't buy monitors that have dpis that are not close to integer multiples of 96.
Maybe I should try 2x gui scaling and 0.75 font size? Never occured to me that I could go below 1 on the scales, it might work.
edit: 2x on wayland is still blurry. 1x + 2x font works but as I said, tiny buttons and stuff.
Still, my point is; why the fuck am I dealing with this?
In mobile where there is market, Google did it.
I stopped trying a couple of years ago (after I got a 4K monitor) to convince myself that desktop Linux is workable.
Linux for desktop = console
X11 apps do that. Firefox can run in Wayland mode, so run it in Wayland mode. vscode and other electron apps do not support wayland yet, so until they do (yes, they are taking their sweet time), you either need to run with integer scaling, or tolerate them being blurry.
In Windows, the shittyness is much worse. Basically any Qt app is unusable with hidpi in Windows.
In my experience, it isn’t. On my setup (200% + 100% screens, Retina MacBook Pro in Boot Camp) KeePassXC, qBitTorrent, Qt Designer all look great on both screens; VLC has slightly weird fonts on the HiDPI screen but still works fine.
Tangentially, I want to mention that the archlinux wiki is a terrific catalog of machines and their known quirks (and sometimes, fixes for the quirks!).
And thats sort of the theme everywhere. Its super fast and I love it, and its mostly stable and mostly great. But tiny bugs shine through all over. I've been totally spoiled by Apple's spit and polish.
For example, I get random graphics bugs after waking from sleep sometimes. Sometimes my mouse cursor is either invisible or for some reason duplicated, so I see a second stationary cursor hovering over my windows. And for some reason my second display doesn't vsync, so I get obvious tearing when I scroll or move windows around.
I use a trackpad, and smooth scrolling works properly in most apps. But Firefox needs an obscure XInput environment variable to make it work. (That trick is only mentioned deep some a bug tracker). Smooth scrolling doesn't work at all in IntelliJ. Intellij also doesn't let me use the Meta key (Cmd / Start) as a shortcut modifier - so my muscle memory for navigation is all messed up and I can't rebind keys to fix it. I hate how Ctrl+C is copy everywhere except the terminal, which needs me to Ctrl+Shift+C instead. Etc etc. Forever.
Its unbelievably responsive compared to my 2016 MBP though. If you haven't upgraded in awhile and you can afford it, its a fantastic time to get a new system. But linux on the desktop still isn't entirely pain free. Way better than it was a few years ago though.
...or use Wayland where smooth scrolling should work by default
In Fedora maybe. Not in most distros. And recall many apps have to run under xwayland for compatibility so all of those apps won't support scaling properly.
It's still not there yet.
Battery life also takes a huge hit on the Dell XPS and HP Elitebooks I've tried, even after trying to apply some tweaks (not that I should need to). For this reason I use Windows+WSL on laptops even though I'd prefer Ubuntu.