Hope it doesn't need fractional DPI scaling, because as far as I know there is no desktop on Linux that supports it, while Windows has supported it for years.
System76 employee (and HiDPI... enthusiast?) here! It does not need non-integer scaling. It's 3200x1800, which is like 1600x900 pixel-doubled. A great resolution for that physical size. This is also why you don't hear us talking about "4K" on that display, because that'd actually be too many pixels and require non-integer scaling for a usable UI.
Glad you guys have thought this through. I bought a Dell XPS 13 Developer Edition in 2015 that was marketed as a perfect Linux laptop and came with Ubuntu preinstalled. It had a 13.3" screen with a resolution of 1080p, so the text and UI was almost unusable. Two years later, there's still no fix.
"Fractional scaling" (I assume you mean e.g. 160 DPI instead of 200 DPI) is no problem even with the older KDE version from F23, just a bit finicky to get started up - but when it runs, it works. Gtk on the other hand seems (seemed? - I don't bother to check regularly) to be stuck at 100 %, 200 %, 300 %, ...
That doesn't scale the UI. And it doesn't even scale the text for many third party apps like Sublime Text, Firefox, and IntelliJ. Windows handles this perfectly.
You can achieve fractional scaling by first setting Gnome hidpi to an integer value and then adjusting the screen with xrandr [0]. It's not great, but it works.
Yes I'm aware of that, it's a poor "solution" that's more like a hack. It renders everything at twice the resolution you need and then scales it down, which is awful for battery life on laptops. And it's difficult to do, barely anyone understands what a command like this does:
No, Gnome only supports integer scaling for UI (@1x, @2x, @3x).
I don't know why they don't support fractional downscaling from @2x like Apple does. I guess nobody expected 13" 1080p laptops and 27" 4K screens seem to become mainstream :(