"Linux" meaning all desktop environments -- including Cinnamon, which is very nice in some ways, but developed by a small team and therefore sort of inherently "caveat emptor" -- will never be "really there yet" because backwards compatibility means that someone will always be able to write an app that screws up. But IMO if the laptop is sold by KDE, it's reasonable for them to sell a HiDPI screen assuming that you can use the computer fully with KDE specifically, because why should KDE have to hold their hardware back to wait for the five-person team behind the Lorem Ipsum toolkit? Of course the definition of "fully" may be subject to review, and one of the problems is that Linux reviewers are too nice: if KDE's VLC replacement works on HiDPI but won't play common formats, the laptop should lose big points for video playback; unfortunately, reviewers let this kind of thing slide too much, and the community suffers for it.