KDE have such settings since KDE3. You can set the font DPI to anything and it work just fine on my Retina mac book pro. It is a single setting and while a spinbox instead of a slider, it works. Of course, there is more to HiDPI than fonts. Being able to drag a window from a standard DPI to an HiDPI one wont "work" (the poor 1080p display will have 2 cm high letters all over it). There is also the problems with pixmaps and custom QPainter widgets that also stay in pixel rather than units. While those problems are important and need a solution, the current situation is good enough for most people.
The issue with dragging a window from between lodpi and hidpi monitors cannot be solved using current X11 window managers. In theory, one could write a compositor that automatically scales windows depending on the monitor they are displayed in, but noone is really interested in doing that work.
Originally Posted by Elv13
Wayland supports this out of the box, so this will become a non-issue in the following months.
If your application has support for both modes you can switch after moving to another screen, but while dragging it will have the old size.
Originally Posted by BlackStar