KWindowSystem has been and is still lacking a platform plugin for OS X. A ReviewBoard RR has been up with a "backport" of the modified KDE4 KWindowSystem implementation that has been used in the MacPorts kdelibs4 port for the last 2 or 3 (or more) years. This is a continuation of that request, still open at https://git.reviewboard.kde.org/r/126291

There's 1 outstanding issue on that RR (https://git.reviewboard.kde.org/r/126291/#comment68028). I have also reintroduced a small change to KWindowSystem::setMainWindow() so that it doesn't cause crashing. Ideally this function would be part of the backend but that's too big of a change to graft on the current patch.

Thoroughly tested up to and include with Qt 5.8.0 and KF5 Frameworks 5.35.0+

The Mac backend code also contains a number of experimental functions that have been deactivated with #ifdefs . At this point I cannot assess their true potential importance; I do not seem to miss them for now but it certainly wouldn't hurt for instance to be able to map windows to applications.
I'd vote to leave in the code in question until it's clearer from feedback whether or not we need it.