I remember this being the reason I gave up playing KSP on my iMac a year or so ago, and it seems the bug hasn't been fixed yet. In the VAB or SPH (but also sometimes while flying an active craft as well), the buttons are rarely clickable (they highlight, but clicking has no effect, sometimes it helps to hold the mouse button down for a few seconds, sometimes it doesn't), the scroll bar on the parts panel seems to get stuck to the pointer and not be able to click off it, the camera randomly spins and changes angles without warning, it's basically impossible to build anything. As soon as you line a part up (after managing to get it clicked and created in the first place) the camera starts spinning on its own. If it spins to the right, sometimes holding down the right arrow button for 3-5 seconds will stop it, but it'll start spinning again after a few seconds. And if you somehow manage to be lucky enough for the camera to stay still for a moment, then you can't place a part because clicking on the attachment point has no effect and it just drags around with the pointer.
Other problem is in flight mode, the same thing happens - the controls randomly keep pitching up, down, yawing left and right. It's as if KSP is picking up wasd messages without touching the keys - and this is only a problem that occurs in KSP, I don't have it with any other games or programs.
Has anyone else encountered this problem?
OS Sierra 10.12.16
Late 2015 27" iMac
3.2GHz i5
8GB RAM
AMD R9 M390 2GB
KSP version 1.3.1.1891 (OSXPlayer)