Possibly a regression, but I could reproduce this issue as far as 5.2, so unsure when it started.
The issue seems to be that the cloth painting tool lock the camera movements during painting, but doesn't unlock them when not painting anymore, causing the skeletal mesh editor to be in locked mode for ever, or at least until the cloth painting tool is ran again and the camera is moved from inside it.
Observed result: The camera doesn't respond to mouse inputs anymore.
Expected result: The camera responds to mouse inputs.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-194162 in the post.