I'm getting this too.
I'm using ACE3, along with a bunch of other mods, though I am fairly certain that ACE3 is the problem.
Always happens right after I return to the 3D editor after a test.
Vanilla it wont freeze.

(just a side note, it seems like it is still registering that the camera is moving, as the coordinates will change, as well as the lines in the left corner).