latest Unity 2018
PM 1.9.1 P5
It always happens at some point. Usually after 10-30 minutes working on a project. Both freehand panning ability, and scroll bars freeze in the PM editor states layout. Almost seems like the editor is constantly updating the current state layout locatoin, when pressing the scroll arrow button once, the states page wiggles back to the original location.
So far the only solution is to restart Unity.
It seems like it may happen when clicking on the editor too early after opening the window.