Playmaker 1.9.0.p15, Unity 2019.1.0f2 (Win x64). Fairly fresh setup, through the Unity Hub.
This bug occurs since I switched to this Unity version. I did update Playmaker a while ago to p15, but the problem persists. I worked around it so far, though maybe I should report it.
What Happens
When editing the FSM in a prefab and making any change, like adding a transition, Unity begins to stutter and alternates the mouse cursor between "busy" (as if saving) and default mouse pointer, making it impossible to work that way. I can stop this stuttering by starting the play mode, which runs normally. After exiting the mode, editing works as usual again -- until next time. The bug is completely consistent and happens each time I edit a prefab's FSM.
Repo Case
Doesn't matter how the prefab FSMs is being accessed or edited: click on an object in the hierarchy, then in Playmaker window > open (prefab). Or click on the prefab in the project window > "open prefab" (inspector). Add e.g. a FINISHED transition, stuttering occurs.