Reported by a user from Reflector.
When Niagara editor is open, in play loop, at the end of the animation, the user will lose focus of his mouse. Additionally, msgs will spam in the log:
(Logslate:fsceneviewport::onfocuslost() reason 3)
This lost of focus then affect the whole editor. A workaround to avoid this issue is to adjust the play range end a bit, then you wont get spammed and wont lose focus.
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-72428 in the post.