On anim-render, a click in timeline stopped render completely.
The reason for this was a bit wacko code to cope with frame-step
feature (steps of multiple frames).
I thought of fixing that, but instead decided to block any operator
in Blender to change a frame while a render is in progress.
Both render engine and UI are accessing (writing to) the same
data then, which is a bad conflict.
Still a serious weakness of threaded render, but I'll keep
trying to allow this as far as possible :)
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