Disable automatic-scrolling button: don't show it "on load"?
This is a tiny detail but I recognised that the button to disable automatic scrolling shows up on load and then disappears, because the video does not play and it is not needed. This could be a little bit irritating.
I would suggest to either a) not show it on load but really only if it is needed OR b) to not at least not fade it out if you hover over it and read the tool-tip (that allows users to check what this is meant for if they see it/recognise it if they enter the Post-Annotator).