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My name is Frank and I’m a member of the Dropbox specialist team. I've tried to reproduce the problem you described in both Chrome and Safari (Operating System: Mac OS Sierra 10.12.6), and wasn't able to reproduce the problem in either browser. Hitting spacebar in the comment field while a video was playing worked correctly for me, in that the video continued to play and a 'space' character was added to the comment text.
Can you try reproducing the issue again and let me know if you are still affected? If you are still able to reproduce this behaviour, can you open a support ticket so we can look into this further?