I don't use the web interface a lot, but I thought you used to be able to do this. Now, when watching a currently recording show, there is no scrub bar and it shows X:XX:XX / LIVE for the time on a show that is currently recording. I can still skip forward 30 sec, but my transcoder isn't zippy and that would be a pain in any event to, say, start watching a football game at the beginning of the second half.
Completed recordings allow scrubbing as expected.
Running 2020.12.02.1531
(edited for clarity)