There can be a lot of different reasons as to why the task bar is popping up when watching a video in fullscreen. It could be an error, third party applications, windows issue, or even the browser. ...

Try using a different browser if you are using chrome.

Have had the same issue, didn't find any errors related to the timing of the popup. Then I just tried another browser other than chrome and the taskbar did not popup. Hopefully google fixes the issue.