Audio desync when scrubbing around videos

[ctd1500]

Description

I was in the process of updating my build of video.js and I noticed major audio desync issues in Firefox when seeking around videos.
My testing shows that the issue seems to be caused by the FastSeek feature #6525

I created two examples built from the Main branch to reproduce this:
FastSeek feature causing desync: https://codesandbox.io/s/videojs-fastseek-test-gvvx0
FastSeek commit reverted, fixes desync: https://codesandbox.io/s/videojs-fastseek-removed-test-94tnc

Steps to reproduce

Seek around the video in the FastSeek feature example and you'll start to see audio desync.
An easily recognizable example would be the bird at 00:20.

Results

Expected

No desync

Actual

Major audio desync

Additional Information

Please include any additional information necessary here. Including the following:

versions

videojs

Main branch

browsers

Firefox 77

OSes

All OSes (tested on Windows 10, and Mac via BrowserStack)

[gkatsev]

Huh, it's with an mp4? We don't handle playback of that ourselves. Sounds like we probably want to limit the functionality to safari and then expand it to our source handlers whenever we get around to implementing iframes in VHS.

[gkatsev]

This is probably something that we should report to Mozilla as well because we don't handle things like mp4 playback. Adding a PR to limit fastSeek to only Safari based browsers.