I noticed this might be an issue when I fixed the last ticket that changed this, but was not able to produce a problem or find any reason why it was wrong. I just suspected it. I left it because I was not sure.

I now know this becomes a problem with lots of clips.

The same expected/actual results from https://bugzilla.mozilla.org/show_bug.cgi?id=897327 happen again if you add a lot of clips and hit play before they are ready.