I just tried the above animation on W10 + uBo + Firefox 83. I went through 3 loops and it seemed to stay in sync. Then I hopped over to check this comment and came back to that tab, and it was out of sync at that time. So I think Tab Switching might be a challenge for Ruffle.

Clearly Ruffle is not perfect, but leagues, furlongs ahead of having to watch old flash content on YouTube via Unregistered Hypercam 2.