[exprL]

The Flash-based player would do exactly the same (with a slight delay) even if it wasn't as obvious from the UI.

To prevent YouTube from switching sources when resizing the content, select a specific quality first (it defaults to “Auto”). I wouldn't be surprised if there were browser extensions that automatically do this (e.g. always choose the highest quality).

[Ntrails]

I think the flash player doesn't change quality on resize and thus was able to keep using buffered quality?

I switched to forcing HTML a while ago so can't test. Either way, I'll try that - thank you :)