In the last month I’ve noticed that when I switch to fullscreen during a YouTube video – it has to rebuffer the whole thing again.

Anyway, I’ve found some more options that can be tweaked – Go to YouTube.com > “username” > Account > Playback settings – then switch to “I have a slow connection”. You can manually switch to a high quality version of the video when watching it, but this option stops this from happening automatically.

YouTube settings