Ah, how annoying it was to fullscreen a YouTube video as it automatically switched to a higher resolution, resulting in a hefty amount of extra buffering...
No more of that, as I noticed today. Bringing a video to full screen does change the quality, but it doesn't do it instantly, thus allowing the viewer to continue watching without interruptions. Once the hi-res buffering is complete in the background, the switch from, say, 360 to 720p is made seamlessly.
I really hope they keep this functionality, as YouTube is known for testing out new features for only a few days before taking them out.
Go ahead and test it out yourself. But know that improvements such as this are not always rolled out to all users at the same time.