having some padding is nice to have a spot to 'grab' when you want to move windows around. Firefox chrome has gotten so small its a hard to find a 'safe' place to grab.
That might suggest that having no titlebars on systems that can display more than one window at a time isn't the best idea, either ;-)
Anyway, at least you can easily turn the title bar back on.
P.S. Am I the only one losing the fight with smaller pixels? As monitors get higher res, I keep having to increase the font size to be able to read them. Even on 4K, twenty-mumble inch monitors, I can only get one 'visible' window at a time.