@dalias It's an obvious downside: in order to grab the edge (without e.g. taking away your ability to grab the app's scrollbar), there has to be a portion of the screen considered the gaps...
At one point, I had a keybinding to toggle the size of the the gaps, but since dropping my monitor scale from 3 to 2.666, the wasted screen real estate hasn't bothered me: most video players have a fullscreen option.