On every machine I've used any Electron app on, there are large gray areas that flash and lag behind the mouse cursor when resizing (it's especially noticeable on Slack). On hidpi displays, half the time there are 1px black lines remaining on the right and bottom sides (probably due to poor rounding). Compare that to native apps like Sublime or Windows Explorer.

Just tested on both a 4k 13" Windows Display and a 2K 15" Mac Display - yes I see what you mean. That is pretty bad, though I've never noticed that before.