I also noticed this artifact after upgrading to ET 3.0.1. It's definitely due to Electron 2.x but in my case (and maybe others) it only occurs after I wake up my NUC from sleep mode. I have tried my NUC on different TV's and I get different results on different models. On Samsung and LG the problem occurs but on a really old Insignia no problems at all.
Happens for me also, when I wake up my HTPC, but only, if it starts before the tv is switched on.
This is because the Intel gfx chip seems to use a different screen mode (that can be adjusted in the registry), if there is no monitor connected (or tv still off) when starting up and then switches screen mode when tv is ready.
This leads to the same as switching refresh rate when ET is running and so you get the gap at right/bottom.
For me it works, if I first switch on tv, then wake up pc -> no gaps.
Give it a try, might work for you too.
With my AMD based htpc this was not a problem, it seemed to already use the right screen mode when turned on, even when the tv was still off.