There is a black line that goes across the screen and through the login button, snippet attached. Every login screen since Blackwood has done this. I never had it before Blackwood.
So changing the resolution when ESO is set to Fullscreen, not Fullscreen Windowed, does not seem to actually change the resolution at all. It stays at 3440x1440.
When I set it to Windowed and drag the window smaller, the line will get smaller until it is no longer visible, but it does stay in the same relative position to the Login button. Unfortunately I can't tell the exact size this happens at.
When I click on Settings on the Login screen to do this, the line also disappears, so it only persists when the login boxes are on screen.