I can't reproduce this right now, but it happened to me when I was playing with binder background colours when I first downloaded Scrivener 3. My solution was to go to either a darker background, or a lighter background--either would make this stop.
So you know where I'm coming from:
I'm a user, not an L&L employee.
Mac Scrivener 3.0.3, MacBook Air 11, MacOS 10.13.6 (High Sierra)
IOS Scrivener 1.1.5, iPhone 6s, iPad Air 2, iOS 12.0