Sometimes red/grey squares appear below/around text labels, like this:
As you can see, all kinds of apps are effected, like the Dock or Calendar. Also, the squares sometimes have a dark red, sometimes grayish color. When I hover with the mouse over the Dock icons the labels start changing their color between red and grey.
When I restart the app (e.g. use killall dock) the squares are gone but they will re-appear later.
It doesn't matter if I use an external display or the integrated display of the Macbook, the problem will occur on both.
Only thing I noticed is that if I attach both my external monitors to it (27" Thunderbolt Display and 27" Cinema Display) and if I drag an app, e.g. Calendar, from the Cinema Display to the Thunderbolt Display, suddenly the squares appear. If I drag the app back to the other monitor, the squares disappear. The rectangles in the Dock appear on both monitors though.
So far that's all that happens. It's not to bad but it's a bit annoying.
Also, I need this computer for work, so if it's a hardware problem I don't want it to die suddenly in the middle of important work.
So far I haven't tried to reinstall macOS yet since, as I said, I need the computer for work and don't want to fiddle around with it to much since it's working smoothly besides the issue with the rectangles.
What do you think could be the issue here? Is it hardware or software? Would macOS reinstall help? Is it a known issue? Do I have to go see the genius bar? Really want to avoid that :-(