@jeremycherfas What model iMac is that? And what version of macOS? My 2014 5k iMac would do this all the time, but hasn’t in a while. I’ve been assuming it’s something they’ve actually fixed in Catalina, but can’t say for sure.

@DaveWoodX it is a mid 2011 running 10.13.6.
Hardware test showed no fault, but it is probably something dodgy with the graphics card. Screen also flickers sometimes when it wakes from sleep.

@jeremycherfas Cool thanks. That means my theory that it was fixed in Catalina is still possible. I doubt it’s your graphics card, and that you’re just hitting the same bug I hit. Can a 2011 iMac run Catalina? Still may not be worth “upgrading”. The red squares are a minor annoyance. Catalina itself can be a major annoyance. 🤣