tiny triangle in middle of screen

jwsapp wrote:

A tiny triangle has appeared in middle of the screen. I tried rebooting, but it persists.

A screen shot would be some insight...

Try a SafeBoot https://support.apple.com/en-us/HT201262 login and compare your results

Takes noticeable longer to get to the login screen, does a 5 minute disk repair before it fully boots up, and certain system caches get cleared and rebuilt, including dynamic loader cache, etc.

Reboot as normal and test. Caches get rebuilt automatically.

Hi,

Try to check your display with Apple Diagnostics if it reports display or graphic related error(VFD001 to VFD007).

Use Apple Diagnostics to test your Mac - Apple Support

I am not seeing a tiny triangle in your screenshot.