Every new login shows the minecraft domain with red and green separated by several pixels. Torches, for example, show double: one red and one green side by side. The landscape is unrecognizable.

The error has just appeared, and is the same for every log-in to Minecraft whether existing domains or a new domain. All other computer functions are OK including the list of Minecraft tools at the bottom of the screen. The error survives computer reboots.

I may have created the initial error by hitting a function key as I logged off one day, but have since pressed all 12 function keys in an unsuccessful effort to undo any such action.