Just solved a very weird problem on my Mac.
When I typed the letter “i” the entire line would get highlighted. Basically my Mac had become useless. I envisaged days or weeks of lost productivity whilst I sent my Mac back to get a replacement.
It turned out to be a setting I’d enabled in Accessibility > Mouse & Trackpad > Enable Mouse Keys.
Why had I done that? To solve another bug on my Mac where the cursor disappears on external monitors. There seem to be several of these ongoing bugs that have never been fixed.