Apparently, this is an undocumented change and not a bug.
When typing, clicking anywhere else on the screen deselects the chat box, and if you don’t realise you’ve done that or aren’t used to it because that never used to be the case, your hotkeys will suddenly start spamming the screen.
Can anyone create an addon to revert this change? Is that even a thing that’s possible?
Prior to 8.3, if I was typing and clicked somewhere, nothing would happen with the cursor in the chat box. It would stay there and I could keep typing while also mouse steering.