Moving left or right with arrow keys in incomplete mentions results in various glitches, from skipping to the beginning of the mention, to the pop-up failing to pop up again when arrowing to the end of an incomplete mention, and even to auto-completed mentions that cannot be deleted. I'm not providing recipes for reproduction as these issues appear readily via a bit of playing around with the left-right arrow keys, but if others can't reproduce them, then I will provide one or two recipes.
I have spent a lot of time trying to identify where in the atwho.js code these issues arise and why, with no success. It is foreign (to me) code, and I just don't have the patience to pore over it enough to understand it sufficiently to be capable of fixing these issues. I continue to be "bugged by this bug".