Kindle Paperwhite newly implements two-finger pinch (smaller font size) and expand (bigger font size). On the way to carrying out the font size change, it pops

Kindle Paperwhite newly implements two-finger pinch (smaller font size) and expand (bigger font size). On the way to carrying out the font size change, it pops up the font-size select window, then closes it. It's possible that you accidentally invoked this functionality.