I'm hoping Duolingo will soon fix this error, and I don't know if it's just my computer (my sister is experiencing this problem too) but every time I strengthen one of the subjects, this white rectangle shows up and flits around the screen. It is extremely annoying and makes learning difficult. Also, I agree with someone's statement about the green for correct being too bright. Please darken that :)
It seems like it might have something to do with Grammarly, if you are using it? In that case, try disabling it and see if it helps.