% tmux -f /dev/null
% vim -u NONE +'set mouse=a' -o <(seq 1 499) <(seq 500 999)

then touch bottom window, then top window, do this a few times, notice that the content in top window scroll when touching bottom window! This problem does not occur in vim running outside tmux, or in vim in tmux through ssh from a computer using mouse click instead of touching.