Vim can't scroll in iTerm2

My vim can't scroll in iTerm2 even set mouse=a has added in .vimrc file.

Within tmux, however, vim's scroll function works correctly.

What's wrong with my vim?

Recent versions of vim support the "SGR" protocol, you can add code bellow to your .vimrc:

set mouse=a
if has("mouse_sgr")
    set ttymouse=sgr
else
    set ttymouse=xterm2
end