It appears that dragging the status line to resize a split is not possible when the Vim option
xterm; it does work when the value is
xterm2 though. The latter value configures Vim to ask for an extended mouse reporting mode that (among other things) provides better dragging support. This extended mode only works with newer versions of xterm (and other compatible terminal emulators, including tmux), so it is not the default value.
You could use something like the following in your
.vimrc to set the option:
if &term =~ '^screen'
" tmux knows the extended mouse mode
(Though, I am not sure how this will affect actual screen instances, which also use a TERM that starts with
When you are outside of tmux, the TERM environment variable is probably an
xterm-ish value, and Vim will probe for the xterm version by using the
t_RV control sequence.