[twooster]

I believe this is because vim manages drawing the entire screen, rather than depending on its terminal emulator to wrap lines, etc. This may be dependent upon configuration, and the setting `t_ut` (which is the background clearing color).

Anyway, using tmux for selection in vim is a bad idea. You can give vim mouse support by `set mouse=a`, which will prevent tmux from capturing selection and scrollback control if you like. Achieving parity between vim, system, and tmux clipboard (and for bonus points, across a network) is left as a "fun" exercise.