If you don't enable mouse support (set -g mouse on) then the terminal process handles mouse and does what you're describing* . When you enable mouse support tm

If you don't enable mouse support (set -g mouse on) then the terminal process handles mouse and does what you're describing* . When you enable mouse support tmux handles selections within panes and doesn't span. * the other possibility also is that your term emulator is configured to grab mouse control without passing it through.