Mouse-mode has been rewritten. There's now no longer options for:
- mouse-resize-pane
- mouse-select-pane
- mouse-select-window
- mode-mouse
Instead there is just one option: 'mouse' which turns on mouse support
# from v2.1
set -gq mouse on
# before v2.1
set -gq mode-mouse on
set -gq mouse-resize-pane on
set -gq mouse-select-pane on
set -gq mouse-select-window on