tmux: disable "mouse-drag-ended--resets-viewport" behavior

[bew]

Can be disabled in recent versions of tmux:
tmux/tmux#140 (near the bottom of the issue)

[bew]

tmux/tmux#140 (comment) is the correct solution
Extract:

set -g mouse on

# disable "release mouse drag to copy and exit copy-mode", ref: https://github.com/tmux/tmux/issues/140
unbind-key -T copy-mode-vi MouseDragEnd1Pane

# since MouseDragEnd1Pane neither exit copy-mode nor clear selection now,
# let single click do selection clearing for us.
bind-key -T copy-mode-vi MouseDown1Pane select-pane\; send-keys -X clear-selection

# this line changes the default binding of MouseDrag1Pane, the only difference
# is that we use `copy-mode -eM` instead of `copy-mode -M`, so that WheelDownPane
# can trigger copy-mode to exit when copy-mode is entered by MouseDrag1Pane
bind -n MouseDrag1Pane if -Ft= '#{mouse_any_flag}' 'if -Ft= \"#{pane_in_mode}\" \"copy-mode -eM\" \"send-keys -M\"' 'copy-mode -eM'

[bew]

Also see tmux rewritting at 2a538c3