Resize a pane

A pane is too narrow for the output you want to read, and dragging the border by hand is slow.

tmux kill-server 2>/dev/null; sleep 1
tmux new-session -d -s w -x 120 -y 30
tmux split-window -h -t w
tmux list-panes -t w -F 'pane #{pane_index}: #{pane_width} columns'
tmux resize-pane -t w:0.0 -R 20
tmux list-panes -t w -F 'after -R 20, pane #{pane_index}: #{pane_width} columns'
tmux resize-pane -t w:0.0 -x 30
tmux list-panes -t w -F 'after -x 30, pane #{pane_index}: #{pane_width} columns'
tmux kill-server

pane 0: 60 columns
pane 1: 59 columns
after -R 20, pane 0: 80 columns
after -R 20, pane 1: 39 columns
after -x 30, pane 0: 30 columns
after -x 30, pane 1: 89 columns

-R, -L, -U and -D move the pane's edge by a number of cells in that direction, and -x and -y set an absolute width or height, as described in the resize-pane entry of the manual. The neighbour gives or takes the space, so the widths always add up to the window less one column for the border. The key bindings are prefix then Ctrl and an arrow for one cell, and Alt and an arrow for five.

Sizes persist until the layout is changed, and select-layout or a window resize will recompute them. A pane cannot shrink below one cell, or grow past what its neighbours can give.