Everything I write, here and elsewhere. Newest first.
join-pane and move-window accept a source in another session of the same server, and the process running in the pane is re-parented, not restarted. A server started in a throwaway session lands in your main workspace without going down.
Relative targets like down-of name a pane by its position, no index lookup. join-pane -h -s picks the pane below the active one and re-attaches it to the right: a stacked pair becomes side by side in one command.
join-pane detaches a pane from wherever it lives and re-attaches it as a split in the current window, processes included. break-pane does the exact opposite: it sends a pane off to its own window.
Mark a pane with Ctrl-A m and the {marked} target resolves to it from any window or session, so a swap or a join needs no index juggling.
tmux window targets accept special tokens for the first, last, previously active, next and previous windows, with one-character aliases. They keep scripts independent of actual index numbers.
A bare number in a tmux pane target means a pane of the current window when that pane exists, otherwise a whole window, and it flips silently. Qualify with window.pane, a pane id, or a window name.
Windows and panes start at 0 by default, so the leftmost one sits under a key at the far end of the keyboard. Set base-index and pane-base-index to 1 in ~/.tmux.conf so key 1 selects the first one.
The mouse is off by default. Add set -g mouse on to ~/.tmux.conf, then drag to select and copy into the tmux buffer. Hold Shift to fall back to the terminal's own selection.
Who is allowed to touch the models, the bill for the hype, and a scroll read after 2,000 years. And much more ...
Use resize-pane with -L/-R/-U/-D to push a border by N cells, or -x/-y to set an exact size. A resize only works when there is a neighbour to trade space with.