** Follow mode
Follow mode is a new minor mode combining windows showing the same
buffer into one tall "virtual window". The windows are typically two
side-by-side windows. Follow mode makes them scroll together as if
they were a unit. To use it, go to a frame with just one window,
split it into two side-by-side windows using C-x 3, and then type M-x
follow-mode.
M-x follow-mode turns off Follow mode if it is already enabled.
To display two side-by-side windows and activate Follow mode, use the command M-x follow-delete-other-windows-and-split.