Horizontal Scrolling Display Selective Display
Follow mode is a minor mode that makes two windows showing the same buffer scroll as one tall ``virtual window.'' To use Follow mode, 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. From then on, you can edit the buffer in either of the two windows, or scroll either one; the other window follows it.
To turn off Follow mode, type M-x follow-mode a second time.Horizontal Scrolling Display Selective Display