Uncommonly useful: I have a file with two functions that I want to not just see side-by-side, but with a diff.

`C-x 4 c' runs the command clone-indirect-buffer-other-window Now you can narrow both buffers to the two different functions (mark and C-x n n) and do M-x ediff-buffers to get a diff

See also https://karthinks.com/software/batteries-included-with-emacs...

Also, rectangles: https://karthinks.com/software/more-batteries-included-with-...