Friday, June 1, 2012

Emacs: Scroll Other Window Up

It often happens that I have two related files open in one emacs frame, and I want to scroll around comparing them.  Of course to scroll back and forth in a single buffer, C-v scrolls you down, and M-v scrolls you up. (In a perverse bit of emacs jargon, C-v's function is called "scroll-up", since the text moves "up" relative to the fixed window -- even though every human wanting to look lower in the file thinks and says "scroll down, scroll down" -- and M-v's function is called "scroll-down".)

To keep two files synchronized as you peruse them, the power tool to use is M-x ediff-buffers, which sets up an interactive diff session to step through the diffs. Within ediff, a simple lowercase "v" scrolls lower in both files at once, and uppercase "V" scrolls higher. Don't forget M-x ediff-revision as a handy tool for comparing a version-controlled file to its latest revision, or for comparing two different revisions interactively.

Ediff is great, but sometimes it is too heavy-handed for the eyeballing I'm trying to do. For one thing, if one of the files contains a large section that is missing in the other, scrolling both files at once means striding through the section in one window while inching down a line at a time in the file that lacks it. Also, navigating ediff can be tedious if there are a great number of changes or reordered chunks, and once your diffs no longer line up sensibly, ediff's highlighting is simply a nuisance.

So there are the old standbys C-v and M-v, plus there is the handy M-C-v, which scrolls lower in the other window. That is, if I have a.txt and b.txt open in a single frame, and the cursor is in a.txt, then M-C-v will show a lower chunk of b.txt, to keep up with C-v in a.txt. Until today, whenever I wanted to move higher in b.txt, I didn't know of a single keystroke to do that, so I always just switched windows and used M-v. I could do C-u - M-C-v, but I never liked that.

Turns out there is a keystroke to scroll higher in b.txt without switching windows: M-C-S-v -- same keys as scrolling lower, plus the shift key. Now, M-C-v is a pretty ergonomic companion to C-v. I lean the base of my left hand on the Ctrl key, hit V with my left index finger, and keep my thumb poised over the Alt key to choose which window to scroll. The shift key just does not fit in with this scheme. With some effort, I can hold it with my left pinkie, but it's not comfortable. So my new solution is to tie that command to C-c M-C-v. True, it's not a single keystroke, but unlike the C-u solution above, it's all in the left hand, and close together without being cramped. The .emacs line is:

  (global-set-key "\C-c\M-\C-v" `scroll-other-window-down)

Yes, the command to go higher in the file -- to scroll up as we humans say -- is "scroll-other-window-down".

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