Discussed last year but I forgot to add it to the tracker: https://lists.gnu.org/archive/html/lilypond-user/2014-04/msg00676.html

I've recently updated the translation of Frescobaldi and I found out a wonderful feature that I had ignored for a lot of time (I don't know when it was introduced..). I'm talking about Window>Split horizontally, which allows to divide the input view in two scrollable panes. This is very handy when writing a polyphonic music where each voice is saved in a variable; after a few bars the distance between the two voices forces you to scroll up and down between the first and the second voice, while inserting new bars. If you split the input view, you can easily jump from first to second voice without scrolling.

What about adding a section in the Usage manual named "Editor tips" under chapter 5? Of course, this section should not be a guide for each editor. Instead it should be a concise list of some editor features that can ease the writing of lilypond code.

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The thread above contain some specific information for Emacs.