Initially I opened Lags when navigating vc-root-diff buffer and the cause of slowness was Emacs opening referenced files by diff.

Back then I used Emacs 27.1. Now I'm on Emacs 28.2 and previous solution:

(setq diff-font-lock-syntax 'nil)

doesn't work:

  • I experience delay of Emacs opening files in the background
  • later I end with lots of buffers with files I personally never opened but Emacs opened them for me, so I spend minutes closing tons of files, they pollute history of recent files (I don't care about robotic opened file, I need only human origin).

UPDATE 2023-04-10 Bug report: Emacs keeps opening related file from vc-diff buffer

  • 1
    Irrelevant to the question: you don't have to quote a nil.
    – shynur
    Apr 9, 2023 at 12:13
  • Seems they are equal: (eq nil 'nil) is t... quote is a macro so the end result could be magical.
    – gavenkoa
    Apr 9, 2023 at 12:18
  • 2
    "quote is a macro" --- quote is a special form.
    – shynur
    Apr 9, 2023 at 12:25
  • This sounds like it might be a candidate for M-x report-emacs-bug, if you don't get a helpful answer here. On the other hand, you say "Modern Emacs", and there's an Emacs with that name which differs from GNU Emacs. Maybe there's a different way to report a bug there, if you meant that particular Emacs.
    – Drew
    Apr 9, 2023 at 14:14
  • Emacs is stock GNU Emacs in Cygwin. Probably need to file a bug report. I thought other people struggle with the issue, like it is a common problem.
    – gavenkoa
    Apr 9, 2023 at 14:18

1 Answer 1


Bug report https://debbugs.gnu.org/cgi/bugreport.cgi?bug=62749

The culprit is:

(which-func-mode 1)

When the cursor moves through the hunks of VC diff the mode automatically opens corresponding file, causing Emacs to hang a lot in Cygwin env ((

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