Bug: org-src-font-lock-fontify-block should be wrapped with save-match-data [9.3.7 (9.3.7-4-gba6ca7-elpaplus @ mixed installation! /mnt/c/Users/toz/Weiterbildung/Soft/Emacs/ and /mnt/c/Users/toz/.emacs.d/elpa/org-plus-contrib-20200615/)

[Tobias Zawada]

~org-src-font-lock-fontify-block~ modifies ~match-data~ through the
fontification of the temporary source buffer. But
~org-src-font-lock-fontify-block~ is also called in
~org-fontify-meta-lines-and-blocks-1~ by
~font-lock-fontify-region~. There it puts the text property
~font-lock-multiline~ on some text from the beginning up to the end of
the last match in the Org buffer. Since the source buffer is smaller than the 
Org buffer
~match-beginning~ is smaller than it should be.

This can slow down editing operations in org-mode with large source blocks to 
an extent to which
org-mode becomes unusable.

An easy workaround is:

#+begin_src emacs-lisp
(defun org+-with-save-match-data (fun &rest args)
  "Run FUN with ARGS but save `match-data'."
  (save-match-data
    (apply fun args)))

(advice-add 'org-src-font-lock-fontify-block :around 
#'org+-with-save-match-data)
#+end_src

Emacs  : GNU Emacs 27.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.20, 
cairo version 1.16.0)
 of 2020-09-19
Package: Org mode version 9.3.7 (9.3.7-4-gba6ca7-elpaplus @ mixed installation! 
/mnt/c/Users/toz/Weiterbildung/Soft/Emacs/ and 
/mnt/c/Users/toz/.emacs.d/elpa/org-plus-contrib-20200615/)