(jit-lock-fontify-now): Don't bind

font-lock-beginning-of-syntax-function to nil.

1 files changed, 9 insertions, 2 deletions

diff --git a/lisp/jit-lock.el b/lisp/jit-lock.el
index 04fd77893db..6b142997f5c 100644
--- a/lisp/jit-lock.el
+++ b/lisp/jit-lock.el
@@ -264,8 +264,15 @@ Defaults to the whole buffer.  END can be out of bounds."
        (widen)
        (unless start (setq start (point-min)))
        (setq end (if end (min end (point-max)) (point-max)))
-       (let ((font-lock-beginning-of-syntax-function nil)
-	     next)
+       ;; This did bind `font-lock-beginning-of-syntax-function' to
+       ;; nil at some point, for an unknown reason.  Don't do this; it
+       ;; can make highlighting slow due to expensive calls to
+       ;; `parse-partial-sexp' in function
+       ;; `font-lock-fontify-syntactically-region'.  Example: paging
+       ;; from the end of a buffer to its start, can do repeated
+       ;; `parse-partial-sexp' starting from `point-min', which can
+       ;; take a long time in a large buffer.
+       (let (next)
 	 (save-match-data
 	   ;; Fontify chunks beginning at START.  The end of a
 	   ;; chunk is either `end', or the start of a region