Re: master 2399541: Remove font-lock toggle from font-lock-update

[Gregory Heytings <gregory@heytings.org>]

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>> I'd much prefer a longer `font-lock-fontify-diwm` which tries to 
>> reproduce more or less the same behavior as your favorite, but by 
>> explicitly testing the different circumstances you care about.
>
> Sure.  It could, for instance, refontify the region (if selected) 
> instead of the entire buffer.  And probably a lot of other DWIM-ish 
> niceness.
>

Here's a patch to do this, and to fix the problem when "too large" buffers 
are refontified (xdisp.c for example).

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From 44332493a8796f692605e7e7ec55390742ff23bc Mon Sep 17 00:00:00 2001
From: Gregory Heytings <gregory@heytings.org>
Date: Thu, 25 Mar 2021 09:50:05 +0000
Subject: [PATCH] Improve font-lock-update and rename it to font-lock-dwim

* lisp/font-lock.el (font-lock-dwim): Renamed from 'font-lock-update'.
Only refontify the region when it is active.  Do not refontify too
large buffers at once.

* lisp/bindings.el (ctl-x-x-map): Adjust the command name.

* etc/NEWS: Adjust the command name.
---
 etc/NEWS          |  4 ++--
 lisp/bindings.el  |  2 +-
 lisp/font-lock.el | 14 +++++++++-----
 3 files changed, 12 insertions(+), 8 deletions(-)

diff --git a/etc/NEWS b/etc/NEWS
index 49a4bb8106..d7b5efaaee 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -94,7 +94,7 @@ useful on systems such as FreeBSD which ships only with "etc/termcap".
 * Changes in Emacs 28.1
 
 +++
-** New command 'font-lock-update', bound to 'C-x x f'.
+** New command 'font-lock-dwim', bound to 'C-x x f'.
 This command updates the syntax highlighting in this buffer.
 
 ** The new NonGNU ELPA archive is enabled by default alongside GNU ELPA.
@@ -255,7 +255,7 @@ The 'C-x x' keymap now holds keystrokes for various buffer-oriented
 commands.  The new keystrokes are 'C-x x g' ('revert-buffer'),
 'C-x x r' ('rename-buffer'), 'C-x x u' ('rename-uniquely'), 'C-x x n'
 ('clone-buffer'), 'C-x x i' ('insert-buffer'), 'C-x x t'
-('toggle-truncate-lines') and 'C-x x f' ('font-lock-update').
+('toggle-truncate-lines') and 'C-x x f' ('font-lock-dwim').
 
 ---
 ** Commands 'set-frame-width' and 'set-frame-height' can now get their
diff --git a/lisp/bindings.el b/lisp/bindings.el
index 6eac528eb6..6f25e9738a 100644
--- a/lisp/bindings.el
+++ b/lisp/bindings.el
@@ -1432,7 +1432,7 @@ if `inhibit-field-text-motion' is non-nil."
 
 (defvar ctl-x-x-map
   (let ((map (make-sparse-keymap)))
-    (define-key map "f" #'font-lock-update)
+    (define-key map "f" #'font-lock-dwim)
     (define-key map "g" #'revert-buffer)
     (define-key map "r" #'rename-buffer)
     (define-key map "u" #'rename-uniquely)
diff --git a/lisp/font-lock.el b/lisp/font-lock.el
index 82915d8c8b..6b351d34ea 100644
--- a/lisp/font-lock.el
+++ b/lisp/font-lock.el
@@ -1120,15 +1120,19 @@ portion of the buffer."
     (funcall font-lock-ensure-function
              (or beg (point-min)) (or end (point-max)))))
 
-(defun font-lock-update (&optional arg)
+(defun font-lock-dwim (&optional arg)
   "Updates the syntax highlighting in this buffer.
-Refontify the accessible portion of this buffer, or enable Font Lock mode
-in this buffer if it is currently disabled.  With prefix ARG, toggle Font
-Lock mode."
+Enable Font Lock mode if it is disabled.  Otherwise, refontify the region
+if it is active, or a large part of the accessible portion of the buffer.
+With prefix ARG, toggle Font Lock mode."
   (interactive "P")
   (save-excursion
     (if (and (not arg) font-lock-mode)
-        (font-lock-fontify-region (point-min) (point-max))
+        (if (use-region-p)
+            (font-lock-fontify-region (region-beginning) (region-end))
+          (font-lock-flush (point-min) (point-max))
+          (font-lock-fontify-region (max (point-min) (- (point) 50000))
+                                    (min (point-max) (+ (point) 50000))))
       (font-lock-unfontify-region (point-min) (point-max))
       (font-lock-mode 'toggle))))
 
-- 
2.30.2