From: Gregory Heytings <gregory@heytings.org>
Date: Thu, 21 Jul 2022 10:37:45 +0000 (+0200)
Subject: Merge branch 'feature/fix-the-long-lines-display-bug'
X-Git-Tag: emacs-29.0.90~1447^2~821
X-Git-Url: http://git.eshelyaron.com/gitweb/?a=commitdiff_plain;h=616da8fa8efa9023f56fa731072d877e2150afbc;p=emacs.git

Merge branch 'feature/fix-the-long-lines-display-bug'
---

616da8fa8efa9023f56fa731072d877e2150afbc
diff --cc lisp/obsolete/longlines.el
index 00000000000,d44a634e2e0..1e2ae698c6f
mode 000000,100644..100644
--- a/lisp/obsolete/longlines.el
+++ b/lisp/obsolete/longlines.el
@@@ -1,0 -1,503 +1,512 @@@
+ ;;; longlines.el --- automatically wrap long lines   -*- lexical-binding: t; -*-
+ 
+ ;; Copyright (C) 2000-2001, 2004-2022 Free Software Foundation, Inc.
+ 
+ ;; Authors:    Kai Grossjohann <Kai.Grossjohann@CS.Uni-Dortmund.DE>
+ ;;             Alex Schroeder <alex@gnu.org>
+ ;;             Chong Yidong <cyd@stupidchicken.com>
+ ;; Maintainer: emacs-devel@gnu.org
+ ;; Obsolete-since: 24.4
+ ;; Keywords: convenience, wp
+ 
+ ;; This file is part of GNU Emacs.
+ 
+ ;; GNU Emacs is free software: you can redistribute it and/or modify
+ ;; it under the terms of the GNU General Public License as published by
+ ;; the Free Software Foundation, either version 3 of the License, or
+ ;; (at your option) any later version.
+ 
+ ;; GNU Emacs is distributed in the hope that it will be useful,
+ ;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+ ;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ ;; GNU General Public License for more details.
+ 
+ ;; You should have received a copy of the GNU General Public License
+ ;; along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.
+ 
+ ;;; Commentary:
+ 
+ ;; Some text editors save text files with long lines, and they
+ ;; automatically break these lines at whitespace, without actually
+ ;; inserting any newline characters.  When doing `M-q' in Emacs, you
+ ;; are inserting newline characters.  Longlines mode provides a file
+ ;; format which wraps the long lines when reading a file and unwraps
+ ;; the lines when saving the file.  It can also wrap and unwrap
+ ;; automatically as editing takes place.
+ 
+ ;; Special thanks to Rod Smith for many useful bug reports.
+ 
+ ;;; Code:
+ ;;; Options
+ 
+ (defgroup longlines nil
+   "Automatic wrapping of long lines when loading files."
+   :group 'fill)
+ 
+ (defcustom longlines-auto-wrap t
+   "Non-nil means long lines are automatically wrapped after each command.
+ Otherwise, you can perform filling using `fill-paragraph' or
+ `auto-fill-mode'.  In any case, the soft newlines will be removed
+ when the file is saved to disk."
+   :type 'boolean)
+ 
+ (defcustom longlines-wrap-follows-window-size nil
+   "Non-nil means wrapping and filling happen at the edge of the window.
+ Otherwise, `fill-column' is used, regardless of the window size.  This
+ does not work well when the buffer is displayed in multiple windows
+ with differing widths.
+ 
+ If the value is an integer, that specifies the distance from the
+ right edge of the window at which wrapping occurs.  For any other
+ non-nil value, wrapping occurs 2 characters from the right edge."
+   :type 'boolean)
+ 
+ (defcustom longlines-show-hard-newlines nil
+   "Non-nil means each hard newline is marked on the screen.
+ \(The variable `longlines-show-effect' controls what they look like.)
+ You can also enable the display temporarily, using the command
+ `longlines-show-hard-newlines'."
+   :type 'boolean)
+ 
+ (defcustom longlines-show-effect (propertize "¶\n" 'face 'escape-glyph)
+   "A string to display when showing hard newlines.
+ This is used when `longlines-show-hard-newlines' is on."
+   :type 'string)
+ 
 -(defcustom longlines-breakpoint-chars " ;,|"
++(defcustom longlines-break-chars " ;,|"
+   "A bag of separator chars for longlines."
++  :version "29.1"
+   :type 'string)
+ 
+ ;;; Internal variables
+ 
+ (defvar longlines-wrap-beg nil)
+ (defvar longlines-wrap-end nil)
+ (defvar longlines-wrap-point nil)
+ (defvar longlines-showing nil)
+ (defvar longlines-decoded nil)
+ 
+ (make-variable-buffer-local 'longlines-wrap-beg)
+ (make-variable-buffer-local 'longlines-wrap-end)
+ (make-variable-buffer-local 'longlines-wrap-point)
+ (make-variable-buffer-local 'longlines-showing)
+ (make-variable-buffer-local 'longlines-decoded)
+ 
+ ;;; Mode
+ 
+ (defvar message-indent-citation-function)
+ 
+ ;;;###autoload
+ (define-minor-mode longlines-mode
+   "Toggle Long Lines mode in this buffer.
+ 
+ When Long Lines mode is enabled, long lines are wrapped if they
+ extend beyond `fill-column'.  The soft newlines used for line
+ wrapping will not show up when the text is yanked or saved to
+ disk.
+ 
+ If the variable `longlines-auto-wrap' is non-nil, lines are
+ automatically wrapped whenever the buffer is changed.  You can
+ always call `fill-paragraph' to fill individual paragraphs.
+ 
+ If the variable `longlines-show-hard-newlines' is non-nil, hard
+ newlines are indicated with a symbol."
+   :lighter " ll"
+   (if longlines-mode
+       ;; Turn on longlines mode
+       (progn
+         (use-hard-newlines 1 'never)
+         (set (make-local-variable 'require-final-newline) nil)
+         (add-to-list 'buffer-file-format 'longlines)
+         (add-hook 'change-major-mode-hook #'longlines-mode-off nil t)
+ 	(add-hook 'before-revert-hook #'longlines-before-revert-hook nil t)
 -        (make-local-variable 'buffer-substring-filters)
+         (make-local-variable 'longlines-auto-wrap)
+ 	(set (make-local-variable 'isearch-search-fun-function)
+ 	     #'longlines-search-function)
+ 	(set (make-local-variable 'replace-search-function)
+ 	     #'longlines-search-forward)
+ 	(set (make-local-variable 'replace-re-search-function)
+ 	     #'longlines-re-search-forward)
 -        (add-to-list 'buffer-substring-filters 'longlines-encode-string)
++        (add-function :filter-return (local 'filter-buffer-substring-function)
++                      #'longlines-encode-string)
+         (when longlines-wrap-follows-window-size
+ 	  (let ((dw (if (and (integerp longlines-wrap-follows-window-size)
+ 			     (>= longlines-wrap-follows-window-size 0)
+ 			     (< longlines-wrap-follows-window-size
+ 				(window-width)))
+ 			longlines-wrap-follows-window-size
+ 		      2)))
+ 	    (set (make-local-variable 'fill-column)
+ 		 (- (window-width) dw)))
+           (add-hook 'window-configuration-change-hook
+                     #'longlines-window-change-function nil t))
+         (let ((buffer-undo-list t)
+               (inhibit-read-only t)
+ 	      (inhibit-modification-hooks t)
+               (mod (buffer-modified-p))
+ 	      buffer-file-name buffer-file-truename)
 -          ;; Turning off undo is OK since (spaces + newlines) is
++          ;; Turning off undo is OK since (separators + newlines) is
+           ;; conserved, except for a corner case in
+           ;; longlines-wrap-lines that we'll never encounter from here
+ 	  (save-restriction
+ 	    (widen)
+ 	    (unless longlines-decoded
+ 	      (longlines-decode-buffer)
+ 	      (setq longlines-decoded t))
+ 	    (longlines-wrap-region (point-min) (point-max)))
+           (set-buffer-modified-p mod))
+         (when (and longlines-show-hard-newlines
+                    (not longlines-showing))
+           (longlines-show-hard-newlines))
+ 
+ 	;; Hacks to make longlines play nice with various modes.
+ 	(cond ((eq major-mode 'mail-mode)
+ 	       (declare-function mail-indent-citation "sendmail" ())
+ 	       (add-hook 'mail-setup-hook #'longlines-decode-buffer nil t)
+ 	       (or mail-citation-hook
+ 		   (add-hook 'mail-citation-hook #'mail-indent-citation nil t))
+ 	       (add-hook 'mail-citation-hook #'longlines-decode-region nil t))
+ 	      ((eq major-mode 'message-mode)
+ 	       (add-hook 'message-setup-hook #'longlines-decode-buffer nil t)
+ 	       (make-local-variable 'message-indent-citation-function)
+ 	       (if (not (listp message-indent-citation-function))
+ 		   (setq message-indent-citation-function
+ 			 (list message-indent-citation-function)))
+ 	       (add-hook 'message-indent-citation-function
+ 			 #'longlines-decode-region t t)))
+ 
+ 	(add-hook 'after-change-functions #'longlines-after-change-function nil t)
+ 	(add-hook 'post-command-hook #'longlines-post-command-function nil t)
+         (when longlines-auto-wrap
+           (auto-fill-mode 0)))
+     ;; Turn off longlines mode
+     (setq buffer-file-format (delete 'longlines buffer-file-format))
+     (if longlines-showing
+         (longlines-unshow-hard-newlines))
+     (let ((buffer-undo-list t)
+ 	  (inhibit-modification-hooks t)
+           (inhibit-read-only t)
+ 	  buffer-file-name buffer-file-truename)
+       (if longlines-decoded
+ 	  (save-restriction
+ 	    (widen)
+ 	    (longlines-encode-region (point-min) (point-max))
+ 	    (setq longlines-decoded nil))))
+     (remove-hook 'change-major-mode-hook #'longlines-mode-off t)
+     (remove-hook 'after-change-functions #'longlines-after-change-function t)
+     (remove-hook 'post-command-hook #'longlines-post-command-function t)
+     (remove-hook 'before-revert-hook #'longlines-before-revert-hook t)
+     (remove-hook 'window-configuration-change-hook
+                  #'longlines-window-change-function t)
+     (when longlines-wrap-follows-window-size
+       (kill-local-variable 'fill-column))
+     (kill-local-variable 'isearch-search-fun-function)
+     (kill-local-variable 'replace-search-function)
+     (kill-local-variable 'replace-re-search-function)
+     (kill-local-variable 'require-final-newline)
 -    (kill-local-variable 'buffer-substring-filters)
++    (remove-function (local 'filter-buffer-substring-function)
++                     #'longlines-encode-string)
+     (kill-local-variable 'use-hard-newlines)))
+ 
+ (defun longlines-mode-off ()
+   "Turn off longlines mode.
+ This function exists to be called by `change-major-mode-hook' when the
+ major mode changes."
+   (longlines-mode 0))
+ 
+ ;;; Showing the effect of hard newlines in the buffer
+ 
+ (defun longlines-show-hard-newlines (&optional arg)
+   "Make hard newlines visible by adding a face.
+ With optional argument ARG, make the hard newlines invisible again."
+   (interactive "P")
+     (if arg
+         (longlines-unshow-hard-newlines)
+       (setq longlines-showing t)
+       (longlines-show-region (point-min) (point-max))))
+ 
+ (defun longlines-show-region (beg end)
+   "Make hard newlines between BEG and END visible."
+   (let* ((pmin (min beg end))
+          (pmax (max beg end))
+          (pos (text-property-not-all pmin pmax 'hard nil))
+ 	 (mod (buffer-modified-p))
+ 	 (buffer-undo-list t)
+ 	 (inhibit-read-only t)
+ 	 (inhibit-modification-hooks t)
+ 	 buffer-file-name buffer-file-truename)
+     (while pos
+       (put-text-property pos (1+ pos) 'display
+ 			 (copy-sequence longlines-show-effect))
+       (setq pos (text-property-not-all (1+ pos) pmax 'hard nil)))
+     (restore-buffer-modified-p mod)))
+ 
+ (defun longlines-unshow-hard-newlines ()
+   "Make hard newlines invisible again."
+   (interactive)
+   (setq longlines-showing nil)
+   (let ((pos (text-property-not-all (point-min) (point-max) 'hard nil))
+ 	(mod (buffer-modified-p))
+ 	(buffer-undo-list t)
+ 	(inhibit-read-only t)
+ 	(inhibit-modification-hooks t)
+ 	buffer-file-name buffer-file-truename)
+     (while pos
+       (remove-text-properties pos (1+ pos) '(display nil))
+       (setq pos (text-property-not-all (1+ pos) (point-max) 'hard nil)))
+     (restore-buffer-modified-p mod)))
+ 
+ ;;; Wrapping the paragraphs
+ 
+ (defun longlines-wrap-region (beg end)
+   "Wrap each successive line, starting with the line before BEG.
+ Stop when we reach lines after END that don't need wrapping, or the
+ end of the buffer."
+   (let ((mod (buffer-modified-p)))
+     (setq longlines-wrap-point (point))
+     (goto-char beg)
+     (forward-line -1)
+     ;; Two successful longlines-wrap-line's in a row mean successive
+     ;; lines don't need wrapping.
+     (while (null (and (longlines-wrap-line)
+ 		      (or (eobp)
+ 			  (and (>= (point) end)
+ 			       (longlines-wrap-line))))))
+     (goto-char longlines-wrap-point)
+     (set-buffer-modified-p mod)))
+ 
+ (defun longlines-wrap-line ()
+   "If the current line needs to be wrapped, wrap it and return nil.
+ If wrapping is performed, point remains on the line.  If the line does
+ not need to be wrapped, move point to the next line and return t."
+   (if (longlines-set-breakpoint fill-column)
+       (progn (insert-before-markers-and-inherit ?\n)
+              nil)
+     (if (longlines-merge-lines-p)
+         (progn (end-of-line)
+      ;; After certain commands (e.g. kill-line), there may be two
+      ;; successive soft newlines in the buffer.  In this case, we
+      ;; replace these two newlines by a single space.  Unfortunately,
+      ;; this breaks the conservation of (spaces + newlines), so we
+      ;; have to fiddle with longlines-wrap-point.
+                (if (or (prog1 (bolp) (forward-char 1)) (eolp))
+ 	           (progn
+ 	             (delete-char -1)
+ 	             (if (> longlines-wrap-point (point))
+ 		         (setq longlines-wrap-point
+ 		               (1- longlines-wrap-point))))
+ 	         (delete-char -1))
+                nil)
+       (forward-line 1)
+       t)))
+ 
+ (defun longlines-set-breakpoint (target-column)
+   "Place point where we should break the current line, and return t.
+ If the line should not be broken, return nil; point remains on the
+ line."
+   (move-to-column target-column)
 -  (let ((non-breakpoint-re (format "[^%s]" longlines-breakpoint-chars)))
 -    (if (and (re-search-forward non-breakpoint-re (line-end-position) t 1)
++  (let ((non-break-re (format "[^%s]" longlines-break-chars)))
++    (if (and (re-search-forward non-break-re (line-end-position) t 1)
+              (> (current-column) target-column))
+         ;; This line is too long.  Can we break it?
+         (or (longlines-find-break-backward)
+             (progn (move-to-column target-column)
+                    (longlines-find-break-forward))))))
+ 
+ (defun longlines-find-break-backward ()
+   "Move point backward to the first available breakpoint and return t.
+ If no breakpoint is found, return nil."
 -  (let ((breakpoint-re (format "[%s]" longlines-breakpoint-chars)))
 -    (when (and (re-search-backward breakpoint-re (line-beginning-position) t 1)
++  (let ((break-re (format "[%s]" longlines-break-chars)))
++    (when (and (re-search-backward break-re (line-beginning-position) t 1)
+                (save-excursion
 -                 (skip-chars-backward longlines-breakpoint-chars
++                 (skip-chars-backward longlines-break-chars
+                                       (line-beginning-position))
+                  (null (bolp))))
+       (forward-char 1)
+       (if (and fill-nobreak-predicate
+                (run-hook-with-args-until-success 'fill-nobreak-predicate))
+           (progn
 -            (skip-chars-backward longlines-breakpoint-chars
++            (skip-chars-backward longlines-break-chars
+                                  (line-beginning-position))
+             (longlines-find-break-backward))
+         t))))
+ 
+ (defun longlines-find-break-forward ()
+   "Move point forward to the first available breakpoint and return t.
+ If no break point is found, return nil."
 -  (let ((breakpoint-re (format "[%s]" longlines-breakpoint-chars)))
 -    (and (re-search-forward breakpoint-re (line-end-position) t 1)
++  (let ((break-re (format "[%s]" longlines-break-chars)))
++    (and (re-search-forward break-re (line-end-position) t 1)
+          (progn
 -           (skip-chars-forward longlines-breakpoint-chars (line-end-position))
++           (skip-chars-forward longlines-break-chars (line-end-position))
+            (null (eolp)))
+          (if (and fill-nobreak-predicate
+                   (run-hook-with-args-until-success 'fill-nobreak-predicate))
+              (longlines-find-break-forward)
+            t))))
+ 
+ (defun longlines-merge-lines-p ()
+   "Return t if part of the next line can fit onto the current line.
+ Otherwise, return nil.  Text cannot be moved across hard newlines."
+   (save-excursion
+     (end-of-line)
+     (and (null (eobp))
+          (null (get-text-property (point) 'hard))
+          (let ((space (- fill-column (current-column))))
+            (forward-line 1)
+            (longlines-set-breakpoint (max 0 (1- space)))))))
+ 
+ (defun longlines-decode-region (&optional beg end)
+   "Turn all newlines between BEG and END into hard newlines.
+ If BEG and END are nil, the point and mark are used."
+   (if (null beg) (setq beg (point)))
+   (if (null end) (setq end (mark t)))
+   (save-excursion
+     (let ((reg-max (max beg end)))
+       (goto-char (min beg end))
+       (while (search-forward "\n" reg-max t)
+ 	(set-hard-newline-properties
+ 	 (match-beginning 0) (match-end 0))))))
+ 
+ (defun longlines-decode-buffer ()
+   "Turn all newlines in the buffer into hard newlines."
+   (longlines-decode-region (point-min) (point-max)))
+ 
+ (defun longlines-encode-region (beg end &optional _buffer)
+   "Remove each soft newline between BEG and END.
+ Hard newlines are left intact.  The optional argument BUFFER exists for
+ compatibility with `format-alist', and is ignored."
+   (save-excursion
+     (let ((reg-max (max beg end))
+ 	  (mod (buffer-modified-p)))
+       (goto-char (min beg end))
+       (while (search-forward "\n" reg-max t)
+ 	(let ((pos (match-beginning 0)))
+ 	  (unless (get-text-property pos 'hard)
+             (remove-text-properties pos (1+ pos) '(hard nil))
+             (delete-region pos (1+ pos)))))
+       (set-buffer-modified-p mod)
+       end)))
+ 
+ (defun longlines-encode-string (string)
 -  "Return a copy of STRING with each soft newline replaced by a space.
++  "Return a copy of STRING with each soft newline removed.
+ Hard newlines are left intact."
 -  (let* ((str (copy-sequence string))
 -         (pos (string-search "\n" str)))
 -    (while pos
 -      (if (null (get-text-property pos 'hard str))
 -          (aset str pos ? ))
 -      (setq pos (string-search "\n" str (1+ pos))))
 -    str))
++  (let ((start 0)
++        (result nil)
++        pos)
++    (while (setq pos (string-search "\n" string start))
++      (unless (= start pos)
++        (push (substring string start pos) result))
++      (when (get-text-property pos 'hard string)
++        (push (substring string pos (1+ pos)) result))
++      (setq start (1+ pos)))
++    (if (null result)
++        (copy-sequence string)
++      (unless (= start (length string))
++        (push (substring string start) result))
++      (apply #'concat (nreverse result)))))
+ 
+ ;;; Auto wrap
+ 
+ (defun longlines-auto-wrap (&optional arg)
+   "Toggle automatic line wrapping.
+ With optional argument ARG, turn on line wrapping if and only if
+ ARG is positive.
+ If automatic line wrapping is turned on, wrap the entire buffer."
+   (interactive "P")
+   (setq arg (if arg
+ 		(> (prefix-numeric-value arg) 0)
+ 	      (not longlines-auto-wrap)))
+   (if arg
+       (progn
+ 	(setq longlines-auto-wrap t)
+ 	(longlines-wrap-region (point-min) (point-max))
+ 	(message "Auto wrap enabled."))
+     (setq longlines-auto-wrap nil)
+     (message "Auto wrap disabled.")))
+ 
+ (defun longlines-after-change-function (beg end _len)
+   "Update `longlines-wrap-beg' and `longlines-wrap-end'.
+ This is called by `after-change-functions' to keep track of the region
+ that has changed."
+   (when (and longlines-auto-wrap (not undo-in-progress))
+     (setq longlines-wrap-beg
+           (if longlines-wrap-beg (min longlines-wrap-beg beg) beg))
+     (setq longlines-wrap-end
+           (if longlines-wrap-end (max longlines-wrap-end end) end))))
+ 
+ (defun longlines-post-command-function ()
+   "Perform line wrapping on the parts of the buffer that have changed.
+ This is called by `post-command-hook' after each command."
+   (when (and longlines-auto-wrap longlines-wrap-beg)
+     (if (or (eq this-command 'yank)
+ 	    (eq this-command 'yank-pop))
+ 	(longlines-decode-region (point) (mark t)))
+     (if longlines-showing
+ 	(longlines-show-region longlines-wrap-beg longlines-wrap-end))
+     (unless (or (eq this-command 'fill-paragraph)
+                 (eq this-command 'fill-region))
+       (longlines-wrap-region longlines-wrap-beg longlines-wrap-end))
+     (setq longlines-wrap-beg nil)
+     (setq longlines-wrap-end nil)))
+ 
+ (defun longlines-window-change-function ()
+   "Re-wrap the buffer if the window width has changed.
+ This is called by `window-configuration-change-hook'."
+   (let ((dw (if (and (integerp longlines-wrap-follows-window-size)
+ 		     (>= longlines-wrap-follows-window-size 0)
+ 		     (< longlines-wrap-follows-window-size (window-width)))
+ 		longlines-wrap-follows-window-size
+ 	      2)))
+     (when (/= fill-column (- (window-width) dw))
+       (setq fill-column (- (window-width) dw))
+       (longlines-wrap-region (point-min) (point-max)))))
+ 
+ ;;; Isearch
+ 
+ (defun longlines-search-function ()
+   (cond
+    ((or isearch-regexp-function isearch-regexp) (isearch-search-fun-default))
+    (isearch-forward #'longlines-search-forward)
+    (t #'longlines-search-backward)))
+ 
+ (defun longlines-search-forward (string &optional bound noerror count)
+   (let ((search-spaces-regexp " *[ \n]"))
+     (re-search-forward (regexp-quote string) bound noerror count)))
+ 
+ (defun longlines-search-backward (string &optional bound noerror count)
+   (let ((search-spaces-regexp " *[ \n]"))
+     (re-search-backward (regexp-quote string) bound noerror count)))
+ 
+ (defun longlines-re-search-forward (string &optional bound noerror count)
+   (let ((search-spaces-regexp " *[ \n]"))
+     (re-search-forward string bound noerror count)))
+ 
+ ;;; Loading and saving
+ 
+ (defun longlines-before-revert-hook ()
+   (add-hook 'after-revert-hook #'longlines-after-revert-hook nil t)
+   (longlines-mode 0))
+ 
+ (defun longlines-after-revert-hook ()
+   (remove-hook 'after-revert-hook #'longlines-after-revert-hook t)
+   (longlines-mode 1))
+ 
+ (add-to-list
+  'format-alist
+  (list 'longlines "Automatically wrap long lines." nil nil
+        #'longlines-encode-region t nil))
+ 
+ ;;; Unloading
+ 
+ (defun longlines-unload-function ()
+   "Unload the longlines library."
+   (save-current-buffer
+     (dolist (buffer (buffer-list))
+       (set-buffer buffer)
+       (longlines-mode-off)))
+   ;; continue standard unloading
+   nil)
+ 
+ (provide 'longlines)
+ 
+ ;;; longlines.el ends here
diff --cc src/composite.c
index 2dee42b5503,4f7b4b0a2c7..b04d34337b4
--- a/src/composite.c
+++ b/src/composite.c
@@@ -1589,15 -1586,12 +1590,20 @@@ find_automatic_composition (ptrdiff_t p
    cur.pos = pos;
    if (NILP (string))
      {
 -      head = backlim < 0 ? BEGV : backlim, tail = ZV, stop = GPT;
 +      if (backlim < 0)
 +	{
 +	  /* This assumes a newline can never be composed.  */
 +	  head = find_newline (pos, -1, 0, -1, -1, NULL, NULL, false);
 +	}
 +      else
 +	head = backlim;
+       /* In buffers with very long lines, this function becomes very
+ 	 slow.  Pretend that the buffer is narrowed to make it fast.  */
+       narrowed_begv = get_narrowed_begv (w);
+       if (narrowed_begv && pos > narrowed_begv)
+ 	head = narrowed_begv;
 +      tail = ZV;
 +      stop = GPT;
        cur.pos_byte = CHAR_TO_BYTE (cur.pos);
        cur.p = BYTE_POS_ADDR (cur.pos_byte);
      }
diff --cc src/insdel.c
index 9b292398537,309ca728a4c..38d5fbda002
--- a/src/insdel.c
+++ b/src/insdel.c
@@@ -1122,8 -1122,7 +1122,8 @@@ insert_from_gap (ptrdiff_t nchars, ptrd
       of this dance.  */
    invalidate_buffer_caches (current_buffer, GPT, GPT);
    record_insert (GPT, nchars);
-   modiff_incr (&MODIFF);
+   modiff_incr (&MODIFF, nchars);
 +  CHARS_MODIFF = MODIFF;
  
    insert_from_gap_1 (nchars, nbytes, text_at_gap_tail);