Merge branch 'feature/improved-locked-narrowing'

6 files changed, 407 insertions, 91 deletions

diff --git a/src/buffer.c b/src/buffer.c
index ac7f4f8e9d4..71be7ed9e13 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -5898,7 +5898,42 @@ this threshold.
 If nil, these display shortcuts will always remain disabled.
 
 There is no reason to change that value except for debugging purposes.  */);
-  XSETFASTINT (Vlong_line_threshold, 10000);
+  XSETFASTINT (Vlong_line_threshold, 50000);
+
+  DEFVAR_INT ("long-line-locked-narrowing-region-size",
+              long_line_locked_narrowing_region_size,
+              doc: /* Region size for locked narrowing in buffers with long lines.
+
+This variable has effect only in buffers which contain one or more
+lines whose length is above `long-line-threshold', which see.  For
+performance reasons, in such buffers, low-level hooks such as
+`fontification-functions' or `post-command-hook' are executed on a
+narrowed buffer, with a narrowing locked with `narrowing-lock'.  This
+variable specifies the size of the narrowed region around point.
+
+To disable that narrowing, set this variable to 0.
+
+See also `long-line-locked-narrowing-bol-search-limit'.
+
+There is no reason to change that value except for debugging purposes.  */);
+  long_line_locked_narrowing_region_size = 500000;
+
+  DEFVAR_INT ("long-line-locked-narrowing-bol-search-limit",
+              long_line_locked_narrowing_bol_search_limit,
+              doc: /* Limit for beginning of line search in buffers with long lines.
+
+This variable has effect only in buffers which contain one or more
+lines whose length is above `long-line-threshold', which see.  For
+performance reasons, in such buffers, low-level hooks such as
+`fontification-functions' or `post-command-hook' are executed on a
+narrowed buffer, with a narrowing locked with `narrowing-lock'.  The
+variable `long-line-locked-narrowing-region-size' specifies the size
+of the narrowed region around point.  This variable, which should be a
+small integer, specifies the number of characters by which that region
+can be extended backwards to make it start at the beginning of a line.
+
+There is no reason to change that value except for debugging purposes.  */);
+  long_line_locked_narrowing_bol_search_limit = 128;
 
   DEFVAR_INT ("large-hscroll-threshold", large_hscroll_threshold,
     doc: /* Horizontal scroll of truncated lines above which to use redisplay shortcuts.
diff --git a/src/dispextern.h b/src/dispextern.h
index 2afbdeabaab..df6134e68f0 100644
--- a/src/dispextern.h
+++ b/src/dispextern.h
@@ -2342,6 +2342,14 @@ struct it
      optimize display.  */
   ptrdiff_t narrowed_zv;
 
+  /* Begin position of the buffer for the locked narrowing around
+     low-level hooks.  */
+  ptrdiff_t locked_narrowing_begv;
+
+  /* End position of the buffer for the locked narrowing around
+     low-level hooks.  */
+  ptrdiff_t locked_narrowing_zv;
+
   /* C string to iterate over.  Non-null means get characters from
      this string, otherwise characters are read from current_buffer
      or it->string.  */
@@ -3405,6 +3413,8 @@ void init_iterator (struct it *, struct window *, ptrdiff_t,
 ptrdiff_t get_narrowed_begv (struct window *, ptrdiff_t);
 ptrdiff_t get_narrowed_zv (struct window *, ptrdiff_t);
 ptrdiff_t get_closer_narrowed_begv (struct window *, ptrdiff_t);
+ptrdiff_t get_locked_narrowing_begv (ptrdiff_t);
+ptrdiff_t get_locked_narrowing_zv (ptrdiff_t);
 void init_iterator_to_row_start (struct it *, struct window *,
                                  struct glyph_row *);
 void start_display (struct it *, struct window *, struct text_pos);
diff --git a/src/editfns.c b/src/editfns.c
index 17dca4708ed..b364f441b53 100644
--- a/src/editfns.c
+++ b/src/editfns.c
@@ -2653,88 +2653,216 @@ DEFUN ("delete-and-extract-region", Fdelete_and_extract_region,
   return del_range_1 (XFIXNUM (start), XFIXNUM (end), 1, 1);
 }
 
-DEFUN ("widen", Fwiden, Swiden, 0, 0, "",
-       doc: /* Remove restrictions (narrowing) from current buffer.
-This allows the buffer's full text to be seen and edited.
+/* Alist of buffers in which locked narrowing is used.  The car of
+   each list element is a buffer, the cdr is a list of triplets (tag
+   begv-marker zv-marker).  The last element of that list always uses
+   the (uninterned) Qoutermost_narrowing tag and records the narrowing
+   bounds that were set by the user and that are visible on display.
+   This alist is used internally by narrow-to-region, widen,
+   narrowing-lock, narrowing-unlock and save-restriction.  */
+static Lisp_Object narrowing_locks;
+
+/* Add BUF with its LOCKS in the narrowing_locks alist.  */
+static void
+narrowing_locks_add (Lisp_Object buf, Lisp_Object locks)
+{
+  narrowing_locks = nconc2 (list1 (list2 (buf, locks)), narrowing_locks);
+}
 
-Note that, when the current buffer contains one or more lines whose
-length is above `long-line-threshold', Emacs may decide to leave, for
-performance reasons, the accessible portion of the buffer unchanged
-after this function is called from low-level hooks, such as
-`jit-lock-functions' or `post-command-hook'.  */)
-  (void)
+/* Remove BUF and its locks from the narrowing_locks alist.  Do
+   nothing if BUF is not present in narrowing_locks.  */
+static void
+narrowing_locks_remove (Lisp_Object buf)
+{
+  narrowing_locks = Fdelq (Fassoc (buf, narrowing_locks, Qnil),
+                           narrowing_locks);
+}
+
+/* Retrieve one of the BEGV/ZV bounds of a narrowing in BUF from the
+   narrowing_locks alist, as a pointer to a struct Lisp_Marker, or
+   NULL if BUF is not in narrowing_locks or is a killed buffer.  When
+   OUTERMOST is true, the bounds that were set by the user and that
+   are visible on display are returned.  Otherwise the innermost
+   locked narrowing bounds are returned.  */
+static struct Lisp_Marker *
+narrowing_lock_get_bound (Lisp_Object buf, bool begv, bool outermost)
+{
+  if (NILP (Fbuffer_live_p (buf)))
+    return NULL;
+  Lisp_Object buffer_locks = assq_no_quit (buf, narrowing_locks);
+  if (NILP (buffer_locks))
+    return NULL;
+  buffer_locks = XCAR (XCDR (buffer_locks));
+  Lisp_Object bounds
+    = outermost
+      ? XCDR (assq_no_quit (Qoutermost_narrowing, buffer_locks))
+      : XCDR (XCAR (buffer_locks));
+  eassert (! NILP (bounds));
+  Lisp_Object marker = begv ? XCAR (bounds) : XCAR (XCDR (bounds));
+  eassert (EQ (Fmarker_buffer (marker), buf));
+  return XMARKER (marker);
+}
+
+/* Retrieve the tag of the innermost narrowing in BUF.  Return nil if
+   BUF is not in narrowing_locks or is a killed buffer.  */
+static Lisp_Object
+narrowing_lock_peek_tag (Lisp_Object buf)
 {
-  if (! NILP (Vrestrictions_locked))
+  if (NILP (Fbuffer_live_p (buf)))
     return Qnil;
-  if (BEG != BEGV || Z != ZV)
-    current_buffer->clip_changed = 1;
-  BEGV = BEG;
-  BEGV_BYTE = BEG_BYTE;
-  SET_BUF_ZV_BOTH (current_buffer, Z, Z_BYTE);
-  /* Changing the buffer bounds invalidates any recorded current column.  */
-  invalidate_current_column ();
-  return Qnil;
+  Lisp_Object buffer_locks = assq_no_quit (buf, narrowing_locks);
+  if (NILP (buffer_locks))
+    return Qnil;
+  Lisp_Object tag = XCAR (XCAR (XCAR (XCDR (buffer_locks))));
+  eassert (! NILP (tag));
+  return tag;
 }
 
+/* Add a LOCK for BUF in the narrowing_locks alist.  */
 static void
-unwind_locked_begv (Lisp_Object point_min)
+narrowing_lock_push (Lisp_Object buf, Lisp_Object lock)
 {
-  SET_BUF_BEGV (current_buffer, XFIXNUM (point_min));
+  Lisp_Object buffer_locks = assq_no_quit (buf, narrowing_locks);
+  if (NILP (buffer_locks))
+    narrowing_locks_add (buf, list1 (lock));
+  else
+    XSETCDR (buffer_locks, list1 (nconc2 (list1 (lock),
+                                          XCAR (XCDR (buffer_locks)))));
 }
 
+/* Remove the innermost lock in BUF from the narrowing_locks alist.
+   Do nothing if BUF is not present in narrowing_locks.  */
 static void
-unwind_locked_zv (Lisp_Object point_max)
+narrowing_lock_pop (Lisp_Object buf)
 {
-  SET_BUF_ZV (current_buffer, XFIXNUM (point_max));
+  Lisp_Object buffer_locks = assq_no_quit (buf, narrowing_locks);
+  if (NILP (buffer_locks))
+    return;
+  if (EQ (narrowing_lock_peek_tag (buf), Qoutermost_narrowing))
+    narrowing_locks_remove (buf);
+  else
+    XSETCDR (buffer_locks, list1 (XCDR (XCAR (XCDR (buffer_locks)))));
 }
 
-/* Internal function for Fnarrow_to_region, meant to be used with a
-   third argument 'true', in which case it should be followed by "specbind
-   (Qrestrictions_locked, Qt)".  */
-Lisp_Object
-narrow_to_region_internal (Lisp_Object start, Lisp_Object end, bool lock)
+static void
+unwind_reset_outermost_narrowing (Lisp_Object buf)
 {
-  EMACS_INT s = fix_position (start), e = fix_position (end);
-
-  if (e < s)
+  struct Lisp_Marker *begv = narrowing_lock_get_bound (buf, true, false);
+  struct Lisp_Marker *zv = narrowing_lock_get_bound (buf, false, false);
+  if (begv != NULL && zv != NULL)
     {
-      EMACS_INT tem = s; s = e; e = tem;
+      SET_BUF_BEGV_BOTH (XBUFFER (buf), begv->charpos, begv->bytepos);
+      SET_BUF_ZV_BOTH (XBUFFER (buf), zv->charpos, zv->bytepos);
     }
+  else
+    narrowing_locks_remove (buf);
+}
 
-  if (lock)
+/* Restore the narrowing bounds that were set by the user, and restore
+   the bounds of the locked narrowing upon return.
+   In particular, this function is called when redisplay starts, so
+   that if a Lisp function executed during redisplay calls (redisplay)
+   while a locked narrowing is in effect, the locked narrowing will
+   not be visible on display.  */
+void
+reset_outermost_narrowings (void)
+{
+  Lisp_Object val, buf;
+  for (val = narrowing_locks; CONSP (val); val = XCDR (val))
     {
-      if (!(BEGV <= s && s <= e && e <= ZV))
-        args_out_of_range (start, end);
+      buf = XCAR (XCAR (val));
+      eassert (BUFFERP (buf));
+      struct Lisp_Marker *begv = narrowing_lock_get_bound (buf, true, true);
+      struct Lisp_Marker *zv = narrowing_lock_get_bound (buf, false, true);
+      if (begv != NULL && zv != NULL)
+        {
+          SET_BUF_BEGV_BOTH (XBUFFER (buf), begv->charpos, begv->bytepos);
+          SET_BUF_ZV_BOTH (XBUFFER (buf), zv->charpos, zv->bytepos);
+          record_unwind_protect (unwind_reset_outermost_narrowing, buf);
+        }
+      else
+        narrowing_locks_remove (buf);
+    }
+}
 
-      if (BEGV != s || ZV != e)
-        current_buffer->clip_changed = 1;
+/* Helper functions to save and restore the narrowing locks of the
+   current buffer in Fsave_restriction.  */
+static Lisp_Object
+narrowing_locks_save (void)
+{
+  Lisp_Object buf = Fcurrent_buffer ();
+  Lisp_Object locks = assq_no_quit (buf, narrowing_locks);
+  if (NILP (locks))
+    return Qnil;
+  locks = XCAR (XCDR (locks));
+  return Fcons (buf, Fcopy_sequence (locks));
+}
 
-      record_unwind_protect (restore_point_unwind, Fpoint_marker ());
-      record_unwind_protect (unwind_locked_begv, Fpoint_min ());
-      record_unwind_protect (unwind_locked_zv, Fpoint_max ());
+static void
+narrowing_locks_restore (Lisp_Object buf_and_saved_locks)
+{
+  if (NILP (buf_and_saved_locks))
+    return;
+  Lisp_Object buf = XCAR (buf_and_saved_locks);
+  Lisp_Object saved_locks = XCDR (buf_and_saved_locks);
+  narrowing_locks_remove (buf);
+  narrowing_locks_add (buf, saved_locks);
+}
 
-      SET_BUF_BEGV (current_buffer, s);
-      SET_BUF_ZV (current_buffer, e);
+static void
+unwind_narrow_to_region_locked (Lisp_Object tag)
+{
+  Fnarrowing_unlock (tag);
+  Fwiden ();
+}
+
+/* Narrow current_buffer to BEGV-ZV with a narrowing locked with TAG.  */
+void
+narrow_to_region_locked (Lisp_Object begv, Lisp_Object zv, Lisp_Object tag)
+{
+  Fnarrow_to_region (begv, zv);
+  Fnarrowing_lock (tag);
+  record_unwind_protect (restore_point_unwind, Fpoint_marker ());
+  record_unwind_protect (unwind_narrow_to_region_locked, tag);
+}
+
+DEFUN ("widen", Fwiden, Swiden, 0, 0, "",
+       doc: /* Remove restrictions (narrowing) from current buffer.
+
+This allows the buffer's full text to be seen and edited, unless
+restrictions have been locked with `narrowing-lock', which see, in
+which case the narrowing that was current when `narrowing-lock' was
+called is restored.  */)
+  (void)
+{
+  Fset (Qoutermost_narrowing, Qnil);
+  Lisp_Object buf = Fcurrent_buffer ();
+  Lisp_Object tag = narrowing_lock_peek_tag (buf);
+
+  if (NILP (tag))
+    {
+      if (BEG != BEGV || Z != ZV)
+        current_buffer->clip_changed = 1;
+      BEGV = BEG;
+      BEGV_BYTE = BEG_BYTE;
+      SET_BUF_ZV_BOTH (current_buffer, Z, Z_BYTE);
     }
   else
     {
-      if (! NILP (Vrestrictions_locked))
-        return Qnil;
-
-      if (!(BEG <= s && s <= e && e <= Z))
-        args_out_of_range (start, end);
-
-      if (BEGV != s || ZV != e)
+      struct Lisp_Marker *begv = narrowing_lock_get_bound (buf, true, false);
+      struct Lisp_Marker *zv = narrowing_lock_get_bound (buf, false, false);
+      eassert (begv != NULL && zv != NULL);
+      if (begv->charpos != BEGV || zv->charpos != ZV)
         current_buffer->clip_changed = 1;
-
-      SET_BUF_BEGV (current_buffer, s);
-      SET_BUF_ZV (current_buffer, e);
+      SET_BUF_BEGV_BOTH (current_buffer, begv->charpos, begv->bytepos);
+      SET_BUF_ZV_BOTH (current_buffer, zv->charpos, zv->bytepos);
+      /* If the only remaining bounds in narrowing_locks for
+         current_buffer are the bounds that were set by the user, no
+         locked narrowing is in effect in current_buffer anymore:
+         remove it from the narrowing_locks alist.  */
+      if (EQ (tag, Qoutermost_narrowing))
+        narrowing_lock_pop (buf);
     }
-
-  if (PT < s)
-    SET_PT (s);
-  if (e < PT)
-    SET_PT (e);
   /* Changing the buffer bounds invalidates any recorded current column.  */
   invalidate_current_column ();
   return Qnil;
@@ -2751,14 +2879,110 @@ When calling from Lisp, pass two arguments START and END:
 positions (integers or markers) bounding the text that should
 remain visible.
 
-Note that, when the current buffer contains one or more lines whose
-length is above `long-line-threshold', Emacs may decide to leave, for
-performance reasons, the accessible portion of the buffer unchanged
-after this function is called from low-level hooks, such as
-`jit-lock-functions' or `post-command-hook'.  */)
+When restrictions have been locked with `narrowing-lock', which see,
+`narrow-to-region' can be used only within the limits of the
+restrictions that were current when `narrowing-lock' was called.  If
+the START or END arguments are outside these limits, the corresponding
+limit of the locked restriction is used instead of the argument.  */)
   (Lisp_Object start, Lisp_Object end)
 {
-  return narrow_to_region_internal (start, end, false);
+  EMACS_INT s = fix_position (start), e = fix_position (end);
+
+  if (e < s)
+    {
+      EMACS_INT tem = s; s = e; e = tem;
+    }
+
+  if (!(BEG <= s && s <= e && e <= Z))
+    args_out_of_range (start, end);
+
+  Lisp_Object buf = Fcurrent_buffer ();
+  if (! NILP (narrowing_lock_peek_tag (buf)))
+    {
+      struct Lisp_Marker *begv = narrowing_lock_get_bound (buf, true, false);
+      struct Lisp_Marker *zv = narrowing_lock_get_bound (buf, false, false);
+      eassert (begv != NULL && zv != NULL);
+      /* Limit the start and end positions to those of the locked
+         narrowing.  */
+      if (s < begv->charpos) s = begv->charpos;
+      if (s > zv->charpos) s = zv->charpos;
+      if (e < begv->charpos) e = begv->charpos;
+      if (e > zv->charpos) e = zv->charpos;
+    }
+
+  /* Record the accessible range of the buffer when narrow-to-region
+     is called, that is, before applying the narrowing.  It is used
+     only by narrowing-lock.  */
+  Fset (Qoutermost_narrowing, list3 (Qoutermost_narrowing,
+                                     Fpoint_min_marker (),
+                                     Fpoint_max_marker ()));
+
+  if (BEGV != s || ZV != e)
+    current_buffer->clip_changed = 1;
+
+  SET_BUF_BEGV (current_buffer, s);
+  SET_BUF_ZV (current_buffer, e);
+
+  if (PT < s)
+    SET_PT (s);
+  if (e < PT)
+    SET_PT (e);
+  /* Changing the buffer bounds invalidates any recorded current column.  */
+  invalidate_current_column ();
+  return Qnil;
+}
+
+DEFUN ("narrowing-lock", Fnarrowing_lock, Snarrowing_lock, 1, 1, 0,
+       doc: /* Lock the current narrowing with TAG.
+
+When restrictions are locked, `narrow-to-region' and `widen' can be
+used only within the limits of the restrictions that were current when
+`narrowing-lock' was called, unless the lock is removed by calling
+`narrowing-unlock' with TAG.
+
+Locking restrictions should be used sparingly, after carefully
+considering the potential adverse effects on the code that will be
+executed within locked restrictions.  It is typically meant to be used
+around portions of code that would become too slow, and make Emacs
+unresponsive, if they were executed in a large buffer.  For example,
+restrictions are locked by Emacs around low-level hooks such as
+`fontification-functions' or `post-command-hook'.
+
+Locked restrictions are never visible on display, and can therefore
+not be used as a stronger variant of normal restrictions.  */)
+  (Lisp_Object tag)
+{
+  Lisp_Object buf = Fcurrent_buffer ();
+  Lisp_Object outermost_narrowing
+    = buffer_local_value (Qoutermost_narrowing, buf);
+  /* If narrowing-lock is called without being preceded by
+     narrow-to-region, do nothing.  */
+  if (NILP (outermost_narrowing))
+    return Qnil;
+  if (NILP (narrowing_lock_peek_tag (buf)))
+    narrowing_lock_push (buf, outermost_narrowing);
+  narrowing_lock_push (buf, list3 (tag,
+                                   Fpoint_min_marker (),
+                                   Fpoint_max_marker ()));
+  return Qnil;
+}
+
+DEFUN ("narrowing-unlock", Fnarrowing_unlock, Snarrowing_unlock, 1, 1, 0,
+       doc: /* Unlock a narrowing locked with (narrowing-lock TAG).
+
+Unlocking restrictions locked with `narrowing-lock' should be used
+sparingly, after carefully considering the reasons why restrictions
+were locked.  Restrictions are typically locked around portions of
+code that would become too slow, and make Emacs unresponsive, if they
+were executed in a large buffer.  For example, restrictions are locked
+by Emacs around low-level hooks such as `fontification-functions' or
+`post-command-hook'.  */)
+  (Lisp_Object tag)
+{
+  Lisp_Object buf = Fcurrent_buffer ();
+  if (EQ (narrowing_lock_peek_tag (buf), tag))
+    narrowing_lock_pop (buf);
+  return Qnil;
 }
 
 Lisp_Object
@@ -2858,11 +3082,12 @@ DEFUN ("save-restriction", Fsave_restriction, Ssave_restriction, 0, UNEVALLED, 0
        doc: /* Execute BODY, saving and restoring current buffer's restrictions.
 The buffer's restrictions make parts of the beginning and end invisible.
 \(They are set up with `narrow-to-region' and eliminated with `widen'.)
-This special form, `save-restriction', saves the current buffer's restrictions
-when it is entered, and restores them when it is exited.
+This special form, `save-restriction', saves the current buffer's
+restrictions, as well as their locks if they have been locked with
+`narrowing-lock', when it is entered, and restores them when it is exited.
 So any `narrow-to-region' within BODY lasts only until the end of the form.
-The old restrictions settings are restored
-even in case of abnormal exit (throw or error).
+The old restrictions settings are restored even in case of abnormal exit
+\(throw or error).
 
 The value returned is the value of the last form in BODY.
 
@@ -2877,6 +3102,7 @@ usage: (save-restriction &rest BODY)  */)
   specpdl_ref count = SPECPDL_INDEX ();
 
   record_unwind_protect (save_restriction_restore, save_restriction_save ());
+  record_unwind_protect (narrowing_locks_restore, narrowing_locks_save ());
   val = Fprogn (body);
   return unbind_to (count, val);
 }
@@ -4518,6 +4744,8 @@ syms_of_editfns (void)
   DEFSYM (Qwall, "wall");
   DEFSYM (Qpropertize, "propertize");
 
+  staticpro (&narrowing_locks);
+
   DEFVAR_LISP ("inhibit-field-text-motion", Vinhibit_field_text_motion,
                doc: /* Non-nil means text motion commands don't notice fields.  */);
   Vinhibit_field_text_motion = Qnil;
@@ -4577,11 +4805,12 @@ This variable is experimental; email 32252@debbugs.gnu.org if you need
 it to be non-nil.  */);
   binary_as_unsigned = false;
 
-  DEFSYM (Qrestrictions_locked, "restrictions-locked");
-  DEFVAR_LISP ("restrictions-locked", Vrestrictions_locked,
-               doc: /* If non-nil, restrictions are currently locked.  */);
-  Vrestrictions_locked = Qnil;
-  Funintern (Qrestrictions_locked, Qnil);
+  DEFVAR_LISP ("outermost-narrowing", Voutermost_narrowing,
+               doc: /* Outermost narrowing bounds, if any.  Internal use only.  */);
+  Voutermost_narrowing = Qnil;
+  Fmake_variable_buffer_local (Qoutermost_narrowing);
+  DEFSYM (Qoutermost_narrowing, "outermost-narrowing");
+  Funintern (Qoutermost_narrowing, Qnil);
 
   defsubr (&Spropertize);
   defsubr (&Schar_equal);
@@ -4674,6 +4903,8 @@ it to be non-nil.  */);
   defsubr (&Sdelete_and_extract_region);
   defsubr (&Swiden);
   defsubr (&Snarrow_to_region);
+  defsubr (&Snarrowing_lock);
+  defsubr (&Snarrowing_unlock);
   defsubr (&Ssave_restriction);
   defsubr (&Stranspose_regions);
 }
diff --git a/src/keyboard.c b/src/keyboard.c
index 811998823cc..b82a5e1a3ef 100644
--- a/src/keyboard.c
+++ b/src/keyboard.c
@@ -1911,9 +1911,9 @@ safe_run_hooks_maybe_narrowed (Lisp_Object hook, struct window *w)
   specbind (Qinhibit_quit, Qt);
 
   if (current_buffer->long_line_optimizations_p)
-    narrow_to_region_internal (make_fixnum (get_narrowed_begv (w, PT)),
-                               make_fixnum (get_narrowed_zv (w, PT)),
-                               true);
+    narrow_to_region_locked (make_fixnum (get_locked_narrowing_begv (PT)),
+                             make_fixnum (get_locked_narrowing_zv (PT)),
+                             hook);
 
   run_hook_with_args (2, ((Lisp_Object []) {hook, hook}),
                       safe_run_hook_funcall);
@@ -12727,8 +12727,9 @@ the error might happen repeatedly and make Emacs nonfunctional.
 
 Note that, when the current buffer contains one or more lines whose
 length is above `long-line-threshold', these hook functions are called
-with the buffer narrowed to a small portion around point, and the
-narrowing is locked (see `narrow-to-region'), so that these hook
+with the buffer narrowed to a small portion around point (whose size
+is specified by `long-line-locked-narrowing-region-size'), and the
+narrowing is locked (see `narrowing-lock'), so that these hook
 functions cannot use `widen' to gain access to other portions of
 buffer text.
 
@@ -12748,8 +12749,9 @@ avoid making Emacs unresponsive while the user types.
 
 Note that, when the current buffer contains one or more lines whose
 length is above `long-line-threshold', these hook functions are called
-with the buffer narrowed to a small portion around point, and the
-narrowing is locked (see `narrow-to-region'), so that these hook
+with the buffer narrowed to a small portion around point (whose size
+is specified by `long-line-locked-narrowing-region-size'), and the
+narrowing is locked (see `narrowing-lock'), so that these hook
 functions cannot use `widen' to gain access to other portions of
 buffer text.
 
diff --git a/src/lisp.h b/src/lisp.h
index 6a24a538172..0f70f60d75c 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -4687,7 +4687,8 @@ extern void save_restriction_restore (Lisp_Object);
 extern Lisp_Object make_buffer_string (ptrdiff_t, ptrdiff_t, bool);
 extern Lisp_Object make_buffer_string_both (ptrdiff_t, ptrdiff_t, ptrdiff_t,
                                             ptrdiff_t, bool);
-extern Lisp_Object narrow_to_region_internal (Lisp_Object, Lisp_Object, bool);
+extern void narrow_to_region_locked (Lisp_Object, Lisp_Object, Lisp_Object);
+extern void reset_outermost_narrowings (void);
 extern void init_editfns (void);
 extern void syms_of_editfns (void);
 
diff --git a/src/xdisp.c b/src/xdisp.c
index 5dcf21dc4ce..0002c3d611c 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -3533,6 +3533,33 @@ get_closer_narrowed_begv (struct window *w, ptrdiff_t pos)
   return max ((pos / len - 1) * len, BEGV);
 }
 
+ptrdiff_t
+get_locked_narrowing_begv (ptrdiff_t pos)
+{
+  if (long_line_locked_narrowing_region_size == 0)
+    return BEGV;
+  int len = long_line_locked_narrowing_region_size / 2;
+  int begv = max (pos - len, BEGV);
+  int limit = long_line_locked_narrowing_bol_search_limit;
+  while (limit)
+    {
+      if (begv == BEGV || FETCH_BYTE (CHAR_TO_BYTE (begv) - 1) == '\n')
+        return begv;
+      begv--;
+      limit--;
+    }
+  return begv;
+}
+
+ptrdiff_t
+get_locked_narrowing_zv (ptrdiff_t pos)
+{
+  if (long_line_locked_narrowing_region_size == 0)
+    return ZV;
+  int len = long_line_locked_narrowing_region_size / 2;
+  return min (pos + len, ZV);
+}
+
 static void
 unwind_narrowed_begv (Lisp_Object point_min)
 {
@@ -4368,16 +4395,16 @@ handle_fontified_prop (struct it *it)
 
       if (current_buffer->long_line_optimizations_p)
         {
-          ptrdiff_t begv = it->narrowed_begv;
-          ptrdiff_t zv = it->narrowed_zv;
+          ptrdiff_t begv = it->locked_narrowing_begv;
+          ptrdiff_t zv = it->locked_narrowing_zv;
           ptrdiff_t charpos = IT_CHARPOS (*it);
           if (charpos < begv || charpos > zv)
             {
-              begv = get_narrowed_begv (it->w, charpos);
-              zv = get_narrowed_zv (it->w, charpos);
+              begv = get_locked_narrowing_begv (charpos);
+              zv = get_locked_narrowing_zv (charpos);
             }
-          narrow_to_region_internal (make_fixnum (begv), make_fixnum (zv), true);
-          specbind (Qrestrictions_locked, Qt);
+          narrow_to_region_locked (make_fixnum (begv), make_fixnum (zv),
+                                   Qfontification_functions);
         }
 
       /* Don't allow Lisp that runs from 'fontification-functions'
@@ -7435,12 +7462,20 @@ reseat (struct it *it, struct text_pos pos, bool force_p)
         {
           it->narrowed_begv = get_narrowed_begv (it->w, window_point (it->w));
           it->narrowed_zv = get_narrowed_zv (it->w, window_point (it->w));
+          it->locked_narrowing_begv
+            = get_locked_narrowing_begv (window_point (it->w));
+          it->locked_narrowing_zv
+            = get_locked_narrowing_zv (window_point (it->w));
         }
       else if ((pos.charpos < it->narrowed_begv || pos.charpos > it->narrowed_zv)
                 && (!redisplaying_p || it->line_wrap == TRUNCATE))
         {
           it->narrowed_begv = get_narrowed_begv (it->w, pos.charpos);
           it->narrowed_zv = get_narrowed_zv (it->w, pos.charpos);
+          it->locked_narrowing_begv
+            = get_locked_narrowing_begv (window_point (it->w));
+          it->locked_narrowing_zv
+            = get_locked_narrowing_zv (window_point (it->w));
         }
     }
 
@@ -16266,7 +16301,6 @@ do { if (! polling_stopped_here) stop_polling ();	\
 do { if (polling_stopped_here) start_polling ();        \
        polling_stopped_here = false; } while (false)
 
-
 /* Perhaps in the future avoid recentering windows if it
    is not necessary; currently that causes some problems.  */
 
@@ -16352,6 +16386,8 @@ redisplay_internal (void)
   FOR_EACH_FRAME (tail, frame)
     XFRAME (frame)->already_hscrolled_p = false;
 
+  reset_outermost_narrowings ();
+
  retry:
   /* Remember the currently selected window.  */
   sw = w;
@@ -36711,10 +36747,11 @@ fontify a region starting at POS in the current buffer, and give
 fontified regions the property `fontified' with a non-nil value.
 
 Note that, when the buffer contains one or more lines whose length is
-above `long-line-threshold', these functions are called with the buffer
-narrowed to a small portion around POS, and the narrowing is locked (see
-`narrow-to-region'), so that these functions cannot use `widen' to gain
-access to other portions of buffer text.  */);
+above `long-line-threshold', these functions are called with the
+buffer narrowed to a small portion around POS (whose size is specified
+by `long-line-locked-narrowing-region-size'), and the narrowing is
+locked (see `narrowing-lock'), so that these functions cannot use
+`widen' to gain access to other portions of buffer text.  */);
   Vfontification_functions = Qnil;
   Fmake_variable_buffer_local (Qfontification_functions);