Various improvements to auto-narrow mode.

* lisp/files.el (auto-narrow-display-length): Improve docstring.
(auto-narrow-widen-automatically): Add 'undo' to the command list.
(auto-narrow-hook): New defcustom.
(auto-narrow-pre-command-function, auto-narrow-post-command-function):
Move initialization code.
(auto-narrow-mode): Do not make the 'auto-narrow-mode' permanent local
anymore.

* lisp/font-lock.el (turn-off-font-lock-mode): New convenience function.

* src/xdisp.c (set_vertical_scroll_bar): Fix typo.
(redisplay_window): Use the new convenience function.

* src/buffer.h (BUFFER_NEEDS_AUTO_NARROWING_P,
BUFFER_AUTO_NARROWED_NON_NARROWED_P): Two new convenience functions.

* src/window.c (Frecenter): Do not recenter auto-narrowed buffers that
are not actually narrowed.

* etc/NEWS: Improvement.

* doc/emacs/display.texi (Auto-Narrowing): Improvement.

3 files changed, 23 insertions, 2 deletions

diff --git a/src/buffer.h b/src/buffer.h
index 19faa844e02..84492737b0e 100644
--- a/src/buffer.h
+++ b/src/buffer.h
@@ -1120,12 +1120,28 @@ BUFFER_CHECK_INDIRECTION (struct buffer *b)
     }
 }
 
+/* Check the state of auto-narrowed buffers.  */
+
+INLINE bool
+BUFFER_NEEDS_AUTO_NARROWING_P (struct buffer *b)
+{
+  return EQ (BVAR (b, auto_narrow__narrowing_state), Qneeded);
+}
+
 INLINE bool
 BUFFER_AUTO_NARROWED_P (struct buffer *b)
 {
   return EQ (BVAR (b, auto_narrow__narrowing_state), Qauto);
 }
 
+INLINE bool
+BUFFER_AUTO_NARROWED_NON_NARROWED_P (struct buffer *b)
+{
+  return BUFFER_AUTO_NARROWED_P (b)
+    && BUF_BEG (b) == BUF_BEGV (b)
+    && BUF_Z (b) == BUF_ZV (b);
+}
+
 /* This structure holds the default values of the buffer-local variables
    that have special slots in each buffer.
    The default value occupies the same slot in this structure
diff --git a/src/window.c b/src/window.c
index af463b90ce6..218b3e6a4f8 100644
--- a/src/window.c
+++ b/src/window.c
@@ -6557,6 +6557,11 @@ and redisplay normally--don't erase and redraw the frame.  */)
   if (buf != current_buffer)
     error ("`recenter'ing a window that does not display current-buffer.");
 
+  /* Refuse to recenter auto-narrowed buffers that are not actually narrowed,
+     as this can be very slow.  */
+  if (BUFFER_AUTO_NARROWED_NON_NARROWED_P (buf))
+    return Qnil;
+
   /* If redisplay is suppressed due to an error, try again.  */
   buf->display_error_modiff = 0;
 
diff --git a/src/xdisp.c b/src/xdisp.c
index be51a0eb136..7821c120ca0 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -18872,7 +18872,7 @@ set_vertical_scroll_bar (struct window *w)
           && NILP (echo_area_buffer[0])))
     {
       struct buffer *buf = XBUFFER (w->contents);
-      if (! BUFFER_AUTO_NARROWED_P (current_buffer))
+      if (! BUFFER_AUTO_NARROWED_P (buf))
         {
           whole = BUF_ZV (buf) - BUF_BEGV (buf);
           start = marker_position (w->start) - BUF_BEGV (buf);
@@ -19142,7 +19142,7 @@ redisplay_window (Lisp_Object window, bool just_this_one_p)
      variables.  */
   set_buffer_internal_1 (XBUFFER (w->contents));
 
-  if (EQ (BVAR (current_buffer, auto_narrow__narrowing_state), Qneeded))
+  if (BUFFER_NEEDS_AUTO_NARROWING_P (current_buffer))
     {
       safe_call (1, Qauto_narrow_mode);
       /* Normally set by auto-narrow-mode, set it here anyway as a safety measure.  */