Actually fix the long lines display bug (bug#56393).

* lisp/files.el (auto-narrow-mode): New minor mode.
(auto-narrow-pre-command-function, auto-narrow-post-command-function):
New auxiliary functions for the minor mode.
(auto-narrow-display-length, auto-narrow-widen-automatically): New
defcustoms for the minor mode.
(auto-narrow--widen-automatically, auto-narrow--isearch-widen-automatically,
auto-narrow--initialized): New internal variables for the minor mode.

* src/buffer.h (struct buffer): New internal variable for the minor mode.
(bset_auto_narrow__narrowing_state, BUFFER_AUTO_NARROWED_P): New
auxiliary functions.

* src/buffer.c (init_buffer_once, syms_of_buffer): New internal variable
for the minor mode. Update the docstring of mode-line-format.

* src/fileio.c (Finsert_file_contents): Detect whether the minor mode should
be entered when the buffer is displayed.
(syms_of_fileio): New defcustom for the minor mode.

* src/keyboard.c (syms_of_keyboard): New hook functions for the minor mode.
(command_loop_1): Execute the hook functions.

* src/xdisp.c (redisplay_window): Enter the minor mode when the buffer is
displayed.
(decode_mode_spec): Indicate when the minor mode is active
in the modeline.  Indicate the buffer position relative to the whole buffer.
(set_vertical_scroll_bar): Indicate the buffer position relative to the
whole buffer.
(syms_of_xdisp): Two new symbols.

* lisp/isearch.el (isearch-widen-automatically): New defcustom.
(isearch-search): Use the new defcustom.

* lisp/bindings.el (mode-line-modes): Do not propertize the indication
in the modeline when the new minor mode is active.

* etc/NEWS: Announce the new minor mode, and remove the unobsoletion
indication for 'longlines-mode'.

* lisp/longlines.el: Reobsolete longlines-mode.

* doc/emacs/display.texi (Auto-Narrowing): New section, describing the new
minor mode.
(Display): Entry for the new section.

* doc/emacs/trouble.texi (Long Lines): Remove the section.
(Lossage): Remove the entry for the Long Lines section.

* doc/emacs/emacs.texi (Top): Remove the entry for the Long Lines section.

5 files changed, 164 insertions, 30 deletions

diff --git a/src/buffer.c b/src/buffer.c
index 509ce51b55e..d5b4292a21d 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -5276,6 +5276,9 @@ init_buffer_once (void)
   XSETFASTINT (BVAR (&buffer_local_flags, tab_line_format), idx); ++idx;
   XSETFASTINT (BVAR (&buffer_local_flags, cursor_type), idx); ++idx;
   XSETFASTINT (BVAR (&buffer_local_flags, extra_line_spacing), idx); ++idx;
+  XSETFASTINT (BVAR (&buffer_local_flags, auto_narrow__narrowing_state), idx);
+  /* Make this one a permanent local.  */
+  buffer_permanent_local_flags[idx++] = 1;
   XSETFASTINT (BVAR (&buffer_local_flags, cursor_in_non_selected_windows), idx); ++idx;
 
   /* buffer_local_flags contains no pointers, so it's safe to treat it
@@ -5662,7 +5665,7 @@ A string is printed verbatim in the mode line except for %-constructs:
   %p -- print percent of buffer above top of window, or Top, Bot or All.
   %P -- print percent of buffer above bottom of window, perhaps plus Top,
         or print Bottom or All.
-  %n -- print Narrow if appropriate.
+  %n -- print Narrow or Auto-Narrow if appropriate.
   %t -- visited file is text or binary (if OS supports this distinction).
   %z -- print mnemonics of keyboard, terminal, and buffer coding systems.
   %Z -- like %z, but including the end-of-line format.
@@ -6363,6 +6366,10 @@ see `display-graphic-p'.
 If value is a floating point number, it specifies the spacing relative
 to the default frame line height.  A value of nil means add no extra space.  */);
 
+  DEFVAR_PER_BUFFER ("auto-narrow--narrowing-state",
+                     &BVAR (current_buffer, auto_narrow__narrowing_state), Qnil,
+                     doc: /* Internal variable used by `auto-narrow-mode'.  */);
+
   DEFVAR_PER_BUFFER ("cursor-in-non-selected-windows",
                      &BVAR (current_buffer, cursor_in_non_selected_windows), Qnil,
                      doc: /* Non-nil means show a cursor in non-selected windows.
diff --git a/src/buffer.h b/src/buffer.h
index 135eaf72d30..19faa844e02 100644
--- a/src/buffer.h
+++ b/src/buffer.h
@@ -561,6 +561,9 @@ struct buffer
      in the display of this buffer.  */
   Lisp_Object extra_line_spacing_;
 
+  /* Narrowing state when auto-narrow mode is in effect.  */
+  Lisp_Object auto_narrow__narrowing_state_;
+
   /* Cursor type to display in non-selected windows.
      t means to use hollow box cursor.
      See `cursor-type' for other values.  */
@@ -832,6 +835,11 @@ bset_width_table (struct buffer *b, Lisp_Object val)
 {
   b->width_table_ = val;
 }
+INLINE void
+bset_auto_narrow__narrowing_state (struct buffer *b, Lisp_Object val)
+{
+  b->auto_narrow__narrowing_state_ = val;
+}
 
 /* BUFFER_CEILING_OF (resp. BUFFER_FLOOR_OF), when applied to n, return
    the max (resp. min) p such that
@@ -1112,6 +1120,12 @@ BUFFER_CHECK_INDIRECTION (struct buffer *b)
     }
 }
 
+INLINE bool
+BUFFER_AUTO_NARROWED_P (struct buffer *b)
+{
+  return EQ (BVAR (b, auto_narrow__narrowing_state), Qauto);
+}
+
 /* 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/fileio.c b/src/fileio.c
index 10d4b8bc15e..bb6f73fe39a 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -4997,6 +4997,16 @@ by calling `format-decode', which see.  */)
            Otherwise start with an empty undo_list.  */
         bset_undo_list (current_buffer, EQ (old_undo, Qt) ? Qt : Qnil);
 
+      if (!noninteractive
+          && !NILP (Vauto_narrow_long_line_threshold)
+          && !NILP (CALLN
+                    (Fgtr,
+                     Fcar (Fcdr (Fbuffer_line_statistics (Fcurrent_buffer ()))),
+                     Vauto_narrow_long_line_threshold)))
+        {
+          bset_auto_narrow__narrowing_state (current_buffer, Qneeded);
+        }
+
       unbind_to (count1, Qnil);
     }
 
@@ -6638,6 +6648,17 @@ This includes interactive calls to `delete-file' and
   /* Lisp function for recursively deleting directories.  */
   DEFSYM (Qdelete_directory, "delete-directory");
 
+  DEFVAR_LISP ("auto-narrow-long-line-threshold",
+               Vauto_narrow_long_line_threshold,
+               doc: /* Line length above which `auto-narrow-mode' is entered.
+When non-nil, and when a file with one or more lines longer than
+`auto-narrow-long-line-threshold' is opened or inserted in a buffer,
+`auto-narrow-mode' is automatically enabled.
+When nil, `auto-narrow-mode' is disabled.  */);
+  Vauto_narrow_long_line_threshold = Qnil;
+
+  DEFSYM (Qauto_narrow_mode, "auto-narrow-mode");
+
   DEFSYM (Qsubstitute_env_in_file_name, "substitute-env-in-file-name");
   DEFSYM (Qget_buffer_window_list, "get-buffer-window-list");
 
diff --git a/src/keyboard.c b/src/keyboard.c
index bed8307b6f2..38aa64774dc 100644
--- a/src/keyboard.c
+++ b/src/keyboard.c
@@ -1290,6 +1290,9 @@ command_loop_1 (void)
 
   if (NILP (Vmemory_full))
     {
+      if (!NILP (Vauto_narrow_post_command_function) && !NILP (Vrun_hooks))
+        safe_run_hooks (Qauto_narrow_post_command_function);
+
       /* Make sure this hook runs after commands that get errors and
          throw to top level.  */
       /* Note that the value cell will never directly contain nil
@@ -1472,6 +1475,8 @@ command_loop_1 (void)
       Vreal_this_command = cmd;
       safe_run_hooks (Qpre_command_hook);
 
+      safe_run_hooks (Qauto_narrow_pre_command_function);
+
       already_adjusted = 0;
 
       if (NILP (Vthis_command))
@@ -1522,6 +1527,8 @@ command_loop_1 (void)
           }
       kset_last_prefix_arg (current_kboard, Vcurrent_prefix_arg);
 
+      safe_run_hooks (Qauto_narrow_post_command_function);
+
       safe_run_hooks (Qpost_command_hook);
 
       /* If displaying a message, resize the echo area window to fit
@@ -12040,6 +12047,11 @@ syms_of_keyboard (void)
   DEFSYM (Qpre_command_hook, "pre-command-hook");
   DEFSYM (Qpost_command_hook, "post-command-hook");
 
+  DEFSYM (Qauto_narrow_pre_command_function,
+          "auto-narrow-pre-command-function");
+  DEFSYM (Qauto_narrow_post_command_function,
+          "auto-narrow-post-command-function");
+
   DEFSYM (Qundo_auto__add_boundary, "undo-auto--add-boundary");
   DEFSYM (Qundo_auto__undoably_changed_buffers,
           "undo-auto--undoably-changed-buffers");
@@ -12604,6 +12616,22 @@ avoid making Emacs unresponsive while the user types.
 See also `pre-command-hook'.  */);
   Vpost_command_hook = Qnil;
 
+  DEFVAR_LISP ("auto-narrow-pre-command-function",
+               Vauto_narrow_pre_command_function,
+               doc: /* Function executed before each command is executed in `auto-narrow-mode'.
+If non-nil, and `auto-narrow-mode' is enabled, the function is
+called before each command is executed, after `pre-command-hook'.
+It is called without arguments.  */);
+  Vauto_narrow_pre_command_function = Qnil;
+
+  DEFVAR_LISP ("auto-narrow-post-command-function",
+               Vauto_narrow_post_command_function,
+               doc: /* Function executed after each command is executed in `auto-narrow-mode'.
+If non-nil, and `auto-narrow-mode' is enabled, the function is
+called after each command is executed, before `post-command-hook'.
+It is called without arguments.  */);
+  Vauto_narrow_post_command_function = Qnil;
+
 #if 0
   DEFVAR_LISP ("echo-area-clear-hook", ...,
                doc: /* Normal hook run when clearing the echo area.  */);
diff --git a/src/xdisp.c b/src/xdisp.c
index 4089525e10f..be51a0eb136 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -18872,11 +18872,20 @@ set_vertical_scroll_bar (struct window *w)
           && NILP (echo_area_buffer[0])))
     {
       struct buffer *buf = XBUFFER (w->contents);
-      whole = BUF_ZV (buf) - BUF_BEGV (buf);
-      start = marker_position (w->start) - BUF_BEGV (buf);
-      /* I don't think this is guaranteed to be right.  For the
-         moment, we'll pretend it is.  */
-      end = BUF_Z (buf) - w->window_end_pos - BUF_BEGV (buf);
+      if (! BUFFER_AUTO_NARROWED_P (current_buffer))
+        {
+          whole = BUF_ZV (buf) - BUF_BEGV (buf);
+          start = marker_position (w->start) - BUF_BEGV (buf);
+          /* I don't think this is guaranteed to be right.  For the
+             moment, we'll pretend it is.  */
+          end = BUF_Z (buf) - w->window_end_pos - BUF_BEGV (buf);
+        }
+      else
+        {
+          whole = BUF_Z (buf) - BUF_BEG (buf);
+          start = marker_position (w->start) - BUF_BEG (buf);
+          end = BUF_Z (buf) - w->window_end_pos - BUF_BEG (buf);
+        }
 
       if (end < start)
         end = start;
@@ -19133,6 +19142,14 @@ 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))
+    {
+      safe_call (1, Qauto_narrow_mode);
+      /* Normally set by auto-narrow-mode, set it here anyway as a safety measure.  */
+      bset_auto_narrow__narrowing_state (current_buffer, Qauto);
+      message1 ("Auto-Narrow mode enabled in current buffer");
+    }
+
   current_matrix_up_to_date_p
     = (w->window_end_valid
        && !current_buffer->clip_changed
@@ -27667,7 +27684,12 @@ decode_mode_spec (struct window *w, register int c, int field_width,
 
     case 'n':
       if (BUF_BEGV (b) > BUF_BEG (b) || BUF_ZV (b) < BUF_Z (b))
-        return " Narrow";
+        {
+          if (! BUFFER_AUTO_NARROWED_P (b))
+            return " Narrow";
+          else
+            return " Auto-Narrow";
+        }
       break;
 
       /* Display the "degree of travel" of the window through the buffer.  */
@@ -27675,17 +27697,27 @@ decode_mode_spec (struct window *w, register int c, int field_width,
       {
         ptrdiff_t toppos = marker_position (w->start);
         ptrdiff_t botpos = BUF_Z (b) - w->window_end_pos;
-        ptrdiff_t begv = BUF_BEGV (b);
-        ptrdiff_t zv = BUF_ZV (b);
+        ptrdiff_t beg, z;
 
-        if (zv <= botpos)
-          return toppos <= begv ? "All" : "Bottom";
-        else if (toppos <= begv)
+        if (! BUFFER_AUTO_NARROWED_P (b))
+          {
+            beg = BUF_BEGV (b);
+            z = BUF_ZV (b);
+          }
+        else
+          {
+            beg = BUF_BEG (b);
+            z = BUF_Z (b);
+          }
+
+        if (z <= botpos)
+          return toppos <= beg ? "All" : "Bottom";
+        else if (toppos <= beg)
           return "Top";
         else
           {
           sprintf (decode_mode_spec_buf, "%2d%%",
-                   percent99 (toppos - begv, (toppos - begv) + (zv - botpos)));
+                   percent99 (toppos - beg, (toppos - beg) + (z - botpos)));
           return decode_mode_spec_buf;
           }
       }
@@ -27694,17 +27726,27 @@ decode_mode_spec (struct window *w, register int c, int field_width,
     case 'p':
       {
         ptrdiff_t pos = marker_position (w->start);
-        ptrdiff_t begv = BUF_BEGV (b);
-        ptrdiff_t zv = BUF_ZV (b);
+        ptrdiff_t beg, z;
 
-        if (w->window_end_pos <= BUF_Z (b) - zv)
-          return pos <= begv ? "All" : "Bottom";
-        else if (pos <= begv)
+        if (! BUFFER_AUTO_NARROWED_P (b))
+          {
+            beg = BUF_BEGV (b);
+            z = BUF_ZV (b);
+          }
+        else
+          {
+            beg = BUF_BEG (b);
+            z = BUF_Z (b);
+          }
+
+        if (w->window_end_pos <= BUF_Z (b) - z)
+          return pos <= beg ? "All" : "Bottom";
+        else if (pos <= beg)
           return "Top";
         else
           {
             sprintf (decode_mode_spec_buf, "%2d%%",
-                     percent99 (pos - begv, zv - begv));
+                     percent99 (pos - beg, z - beg));
             return decode_mode_spec_buf;
           }
       }
@@ -27714,16 +27756,26 @@ decode_mode_spec (struct window *w, register int c, int field_width,
       {
         ptrdiff_t toppos = marker_position (w->start);
         ptrdiff_t botpos = BUF_Z (b) - w->window_end_pos;
-        ptrdiff_t begv = BUF_BEGV (b);
-        ptrdiff_t zv = BUF_ZV (b);
+        ptrdiff_t beg, z;
 
-        if (zv <= botpos)
-          return toppos <= begv ? "All" : "Bottom";
+        if (! BUFFER_AUTO_NARROWED_P (b))
+          {
+            beg = BUF_BEGV (b);
+            z = BUF_ZV (b);
+          }
+        else
+          {
+            beg = BUF_BEG (b);
+            z = BUF_Z (b);
+          }
+
+        if (z <= botpos)
+          return toppos <= beg ? "All" : "Bottom";
         else
           {
             sprintf (decode_mode_spec_buf,
-                     &"Top%2d%%"[begv < toppos ? sizeof "Top" - 1 : 0],
-                     percent99 (botpos - begv, zv - begv));
+                     &"Top%2d%%"[beg < toppos ? sizeof "Top" - 1 : 0],
+                     percent99 (botpos - beg, z - beg));
             return decode_mode_spec_buf;
           }
       }
@@ -27734,15 +27786,25 @@ decode_mode_spec (struct window *w, register int c, int field_width,
       {
         ptrdiff_t toppos = marker_position (w->start);
         ptrdiff_t botpos = BUF_Z (b) - w->window_end_pos;
-        ptrdiff_t begv = BUF_BEGV (b);
-        ptrdiff_t zv = BUF_ZV (b);
+        ptrdiff_t beg, z;
         int top_perc, bot_perc;
 
-        if ((toppos <= begv) && (zv <= botpos))
+        if (! BUFFER_AUTO_NARROWED_P (b))
+          {
+            beg = BUF_BEGV (b);
+            z = BUF_ZV (b);
+          }
+        else
+          {
+            beg = BUF_BEG (b);
+            z = BUF_Z (b);
+          }
+
+        if ((toppos <= beg) && (z <= botpos))
           return "All   ";
 
-        top_perc = toppos <= begv ? 0 : percent99 (toppos - begv, zv - begv);
-        bot_perc = zv <= botpos ? 100 : percent99 (botpos - begv, zv - begv);
+        top_perc = toppos <= beg ? 0 : percent99 (toppos - beg, z - beg);
+        bot_perc = z <= botpos ? 100 : percent99 (botpos - beg, z - beg);
 
         if (top_perc == bot_perc)
           sprintf (decode_mode_spec_buf, "%d%%", top_perc);
@@ -35830,6 +35892,8 @@ be let-bound around code that needs to disable messages temporarily. */);
   DEFSYM (Qinhibit_point_motion_hooks, "inhibit-point-motion-hooks");
   DEFSYM (Qeval, "eval");
   DEFSYM (QCdata, ":data");
+  DEFSYM (Qneeded, "needed");
+  DEFSYM (Qauto, "auto");
 
   /* Names of text properties relevant for redisplay.  */
   DEFSYM (Qdisplay, "display");