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.

1 files changed, 14 insertions, 0 deletions

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