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, 8 insertions, 1 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.