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

* src/dispextern.h (struct it): New 'narrowed_begv' field.

* src/dispextern.h (WITH_NARROWED_BEGV): New macro.

* src/xdisp.c (get_narrowed_begv): New function.
(init_iterator): Initilize the 'narrowed_begv' field.
(back_to_previous_line_start, get_visually_first_element,
move_it_vertically_backward): Use the new macro.

* src/dispextern.h: Prototype of 'get_narrowed_begv'.

* src/window.c (window_body_height): Make it externally visible.

* src/window.h: Prototype of 'window_body_height'.

* src/composite.c (find_automatic_composition): Optimize display in buffers
with very long lines with 'get_narrowed_begv'.

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

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

* 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, 50 insertions, 7 deletions

diff --git a/src/composite.c b/src/composite.c
index 4d69702171f..d8998b5a1f3 100644
--- a/src/composite.c
+++ b/src/composite.c
@@ -1576,6 +1576,7 @@ find_automatic_composition (ptrdiff_t pos, ptrdiff_t limit, ptrdiff_t backlim,
   Lisp_Object window;
   struct window *w;
   bool need_adjustment = 0;
+  ptrdiff_t narrowed_begv;
 
   window = Fget_buffer_window (Fcurrent_buffer (), Qnil);
   if (NILP (window))
@@ -1586,6 +1587,11 @@ find_automatic_composition (ptrdiff_t pos, ptrdiff_t limit, ptrdiff_t backlim,
   if (NILP (string))
     {
       head = backlim < 0 ? BEGV : backlim, tail = ZV, stop = GPT;
+      /* In buffers with very long lines, this function becomes very
+         slow.  Pretend that the buffer is narrowed to make it fast.  */
+      narrowed_begv = get_narrowed_begv (w);
+      if (pos > narrowed_begv)
+        head = narrowed_begv;
       cur.pos_byte = CHAR_TO_BYTE (cur.pos);
       cur.p = BYTE_POS_ADDR (cur.pos_byte);
     }
diff --git a/src/dispextern.h b/src/dispextern.h
index ca7834dec55..2edf4b73f81 100644
--- a/src/dispextern.h
+++ b/src/dispextern.h
@@ -2332,6 +2332,10 @@ struct it
      with which display_string was called.  */
   ptrdiff_t end_charpos;
 
+  /* Alternate begin position of the buffer, which is used to optimize
+     display (see the WITH_NARROWED_BEGV macro below).  */
+  ptrdiff_t narrowed_begv;
+
   /* C string to iterate over.  Non-null means get characters from
      this string, otherwise characters are read from current_buffer
      or it->string.  */
@@ -2813,6 +2817,18 @@ struct it
     reset_box_start_end_flags ((IT));                   \
   } while (false)
 
+/* Execute STATEMENT with a temporarily narrowed buffer.  */
+
+#define WITH_NARROWED_BEGV(STATEMENT)                               \
+  do {                                                              \
+    ptrdiff_t obegv = BEGV;                                         \
+    if (it->narrowed_begv)                                          \
+      SET_BUF_BEGV (current_buffer, it->narrowed_begv);             \
+    STATEMENT;                                                      \
+    if (it->narrowed_begv)                                          \
+      SET_BUF_BEGV (current_buffer, obegv);                         \
+  } while (0)
+
 /* Bit-flags indicating what operation move_it_to should perform.  */
 
 enum move_operation_enum
@@ -3396,6 +3412,7 @@ void mark_window_display_accurate (Lisp_Object, bool);
 void redisplay_preserve_echo_area (int);
 void init_iterator (struct it *, struct window *, ptrdiff_t,
                     ptrdiff_t, struct glyph_row *, enum face_id);
+ptrdiff_t get_narrowed_begv (struct window *w);
 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/window.c b/src/window.c
index af463b90ce6..61ca9feb64d 100644
--- a/src/window.c
+++ b/src/window.c
@@ -1028,7 +1028,7 @@ window_body_unit_from_symbol (Lisp_Object unit)
 /* Return the number of lines/pixels of W's body.  Don't count any mode
    or header line or horizontal divider of W.  Rounds down to nearest
    integer when not working pixelwise. */
-static int
+int
 window_body_height (struct window *w, enum window_body_unit pixelwise)
 {
   int height = (w->pixel_height
diff --git a/src/window.h b/src/window.h
index 298a80a5366..c63b1b24d4f 100644
--- a/src/window.h
+++ b/src/window.h
@@ -1193,6 +1193,7 @@ enum window_body_unit
     WINDOW_BODY_IN_REMAPPED_CHARS
   };
 extern int window_body_width (struct window *w, enum window_body_unit);
+extern int window_body_height (struct window *w, enum window_body_unit);
 enum margin_unit { MARGIN_IN_LINES, MARGIN_IN_PIXELS };
 extern int window_scroll_margin (struct window *, enum margin_unit);
 extern void temp_output_buffer_show (Lisp_Object);
diff --git a/src/xdisp.c b/src/xdisp.c
index 4089525e10f..e130b23d9a1 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -3425,6 +3425,8 @@ init_iterator (struct it *it, struct window *w,
         }
     }
 
+  it->narrowed_begv = get_narrowed_begv (w);
+
   /* If a buffer position was specified, set the iterator there,
      getting overlays and face properties from that position.  */
   if (charpos >= BUF_BEG (current_buffer))
@@ -3491,6 +3493,19 @@ init_iterator (struct it *it, struct window *w,
   CHECK_IT (it);
 }
 
+/* Compute a suitable value for BEGV that can be used temporarily, to
+   optimize display, for the buffer in window W.  */
+
+ptrdiff_t
+get_narrowed_begv (struct window *w)
+{
+  int len, begv;
+  len = (1 + ((window_body_width (w, WINDOW_BODY_IN_CANONICAL_CHARS) *
+               window_body_height (w, WINDOW_BODY_IN_CANONICAL_CHARS)) /
+              10000)) * 10000;
+  begv = max ((PT / len - 2) * len, BEGV);
+  return begv == BEGV ? 0 : begv;
+}
 
 /* Initialize IT for the display of window W with window start POS.  */
 
@@ -6992,7 +7007,8 @@ back_to_previous_line_start (struct it *it)
   ptrdiff_t cp = IT_CHARPOS (*it), bp = IT_BYTEPOS (*it);
 
   dec_both (&cp, &bp);
-  IT_CHARPOS (*it) = find_newline_no_quit (cp, bp, -1, &IT_BYTEPOS (*it));
+  WITH_NARROWED_BEGV (IT_CHARPOS (*it) =
+                      find_newline_no_quit (cp, bp, -1, &IT_BYTEPOS (*it)));
 }
 
 
@@ -8623,7 +8639,9 @@ get_visually_first_element (struct it *it)
 {
   bool string_p = STRINGP (it->string) || it->s;
   ptrdiff_t eob = (string_p ? it->bidi_it.string.schars : ZV);
-  ptrdiff_t bob = (string_p ? 0 : BEGV);
+  ptrdiff_t bob;
+
+  WITH_NARROWED_BEGV (bob = (string_p ? 0 : BEGV));
 
   if (STRINGP (it->string))
     {
@@ -8663,9 +8681,10 @@ get_visually_first_element (struct it *it)
       if (string_p)
         it->bidi_it.charpos = it->bidi_it.bytepos = 0;
       else
-        it->bidi_it.charpos = find_newline_no_quit (IT_CHARPOS (*it),
-                                                    IT_BYTEPOS (*it), -1,
-                                                    &it->bidi_it.bytepos);
+        WITH_NARROWED_BEGV (it->bidi_it.charpos =
+                            find_newline_no_quit (IT_CHARPOS (*it),
+                                                  IT_BYTEPOS (*it), -1,
+                                                  &it->bidi_it.bytepos));
       bidi_paragraph_init (it->paragraph_embedding, &it->bidi_it, true);
       do
         {
@@ -10583,7 +10602,7 @@ move_it_vertically_backward (struct it *it, int dy)
           ptrdiff_t cp = IT_CHARPOS (*it), bp = IT_BYTEPOS (*it);
 
           dec_both (&cp, &bp);
-          cp = find_newline_no_quit (cp, bp, -1, NULL);
+          WITH_NARROWED_BEGV (cp = find_newline_no_quit (cp, bp, -1, NULL));
           move_it_to (it, cp, -1, -1, -1, MOVE_TO_POS);
         }
       bidi_unshelve_cache (it3data, true);