bug#26742: Display bug with composed strings

[YAMAMOTO Mitsuharu]

> If I create a file temp.txt containing the following:

> -*- prettify-symbols-alist: (("R_PO" 8477 (Br . cl) 8804)); -*-
> !!! R_PO !!!

> and run ‘emacs-24.5 -Q temp.txt -f prettify-symbols-mode’, I observe
> a surprising display bug: when I move the point across the second
> line, as soon as the point reaches the blank space after “R_PO”, I
> see a second ℝ displayed instead of the third “!”.

On the Mac port, the filled box cursor disappears if I move it on
the composite characters.  The patch below seems to work for this
bug.  Could you try if it also solves the problems you observe?

                                     YAMAMOTO Mitsuharu
                                mituharu@math.s.chiba-u.ac.jp

diff --git a/src/xdisp.c b/src/xdisp.c
index e3315c4..12ae202 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -24698,6 +24698,20 @@ set_glyph_string_background_width (struct glyph_string 
*s, int start, int last_x
 }
 
 
+/* Return glyph string that shares background with glyph string S and
+   whose `background_width' member has been set.  */
+
+static struct glyph_string *
+glyph_string_containing_background_width (struct glyph_string *s)
+{
+  if (s->cmp)
+    while (s->cmp_from)
+      s = s->prev;
+
+  return s;
+}
+
+
 /* Compute overhangs and x-positions for glyph string S and its
    predecessors, or successors.  X is the starting x-position for S.
    BACKWARD_P non-zero means process predecessors.  */
@@ -25025,7 +25039,10 @@ draw_glyphs (struct window *w, int x, struct glyph_row 
*row,
   i = start;
   BUILD_GLYPH_STRINGS (i, end, head, tail, hl, x, last_x);
   if (tail)
-    x_reached = tail->x + tail->background_width;
+    {
+      s = glyph_string_containing_background_width (tail);
+      x_reached = s->x + s->background_width;
+    }
   else
     x_reached = x;
 
@@ -25180,6 +25197,9 @@ draw_glyphs (struct window *w, int x, struct glyph_row 
*row,
          compute_overhangs_and_x (h, tail->x + tail->width, 0);
          append_glyph_string_lists (&head, &tail, h, t);
        }
+      tail = glyph_string_containing_background_width (tail);
+      if (clip_tail)
+       clip_tail = glyph_string_containing_background_width (clip_tail);
       if (clip_head || clip_tail)
        for (s = head; s; s = s->next)
          {