emacs-27 f425a77: Fix display artifacts when 'display' properties cover newlines

[Eli Zaretskii]

branch: emacs-27
commit f425a774c23ef683e3c052782a497ea30c0ebd10
Author: Eli Zaretskii <eliz@gnu.org>
Commit: Eli Zaretskii <eliz@gnu.org>

    Fix display artifacts when 'display' properties cover newlines
    
    * src/xdisp.c (pos_visible_p): Set glyph_row of scratch iterators
    to NULL, to avoid producing glyphs while we figure out the layout.
    (Bug#44826)
---
 src/xdisp.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/src/xdisp.c b/src/xdisp.c
index c9175a6..e65505b 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -1700,6 +1700,7 @@ pos_visible_p (struct window *w, ptrdiff_t charpos, int 
*x, int *y,
                     from a display vector, we need to consume all of
                     the glyphs from that display vector.  */
                  start_display (&it2, w, top);
+                 it2.glyph_row = NULL;
                  move_it_to (&it2, charpos - 1, -1, -1, -1, MOVE_TO_POS);
                  /* If we didn't get to CHARPOS - 1, there's some
                     replacing display property at that position, and
@@ -1823,6 +1824,7 @@ pos_visible_p (struct window *w, ptrdiff_t charpos, int 
*x, int *y,
                     of the display line where the display string
                     begins.  */
                  start_display (&it3, w, top);
+                 it3.glyph_row = NULL;
                  move_it_to (&it3, -1, 0, top_y, -1, MOVE_TO_X | MOVE_TO_Y);
                  /* If it3_moved stays false after the 'while' loop
                     below, that means we already were at a newline