emacs-31 165152e7cdc: Fix an infloop during cursor movement

branch: emacs-31
commit 165152e7cdc7802735004b81c70219c4ae6002ab
Author: Eli Zaretskii <[email protected]>
Commit: Eli Zaretskii <[email protected]>

    Fix an infloop during cursor movement
    
    * src/xdisp.c (move_it_vertically_backward): Prevent infloop due
    to overlays with embedded newlines at BOB.  (Bug#81567)
---
 src/xdisp.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/src/xdisp.c b/src/xdisp.c
index 5377d5e8ada..b3227ce4937 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -11298,10 +11298,18 @@ move_it_vertically_backward (struct it *it, int dy)
      y-distance.  */
   SAVE_IT (it2, *it, it2data);
   it2.max_ascent = it2.max_descent = 0;
+  ptrdiff_t to_pos = start_pos;
   do
     {
+      /* Avoid inflooping of there's a large display string with several
+         embedded newlines, which makes move_it_to stop after START_POS
+         but still inside a display or overlay string.  */
+      if (IT_CHARPOS (it2) >= start_pos)
+	to_pos = -1;
       move_it_to (&it2, start_pos, -1, -1, it2.vpos + 1,
-		  MOVE_TO_POS | MOVE_TO_VPOS);
+		  (to_pos > 0
+		    ? (MOVE_TO_POS | MOVE_TO_VPOS)
+		   : MOVE_TO_VPOS));
     }
   while (!(IT_POS_VALID_AFTER_MOVE_P (&it2)
 	   /* If we are in a display string which starts at START_POS,