Avoid slow redisplay under 'visual' mode of line numbers

* src/xdisp.c (display_count_lines_visually): Avoid very slow
redisplay when this function is invoked very far from point.
Reported by Alex <agrambot@gmail.com>.

diff --git a/src/xdisp.c b/src/xdisp.c
index aeccac2cb16f786ebcdae2884fe7bd8339884c4f..7bbe9d9ca3e59b678a6e4a9c3b6a980a6b94c7bd 100644 (file)
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -20783,7 +20783,14 @@ display_count_lines_visually (struct it *it)
       /* Need to disable visual mode temporarily, since otherwise the
         call to move_it_to will cause infinite recursion.  */
       specbind (Qdisplay_line_numbers, Qrelative);
-      move_it_to (&tem_it, to, -1, -1, -1, MOVE_TO_POS);
+      /* Some redisplay optimizations could invoke us very far from
+        PT, which will make the caller painfully slow.  There should
+        be no need to go too far beyond the window's bottom, as any
+        such optimization will fail to show point anyway.  */
+      move_it_to (&tem_it, to, -1,
+                 tem_it.last_visible_y
+                 + (SCROLL_LIMIT + 10) * FRAME_LINE_HEIGHT (tem_it.f),
+                 -1, MOVE_TO_POS | MOVE_TO_Y);
       unbind_to (count, Qnil);
       return IT_CHARPOS (*it) <= PT ? -tem_it.vpos : tem_it.vpos;
     }