Fix posn-at-point at beginning of a display string

* src/xdisp.c (pos_visible_p): Account for the line-number width
when the display string at CHARPOS ends in a newline.  (Bug#42039)

1 files changed, 14 insertions, 4 deletions

diff --git a/src/xdisp.c b/src/xdisp.c
index a280b48de99..15901af9a8d 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -1782,6 +1782,12 @@ pos_visible_p (struct window *w, ptrdiff_t charpos, int *x, int *y,
                   start_display (&it3, w, top);
                   if (start > CHARPOS (top))
                     move_it_to (&it3, start - 1, -1, -1, -1, MOVE_TO_POS);
+                  /* Record the line-number width, if any.  Do it here,
+                     before subsequent calls to start_display etc. reset
+                     the line_number_produced_p flag, and we can no
+                     longer be sure we are not using stale info.  */
+                  int lnum_pixel_width =
+                    it3.line_number_produced_p ? it3.lnum_pixel_width : 0;
                   /* Move forward one more line if the position before
                      the display string is a newline or if it is the
                      rightmost character on a line that is
@@ -1850,10 +1856,14 @@ pos_visible_p (struct window *w, ptrdiff_t charpos, int *x, int *y,
                   top_x = it3.current_x - it3.pixel_width;
                   /* Account for line-number display, if IT3 still
                      didn't.  This can happen if START - 1 is the
-                     first character on its display line.  */
-                  if (!it3.line_number_produced_p
-                      && it.line_number_produced_p)
-                    top_x += it.lnum_pixel_width;
+                     first or the last character on its display line.  */
+                  if (!it3.line_number_produced_p)
+                    {
+                      if (lnum_pixel_width > 0)
+                        top_x += lnum_pixel_width;
+                      else if (it.line_number_produced_p)
+                        top_x += it.lnum_pixel_width;
+                    }
                   /* Normally, we would exit the above loop because we
                      found the display element whose character
                      position is CHARPOS.  For the contingency that we