Fix bug #17905 with display of point in partially visible line at end of window.

 src/xdisp.c (redisplay_window): If redisplay of a window ends up
 with point in a partially visible line at end of the window, make
 sure the amended position of point actually has smaller Y
 coordinate; if not, give up and scroll the display.
 src/window.c (window_scroll_pixel_based): When point ends up at the
 last fully visible line, don't let move_it_to stop at the left
 edge of the line and dupe us into thinking point is inside the
 scroll margin.

1 files changed, 26 insertions, 0 deletions

diff --git a/src/window.c b/src/window.c
index 8e8252d3b76..5e9dd0ec718 100644
--- a/src/window.c
+++ b/src/window.c
@@ -5167,6 +5167,32 @@ window_scroll_pixel_based (Lisp_Object window, int n, bool whole, int noerror)
       charpos = IT_CHARPOS (it);
       bytepos = IT_BYTEPOS (it);
 
+      /* If PT is in the screen line at the last fully visible line,
+         move_it_to will stop at X = 0 in that line, because the
+         required Y coordinate is reached there.  See if we can get to
+         PT without descending lower in Y, and if we can, it means we
+         reached PT before the scroll margin.  */
+      if (charpos != PT)
+        {
+          struct it it2;
+          void *it_data;
+
+          it2 = it;
+          it_data = bidi_shelve_cache ();
+          move_it_to (&it, PT, -1, -1, -1, MOVE_TO_POS);
+          if (IT_CHARPOS (it) == PT && it.current_y == it2.current_y)
+            {
+              charpos = IT_CHARPOS (it);
+              bytepos = IT_BYTEPOS (it);
+              bidi_unshelve_cache (it_data, 1);
+            }
+          else
+            {
+              it = it2;
+              bidi_unshelve_cache (it_data, 0);
+            }
+        }
+
       /* See if point is on a partially visible line at the end.  */
       if (it.what == IT_EOB)
         partial_p = it.current_y + it.ascent + it.descent > it.last_visible_y;