Fix subtle problem with scroll-down when scroll-margin is nonzero

* src/window.c (window_scroll_pixel_based): Account for
scroll-margin when scrolling down, i.e. moving window-start
towards the beginning of the buffer.  Reported by zhang cc
<ccsmile2008@outlook.com> in
http://lists.gnu.org/archive/html/emacs-devel/2017-12/msg00894.html.

1 files changed, 3 insertions, 2 deletions

diff --git a/src/window.c b/src/window.c
index 504dcd38357..20a6020a3b7 100644
--- a/src/window.c
+++ b/src/window.c
@@ -5397,12 +5397,13 @@ window_scroll_pixel_based (Lisp_Object window, int n, bool whole, bool noerror)
       if (it.what == IT_EOB)
         partial_p =
           it.current_y + it.ascent + it.descent
-          > it.last_visible_y - WINDOW_HEADER_LINE_HEIGHT (w);
+          > it.last_visible_y - this_scroll_margin - WINDOW_HEADER_LINE_HEIGHT (w);
       else
         {
           move_it_by_lines (&it, 1);
           partial_p =
-            it.current_y > it.last_visible_y - WINDOW_HEADER_LINE_HEIGHT (w);
+            it.current_y
+            > it.last_visible_y - this_scroll_margin - WINDOW_HEADER_LINE_HEIGHT (w);
         }
 
       if (charpos == PT && !partial_p