Fix bug #10192 with assertion violation when scrolling.

 src/xdisp.c (redisplay_window): Don't let `margin' become negative.
 This could happen when scroll-margin is zero and scroll-*-aggressively
 is a small float number.

2 files changed, 2 insertions, 1 deletions

diff --git a/src/ChangeLog b/src/ChangeLog
index efba68d99d6..61b44593062 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -3,6 +3,7 @@
         * xdisp.c (handle_invisible_prop): If the invisible text ends just
         before a newline, prepare the bidi iterator for consuming the
         newline, and keep the current paragraph direction.  (Bug#10183)
+        (redisplay_window): Don't let `margin' become negative.  (Bug#10192)
 
 2011-12-02  Juri Linkov  <juri@jurta.org>
 
diff --git a/src/xdisp.c b/src/xdisp.c
index b9741d97d30..7708b9c40fa 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -15617,7 +15617,7 @@ redisplay_window (Lisp_Object window, int just_this_one_p)
               pt_offset = float_amount * WINDOW_BOX_TEXT_HEIGHT (w);
               if (pt_offset == 0 && float_amount > 0)
                 pt_offset = 1;
-              if (pt_offset)
+              if (pt_offset && margin > 0)
                 margin -= 1;
             }
           /* Compute how much to move the window start backward from