Don't apply Arabic shaping to vertical text.

src/hb-ot-shape-complex-private.hh

diff --git a/src/hb-ot-shape-complex-private.hh b/src/hb-ot-shape-complex-private.hh
index e268933..4dc4f58 100644
--- a/src/hb-ot-shape-complex-private.hh
+++ b/src/hb-ot-shape-complex-private.hh
@@ -179,9 +179,12 @@
     case HB_SCRIPT_PSALTER_PAHLAVI:
 
       /* For Arabic script, use the Arabic shaper even if no OT script tag was found.
-       * This is because we do fallback shaping for Arabic script (and not others). */
-      if (planner->map.chosen_script[0] != HB_OT_TAG_DEFAULT_SCRIPT ||
-	  planner->props.script == HB_SCRIPT_ARABIC)
+       * This is because we do fallback shaping for Arabic script (and not others).
+       * But note that Arabic shaping is applicable only to horizontal layout; for
+       * vertical text, just use the generic shaper instead. */
+      if ((planner->map.chosen_script[0] != HB_OT_TAG_DEFAULT_SCRIPT ||
+	   planner->props.script == HB_SCRIPT_ARABIC) &&
+	  HB_DIRECTION_IS_HORIZONTAL(planner->props.direction))
 	return &_hb_ot_complex_shaper_arabic;
       else
 	return &_hb_ot_complex_shaper_default;