diff --git a/ui/jquery.ui.position.js b/ui/jquery.ui.position.js
index eef8a95c9..2e2fe215a 100644
--- a/ui/jquery.ui.position.js
+++ b/ui/jquery.ui.position.js
@@ -360,7 +360,7 @@ $.ui.position = {
 			var within = data.within,
 				withinOffset = within.offset.left + within.scrollLeft,
 				outerWidth = within.width,
-				offsetLeft = within.isWindow ? 0 : within.offset.left,
+				offsetLeft = within.isWindow ? within.scrollLeft : within.offset.left,
 				collisionPosLeft = position.left - data.collisionPosition.marginLeft,
 				overLeft = collisionPosLeft - offsetLeft,
 				overRight = collisionPosLeft + data.collisionWidth - outerWidth - offsetLeft,
@@ -395,7 +395,7 @@ $.ui.position = {
 			var within = data.within,
 				withinOffset = within.offset.top + within.scrollTop,
 				outerHeight = within.height,
-				offsetTop = within.isWindow ? 0 : within.offset.top,
+				offsetTop = within.isWindow ? within.scrollTop : within.offset.top,
 				collisionPosTop = position.top - data.collisionPosition.marginTop,
 				overTop = collisionPosTop - offsetTop,
 				overBottom = collisionPosTop + data.collisionHeight - outerHeight - offsetTop,