Position: Avoid reading overflow css on documents

Fixes #9533
Closes gh-1072

Author: meyertee

tests/unit/position/position_core.js

@@ -575,7 +575,14 @@ test( "collision: flip, with margin", function() {
 });
 
 test( "within", function() {
-	expect( 6 );
+	expect( 7 );
+
+	collisionTest({
+		within: document
+	}, {
+		top: 10,
+		left: 10
+	}, "within document" );
 
 	collisionTest({
 		within: "#within",

ui/jquery.ui.position.js

@@ -89,8 +89,10 @@ $.position = {
 		return (cachedScrollbarWidth = w1 - w2);
 	},
 	getScrollInfo: function( within ) {
-		var overflowX = within.isWindow ? "" : within.element.css( "overflow-x" ),
-			overflowY = within.isWindow ? "" : within.element.css( "overflow-y" ),
+		var overflowX = within.isWindow || within.isDocument ? "" :
+				within.element.css( "overflow-x" ),
+			overflowY = within.isWindow || within.isDocument ? "" :
+				within.element.css( "overflow-y" ),
 			hasOverflowX = overflowX === "scroll" ||
 				( overflowX === "auto" && within.width < within.element[0].scrollWidth ),
 			hasOverflowY = overflowY === "scroll" ||
@@ -102,10 +104,12 @@ $.position = {
 	},
 	getWithinInfo: function( element ) {
 		var withinElement = $( element || window ),
-			isWindow = $.isWindow( withinElement[0] );
+			isWindow = $.isWindow( withinElement[0] ),
+			isDocument = !!withinElement[ 0 ] && withinElement[ 0 ].nodeType === 9;
 		return {
 			element: withinElement,
 			isWindow: isWindow,
+			isDocument: isDocument,
 			offset: withinElement.offset() || { left: 0, top: 0 },
 			scrollLeft: withinElement.scrollLeft(),
 			scrollTop: withinElement.scrollTop(),