Position: positioning within document raises error

jQuery raises an error when trying to position an element within the document:

$( "#elx" ).position({
	my: "left top",
	at: "right bottom",
	of: "#parent",
	within : document
});

The reason is that jquery-ui tries to access '$(document).css( "overflow-x" )' which causes an error in jQuery. Maybe that's a bug in jQuery itself, not sure if it's legal to access css of document. Either way I created a fix for it in jquery-ui.

Pull request will follow shortly.

The underlying reason in jQuery is that it tries to map the style property in the 'css' function to a vendor prefixed property in this line, causing the error in the 'vendorPropName' function:

// Make sure that we're working with the right name
name = jQuery.cssProps[ origName ] || ( jQuery.cssProps[ origName ] = vendorPropName( elem.style, origName ) );

Because the vendor-prefixed names are cached, the error does not occur when the style-prop "overflow-x" is used on another element earlier, which makes the bug especially hard to catch!