Selectable: Account for scrolling on the helper's initial position. F…

…ixed #8915 - Selectable: Lasso offset issue with scrolling.

tests/unit/selectable/selectable_events.js

@@ -35,4 +35,32 @@ test( "stop", function() {
 	});
 });
 
+test( "mousedown: initial position of helper", function() {
+	expect( 2 );
+
+	var contentToForceScroll, helper,
+		element = $("#selectable1").selectable();
+
+	contentToForceScroll = $("<div>").css({
+		height: "10000px",
+		width: "10000px"
+	});
+
+	contentToForceScroll.appendTo("body");
+	$( window ).scrollTop( 1 ).scrollLeft( 1 );
+	element.simulate( "mousedown", {
+		clientX: 10,
+		clientY: 10
+	});
+
+	helper = $(".ui-selectable-helper");
+	equal( helper.css("top"), "11px", "Scroll top should be accounted for." );
+	equal( helper.css("left"), "11px", "Scroll left should be accounted for." );
+
+	// Cleanup
+	element.simulate("mouseup");
+	contentToForceScroll.remove();
+	$( window ).scrollTop( 0 ).scrollLeft( 0 );
+});
+
 })( jQuery );

ui/jquery.ui.selectable.js

@@ -88,8 +88,8 @@ $.widget("ui.selectable", $.ui.mouse, {
 		$(options.appendTo).append(this.helper);
 		// position helper (lasso)
 		this.helper.css({
-			"left": event.clientX,
-			"top": event.clientY,
+			"left": event.pageX,
+			"top": event.pageY,
 			"width": 0,
 			"height": 0
 		});