Selectable: lasso offset issue with scrolling

I saw #4377, it's similar, but not the same.

This bug is there without the appendTo problem.

Test case:

• Go to: http://jqueryui.com/resources/demos/selectable/display-grid.html (the selectable example page's iframe)
• Resize the window so that you get the scrollbars
• Scroll down and to the right a little
• Click (but don't move the mouse!)

What you get: A little 1x1 point over and to the left of the mouse, which is the lasso, before the resizeing.

The problem is, that it is initialized like this:

		// position helper (lasso)
		this.helper.css({
			"left": event.clientX,
			"top": event.clientY,
			"width": 0,
			"height": 0
		});

But according to W3 and maybe the more clearly sentenced Mozilla MSDN:

"For example, clicking in the top-left corner of the client area will always result in a mouse event with a clientX value of 0, regardless of whether the page is scrolled horizontally."

Solution:

So we need the scrolling offset added to it.

Workaround (one can use this before the official solution, put this before your selectable() initialization):

        $.widget("ui.selectable", $.ui.selectable, {
            _mouseStart: function(event) {
                
                this._super(event);
    
                this.helper.css({
                    "left": event.clientX + $(window).scrollLeft(),
                    "top": event.clientY + $(window).scrollTop()
                });
            }
        });