In this case when the user clicks on the scrollbar, mouseup event is raised immediately after mousedown, before mouse button is released. Here is the workaround:
<script type="text/javascript">
	var rfdPrototype = Telerik.Web.UI.RadFormDecorator.prototype;
	var mouseDownHandler = rfdPrototype._selectBodyMouseDownHandler;
	rfdPrototype._selectBodyMouseDownHandler = function (ev) {
		var upHandler = this._selectBodyClickDelegate;
		if (Telerik.Web.Browser.ie && upHandler) {
			$telerik.removeHandler(document, "mouseup", upHandler);
			setTimeout(function () {
				$telerik.addHandler(document, "mouseup", upHandler);
			}, 0);
		}
		mouseDownHandler.call(this, ev);
	}
</script>