Well, it seems to be all browsers problem: when you click on a scrollbar, a mouse event is fired. This is the real problem, jQuery UI just doesn't solve it. Let's fix it on our own in the jQuery UI .js file (not applied to the min version as it should be obfuscated AFAIK).
Add this condition
if (event.pageX > $(event.target)[0].clientWidth + $(event.target).offset().left)
return;
right after the
_mouseDown: function(event) {
I have seen a lot of such hacks with HasScrollbar()
detectors, but don't get why didn't they just sum up client width (that is without scrollbar) and offset to make it relative to the document and compare with pageX. For me it works perfectly.