Determine whether user clicking scrollbar or content (onclick for native scroll bar)

Solved:

A shortest scrollbar click detection I could come up with, tested on IE, Firefox, Chrome.

var clickedOnScrollbar = function(mouseX){
  if( $(window).outerWidth() <= mouseX ){
    return true;
  }
}

$(document).mousedown(function(e){
  if( clickedOnScrollbar(e.clientX) ){
    alert("clicked on scrollbar");
  }
});

Working example: https://jsfiddle.net/s6mho19z/

You may probably use this hack.

You could try hijacking the mousedown and mouseup events and avoiding them when click on a scrollbar with your custom powered function.

$.fn.mousedown = function(data, fn) {
    if ( fn == null ) {
        fn = data;
        data = null;
    }    
    var o = fn;
    fn = function(e){
        if(!inScrollRange(e)) {
            return o.apply(this, arguments);
        }
        return;
    };
    if ( arguments.length > 0 ) {
        return this.bind( "mousedown", data, fn );
    } 
    return this.trigger( "mousedown" );
};

And the inverse for mousedownScroll and mouseupScroll events.

$.fn.mousedownScroll = function(data, fn) {
    if ( fn == null ) {
        fn = data;
        data = null;
    }    
    var o = fn;
    fn = function(e){
        if(inScrollRange(e)) {
            e.type = "mousedownscroll";
            return o.apply(this, arguments);
        }
        return;
    };
    if ( arguments.length > 0 ) {
        return this.bind( "mousedown", data, fn );
    } 
    return this.trigger( "mousedown" );
};

By the way, I think the scrollbar width is an OS setting.

Use following solution to detect if user clicked mouse over element's scrollbar. Didn't test how it works with window's scrollbar. I guess Pete's solution works better with window scrolls.

window.addEventListener("mousedown", onMouseDown);

function onMouseDown(e) {
  if (e.offsetX > e.target.clientWidth || e.offsetY > e.target.clientHeight) 
  {
      // mouse down over scroll element
   }
}