fix(topbar) don't rely on scrollTop() for sticky

When top bar is sticky:
```
<div class="sticky">
  <top-bar>...</top-bar>
</div>
```

All browsers will throw an ```Uncaught TypeError: undefined is not a function``` for ```win.scrollTop()```.

This is caused by the fact that ```scrollTop()``` is not a native JS function but a jQuery function.  Instead, we should be using ```$window.scrollY``` which is supported cross browser (> IE8).