Dropdown closes after using the scrollbar to scroll to the desired item

[mqg734]

Might be similar to issue #12.

1. Go to demo page https://www.polymer-project.org/components/core-dropdown/demo.html
2. In the "Scrolling and margin" demo, open either dropdown
3. Scroll (by dragging the scrollthumb instead of the mousewheel) to United States and try to select United States.

Result: The dropdown closes when you release the mouse after scrolling to the desired item.
Expected: The dropdown remains open after using the scrollbar.

Workaround: Mousewheel scroll seems to work ok.

[omerts]

The problem is that tapping the dropdown itself is triggering the same "toggle" callback that tapping the menu icon triggers.
You can add the "layered" attribute to your dropdown, which will result in your dropdown being in a different layer, causing it to stop triggering its parent's toggle callback.

For polymer's developers maybe you could stop the tap event from propagating to the core-dropdown-base (or any other parent element toggling a dropdown).

[W64BugBear]

I dicovered that (a) the event detail of the typ event is null when you click on the scrollbar and (b) you can determine the first element of the event propagation path.
As sugested in issue #12 i changed the toggleOverlay function of the core-dropdown-base element to the following:

this.$.coreDropdownElementId.toggleOverlay = function (e, detail, sender) {
    if (detail !== null || e.path[0].localName !== 'div' || e.path[0].id !== 'scroller') {
        this.opened = !this.opened;
    }
}

It feels like a dirty hack to me and I did not dig into the polymer sources deep enough to suggest this as a fix for this issue, but it works in my use case for now and I hope that someone with more knowledge about this topic will implement an appropriate fix for the dropdown scrollbar in a future version (currently using Polymer 0.5.4).