 </core-menu-button>

Style the drop-down by using the `core-menu-button::shadow #dropdown` selector:

    /* position this drop-down below the button, and style it white on black. */
    #btn::shadow #dropdown {
        top: 38px;
        color: #fff;
        background: #000;
    }

@class core-menu-button
@status beta
@homepage github.io
-->

<!--
Fired when an item's selection state is changed. This event is fired both
when an item is selected or deselected. The `isSelected` detail property
contains the