##<iron-dropdown>
<iron-dropdown> is a generalized element that is useful when you have
hidden content (.dropdown-content) that is revealed due to some change in
state that should cause it to do so.
Note that this is a low-level element intended to be used as part of other composite elements that cause dropdowns to be revealed.
Examples of elements that might be implemented using an iron-dropdown
include comboboxes, menubuttons, selects. The list goes on.
The <iron-dropdown> element exposes attributes that allow the position
of the .dropdown-content relative to the .dropdown-trigger to be
configured.
<iron-dropdown horizontal-align="right" vertical-align="top">
<div class="dropdown-content">Hello!</div>
</iron-dropdown>In the above example, the <div> with class .dropdown-content will be
hidden until the dropdown element has opened set to true, or when the open
method is called on the element.