A custom widget built by composition of Autocomplete and Button. You can either type something into the field to get filtered suggestions based on your input, or use the button to get the full list of selections.
The input is read from an existing select-element for progressive enhancement, passed to Autocomplete with a customized source-option.
This is not a supported or even complete widget. Its purely for demoing what autocomplete can do with a bit of customization. For a detailed explanation of how the widget works, check out this Learning jQuery article.