I found if you change the css from claro to tundra (//js.arcgis.com/3.16/dijit/themes/tundra/tundra.css), the dropdown for the pattern works. It would be nice to fix in claro because the widget doesn't look that great with tundra. Also there are only a few of the line patterns available in the API in the widget.
I just noticed that the source code for the widgets are available:
arcgis-js-api/dijit at master · Esri/arcgis-js-api · GitHub