Ah. Yes, that helped and makes sense considering what control it is; it was a different spinner from what I had in mind (there is no example image in npm), I was more thinking along the lines of a swing spinner (text box with up and down arrows). But it’s rotating happily now.
Okay, so the light picker won’t work. But I was trying https://www.npmjs.com/package/@elifent/paper-datepicker as an alternative. And there are more. Which brings me to the remark “the … should be in your page remark”. Of course! Duh. I was so focused on the Java side I did not bother to check the HTML. Tunnel vision. The paper-datepicker tag is also present, but its display is set to none! Removing that makes it visible. Now, I still think it is weird for a component to set itself to not visible per default, so I kinda defend myself with that. DatePickerLight has no such easy solution, unfortunately.
Next would be programming against that component; getElement and setArgument you say, huh? 