How do I render the absolute URL of a referenced entity in a simplenews newsletter? When I send a node as a simplenews newsletter I use the node's Email: HTML display. 
One of the fields in this view is a label of a referenced entity to be rendered as a Link to the referenced entity.
Drupal renders it as a link with a relative / site-local URL. I want it to be rendered as an absolute / external URL.

For what it's worth:
In the render array the link is stored in
$variable['items']['0']['content']['#url'] which is a Drupal\Core\Url with a protected external property of false  

I can't see anything in the Url interface that would change external to true or indeed whether this is the thing to do.
external and absolute are different:

external is an url without internal route. You can't change this property. This is set when you create an url.

absolute is an option, which you can set at any time in the url object:

$url-&gt;setAbsolute();


Or set to false:

$url-&gt;setAbsolute(FALSE);


And this will change how the url is generated absolute or relative.