converse.js: Singleton mode doesn't work

Describe the bug The Prosody web chat is using the embedded view mode

To Reproduce Steps to reproduce the behavior:

1. Configure with both singleton:true and view_mode:"embedded"
2. ???
3. Blank page

Roug minimal config along these lines:

converse.initialize({
"auto_join_rooms" : ["group@chat.example"],
"auto_login" : true,
"blacklisted_plugins" : [
	"converse-controlbox",
],
"bosh_service_url" : "/bosh",
"discover_connection_methods" : false,
"jid" : "anonymous.sasl.chat.example",
"singleton" : true,
"view_mode": "embedded"
});

Setting singleton:false (or commenting it out) makes it sorta work, but the single chat does not take up the entire viewport, leaving space roughly the size of the missing controlbox.

More complete example with HTML etc in the form of an attempt to upgrade chat.prosody.im to Converse.js 8.x

Expected behavior Expected the Prosody web chat but with Converse.js 8.x goodies 😃

Screenshots

Imagine a blank page.

Environment (please complete the following information):

• Firefox 78 ESR (also tested on FF 91 earlier)
• Converse.js v8.0.0

Additional context Add any other context about the problem here.