David, is there any manual test coverage for it?

Yep! Here's the manual test case:

Can mirror a pop-up window
1. Open dev tools console from google.com and enter:
2. Right click on the pop-up window and select “Cast” then select a receiver to Cast to.
3. Verify the pop-up window is mirrored to the receiver.

oops, the remainder of step #1 is:

let params = `scrollbars=no,resizable=no,status=no,location=no,toolbar=no,menubar=no,
width=600,height=300,left=100,top=100`;

open('/', 'test', params);