I found what I was talking about: stackoverflow.com/a/11614605. It was a feature that the Hangouts extension could use, but the user had to manually enable it in the browser settings for any other extensions to use it.
The apps feature is still there just with a different name. It’s labeled as “create shortcut”, and you have to check the box to open a new window. I use it just because Firefox doesn’t have a similar feature.