What if a reference contains a link to another page on the same wiki? The standard behavior for such links (when shown as part of the page, not in a popup) is to open in the same browser tab, and not open a new one. Only external links open in new tabs. The definition of "external" in MediaWiki core might be a little more complicated, but essentially it's "links to another domain".
As of now, all links in a ReferencePreview popup open a new tab. This might not be what users expect. This ticket here is to discuss this, and find a solution that fits the users expectations best.