It was implemented on the latest commit to Authentic Theme, which will be shipped with the next Webmin 2.103 release.
On the other hand it could be very simple.
It may sound simple but theme is extremely complex. For instance implementing this feature to work correctly on page full reload, after login and returning back to the Virtualmin tab was very time consuming. Besides, the last used virtual server could be simply deleted and we needed to handle it too.