In https://github.com/HydraWiki/Tabber/issues/9, rimas-kudelis wrote:

It would be great if changes from this repository were merged upstream. I was recently upgrading a couple MediaWiki installations, and I missed the fact that currently Tabber lives in this repository, and checked out from Wikimedia's git instead, just because the repository was available there. Even though the extension page points to this repository, not everyone will go and check during upgrade. Now that commits to this repository are relatively rare, the necessity for a fork here is even less obvious.