Many organisations have requirements for long-term sustainable software systems and associated communities. In this paper we consider long-term sustainability of Open Source software communities in Open Source projects involving a fork. There is currently a lack of studies in the literature that address how specific Open Source software communities are affected by a fork. We report from a case study aiming to investigate the developer community around the LibreOffice project, which is a fork from the OpenOffice.org project. The results strongly suggest a long-term sustainable community and that there are no signs of stagnation in the project 15 months after the fork. Our analysis provides details on the LibreOffice developer community and how it has evolved from the OpenOffice.org community with respect to project activity, long-term involvement of committers, and organisational influence over time. The findings from our analysis of the LibreOffice project make an important contribution towards a deeper understanding of challenges regarding long-term sustainability of Open Source software communities.