Hi,
the idea is to use a 'master' branch as a preparation for the next 'screen-v5', i.e. a development branch for Screen v5.x.x releases.
Many people don't like this idea and situation with different code bases in different branches (including me).
I think some discussion will take place before we're going to start working on v5.0.0 release.
If you're going to start working on code, please use the 'screen-v4' branch. Code from this branch is used to generate tarballs[1] that maintainers use for creating Screen-packages for GNU/Linux and other systems.