I just stumbled across the issue with isabs(). I'd also say that Mark Hammond already provided the typical use case for this, i.e. that you allow relative paths for convenience when storing them or putting them on the commandline, but for actual use you first convert them so that their meaning doesn't change under your feet, e.g. because they depend on volatile things like the current working directory or drive.

If you assume that is the intention or typical use case, then calling isabs() to determine if the path is stable doesn't work. Of course, the wording of the documentation then needs to change, too, as it explicitly says "after chopping off a potential drive letter".

Concerning the behaviour of path.join() and support for "\\server\share" paths, I'm not sure, but I think that these are issues that can be discussed/changed separately.