• _path_join('//server/share', 'spam') should return "//server/share/spam", not "//server/sharespam"
• _path_join("C:/", "spam") should return "C:/spam", not "C:spam"
• _path_join("C:", "") should return "C:", not "C:\".
• _path_join("C:", "D:") should return "D:", not "D:\"

I don't like the change to make _path_splitroot() shift the separator into the path. It goes against what I expect given the function name, in terms of separating the root from the rest of the path. The root variable here actually has no root path, which is counterintuitive.

As it stands, the following fixes the above case by shifting the separator back into root. The UNC + relative case has to be handled specially at the end.

    if path and path[0].startswith(path_sep_tuple):
        root += path[0][0]
        path[0] = path[0][1:]
    tail = path_sep.join(c for p in path if (c := p.rstrip(path_separators)))
    # Fix up appending a relative path to a UNC root
    if (tail and root.startswith(path_sep_tuple) and
          not root.endswith(path_sep_tuple) and
          not tail.startswith(path_sep_tuple)):
        root += path_sep
    return root + tail

Maybe change the name to nt._path_splitdrive, since that makes more sense now that the root is shifted over to the path side. Implement the POSIX fallback as _path_splitdrive, which only returns str, just like the builtin function. Move the above bytes support code into a common implementation of ntpath.splitdrive(), which is likely the only place it will ever be needed. _path_splitdrive() will be used directly when bytes support isn't required.

I'm sure you know the os.fspath() call isn't necessary since it's handled for a path_t argument in C, so why is it called explicitly here?

The isinstance at 69 needs to recognise values for p that have __fspath__ that returns bytes, as well as actual bytes, which is why we fspath once instead of doing it multiple times.

The isinstance at 69 needs to recognise values for p that have __fspath__ that returns bytes, as well as actual bytes, which is why we fspath once instead of doing it multiple times.

Sigh... sorry about that. :-$