Earlier quoted context omitted.
I'm using case-sensitive HFS+. When I last reinstalled macOS on my laptop it was one of the options available. I thought about it and thought a case-sensitive file system was more sensible. Any problems with case-sensitive file systems must be solved if we want a smooth migration to APFS, which is case-sensitive. I've already reported or helped fix a handful of minor bugs related to case-sensitivity.
I can't see the advantage to having two files open and Open in the same directory. Why would you want that?
(My preference is to leave the root partition case-insensitive, since some apps require that, but to keep all my data and source code on a case-sensitive partition.)