I seem to have gotten stuck in MacOS / Xcode purgatory.
I'm on OS Sierra/10.12.6.
When I try to update, I get a warning, that there's an issue with upgrading vs DisplayLink and so it cannot be done.
That directly conflicts with Xcode which (as of now, on App Store) only works on 10.13.2 or later.
I cannot find a place to download an earlier version of Xcode. I cannot upgrade to High Sierra.
How can I get this happening?
You can get it here at this URL: https://developer.apple.com/download/more/ to download a version of Xcode you want. Specifically, get 9.2. This is the recent version that works with mac OS Sierra.
Edit: Xcode 9.2.1 is no longer an available version. 9.2 works.