[visual-studio-code] Cannot edit in read-only editor VS Code

I received this error during a code compare with previous version and it wasn't letting me edit the current version in the Right-Window. Unrelated to what I suspect OP's issue but this was the first thread that came up for my search and the error was the same. anyway...

My issue was that the particular file was 'Staged' in my source control at the time. This appears to restrict editing by opening an 'index' version for the compare.

Solution: Un-stage the file, and reopen the comparative window.