If we're looking at the same thing, GitHub's vertical view of multiple files in a pull request is not an editor, it just views files. If you choose to edit a file you get one file in an editor window.

You could fairly easily write a command line utility to view multiple files with headers and line numbers, using the normal utilities. You could add syntax highlighting with one of the command line syntax highlighting utilities. With a little more work you could do the same thing on a web page, like GitHub does.