Thanks for the PR, I can not guarantee a speedy reply on this, please be patient.

I've made some good progress on this, thanks to @eamodio's help.
The only issue remaining is you can't go back to a diff editor (I'm not sure why history doesn't support it). But this isn't related to this PR, it doesn't seem you could ever do that.

This also sets up the unused ctrl/cmd + k, shift + O to open either side.

Here's a gif:

Edit:
For some reason diff views get removed from the stack when you navigate away from one. Compare Selected always opens a new tab so its easy to go back, but once that tab is closed you can never reopen that diff in the history.

When changing that to group editor it seems the diffEditor is disposed once closed and cannot be re-opened. I assume this was intentional.

As the above bug seems to have always been there and is separate to this, ive raised a new issue

Awaiting @bpasero to advise.

@bpasero you're right that looks like a mistake, I think I was experimenting there and didn't change it back before committing.
I've both fixed it and rebased.

Hey @bpasero let me know what you think of the changes, I've hopefully addressed both of your points above.

Thanks. I cleaned it up a little.

Thanks for reviewing @bpasero @alexdima

Are you able to offer any guidance to #166805? It feels a bit broken that you can navigate to a diff side but can’t go back.
Is there a reason diffEditors were never added to the history stack? It would be useful to fix this so navigation properly works (for reviewing changes between diff and full file).

I think we only keep editors in the history that can be addressed via plain old JS objects (e.g. { resource: <uri> }), not fully heavy typed editor inputs. We cannot know for sure that a closed editor can be restored from its EditorInput because it is disposed after being closed.