To reproduce the problem:

1. Go to the Pull Requests view.
2. Expand a Pull Request there. Open a diff. Keep this diff open, make sure it's not a preview (ctrl + K, ctrl + enter), and put your cursor in another editor so that the diff is no longer the active editor.
3. Decide you now want to check out the PR and right click -> checkout PR.
4. From your tabs or your currently open editors, you navigate back to the diff in step 2.
5. Intellisense does not work in this diff. This is happening because the diff came from the "Pull Requests" view instead of the "Changes in Pull Request" view.

This is confusing and unintuitive behavior. We should "upgrade" or close (+ maybe reopen) the already open diff so that users don't end up in what appears to be a broken state.