Private PR comments or notes

When reviewing a pull request, it'd be useful to have the option to comment on a diff privately (only viewable by you, the commenter). This could be as a private line/file comment, or a form of bookmarking/tagging with intent to come back to it later in the review process.

Pull requests present the touched files alphabetically, but during peer review the reviewer often has to traverse through referenced files (import/export) to gain a holistic understanding. This usually involves scrolling up/down the PR page, or using the file tree (depending on the UI mode) to traverse between files.

As a reviewer, sometimes you notice something that doesn't look quite right, but you need to double check code elsewhere to confirm. In these cases you can either double check immediately, or write a note in a notepad to come back to that piece later, and continue reviewing.

An inlined digital note mechanism would be useful. It would allow you to jot down your thoughts at the time, without spamming others with your incomplete assessment.

I envision private notes would either be replaced with finalised comments (once validated), or deleted (in the event your initial thought/interpretation was wrong).