Manually mark file in pull request as unread

When reviewing large pull requests, I'm often switching between files. When I'm just looking something up in a file, I haven't reviewed the whole file and would like it to remain 'unread' (bold). This way, I have a better overview of which files still need reviewing. It would, therefore, help if there is an option to manually mark a file as 'unread', similar to e-mail.