Viewed diff file shortcut should provide feedback if the file is not visible

The following discussion from !137912 (merged) should be addressed:

• @slashmanov started a discussion:

  issue(ux): if a file is not in a viewport we can still toggle its viewed state. This might be confusing. To fix that we could show a notice saying which file was collapsed and providing a button to scroll to that file.

  @mle I don't think that's blocking since the feature is working as expected but we might want to do that in a follow-up.