For diff comments that were on new or deleted files, the heading for the file

would spill outside of the review box. This is fixed by adding a

display: grid to the diff table and to the filename row in the table for

diffs that appear in review comments. This problem didn't appear for diff

comments on existing files because we were already specifically giving

them a grid display.

Looked at diff comments on new files in mobile and desktop.
Looked at diff comments on existing files in mobile and desktop.
Looked at the diff viewer in mobile and desktop, made sure it was the

  same as before.