If a file is deleted in a changeset, ReviewBoard shows this message in the diff section of the file: "This file was deleted. The content cannot be displayed.". (e.g. https://reviewboard.mozilla.org/r/1629/diff/#1 ) It seems important to keep an eye on what the file contained before deletion. For instance, if one is moving code from one file to another (as in the example), then the reviewer should be able to compare the new code and the code which was present in the previous file, to be sure nothing is missing.
Associated Review Board bug: https://code.google.com/p/reviewboard/issues/detail?id=3783
They moved the reviewboard issue tracker to splat. Here is the new link: https://hellosplat.com/s/beanbag/tickets/3783/
Note that our top reviewer :smaug mentioned this as a blocker for reviewers to work with MozReview (bug 1191597 comment 22).
(I meant just certain types of reviews, but this is indeed among the mozreview bugs keeping me to use bugzilla's 'details' )
This was apparently fixed in the 2.6 branch, but I think it's now scheduled for 3.0. https://github.com/reviewboard/reviewboard/commit/5b87238010c304c5215fde576a0a514afa745d38