What version are you running?
2.5.2
What's the URL of the page this enhancement relates to, if any?
.../r/####/diff/#
Describe the enhancement and the motivation for it.
When reviewing it can be useful to jump back and forth between specific files or parts of the patch. Right now this requires scrolling all the way to the top of the page and clicking the desired file, or remembering where it is (which can be difficult in a large diff).
It would be useful to have an easier way to call up which files are part of the diff from anywhere on the page.

Please provide any additional information below.
First reported as a desired feature of MozReview (Mozilla's customized Review Board instance) here: https://bugzilla.mozilla.org/show_bug.cgi?id=1238120