What version are you running?
2.5.2
What's the URL of the page this enhancement relates to, if any?
.../r/####/
Describe the enhancement and the motivation for it.
When viewing a diff comment on the reviews page, right now the 4 clickable things that expand on hover are:

show 20 previous lines
show all previous lines
show all next lines
show 20 next lines

None of these options provide a quick one-click way to expand "context", which is what I want to see when looking at an issue.
It would be nice if there was a clickable button (that didn't require hovering, or waiting for expando animations or anything) that was just "Show [N] Context", to show N previous and following lines.  Ideally N would be a user preference.
Please provide any additional information below.
Originally filed against MozReview here: https://bugzilla.mozilla.org/show_bug.cgi?id=1243474