The existing implementation of diff expansion in reviews used

JavaScript-side animations for expanding the collapsed expansion

controls in a diff on hover. As the controls expanded, the content below

the diff would get pushed down. This caused a lot of rendering on the

page, which could be slow and made scrolling through the review request

page a bit more tedious.
The new implementation uses CSS transforms and transitions to shift and

scale the controls and the diff header, which allows us to expand

outward from the diff without impacting the position of the diff itself

or any of the content below it on the page. This is much faster,

resulting in a smoother animation and less jumpiness on the page.
To do this, we begin the diff fragments in a collapsed state, scaling

down the controls and shifting the diff header down by the same amount.

We do this without transitions. After it's been loaded and scaled down,

transitions are enabled, which will then start taking place when the

user hovers.

Tested on Chrome, Firefox, and IE9+.
Tested with transitions fully disabled to make sure functionality

still worked.
Tested with diffs containing headers just above, just below, both

above and below, and neither above nor below.