The recent work on expanded diff fragments regressed the visibility of

the controls when expanded. Controls should always appear when expanded

(even on mouseleave), and should default to being hidden again when

collapsing. However, the recent changes resulted in the controls

immediately collapsing after expansion, and sometimes showing after

collapsing.
We now store some state as to whether the fragment has expanded, and we

use this to determine whether controls can be hidden. The rendering

order has also changed to better ensure that state is correctly set

before/after expanding and collapsing diffs.

Expanded diffs and saw that the controls remained on-screen. Collapsed

and saw that they properly disappeared.