Fix the size of the floating collapse button in the diff viewer.
The floating collapse button, when in position: fixed mode, was

appearing as wide as the remainder of the diff viewer. This was due to

not specifying any width, which the old one did (as it was an <img/>

tag). We now specify that width/height appropriately.

Expanded a large chunk and scrolled around. The collapse button

remained the same size when switching modes.