The diff chunk selector is now cleaner, both visually and code-wise.

The big borders are gone. Instead, we have highlighting on the side,
which no longer has a "jumpy" effect like the borders have. They also
don't have the offset problems, nor do they cover up content.

The code is a bit cleaner. This class is no longer a singleton, and
instead is owned by the page. Recalculating the positions and sizes is
less expensive, and done as infrequently as possible.

The result is a lighter-weight, less noisy chunk selector.

Played around with the selector on different browsers. The results were

consistent.
Jumped all around the page, back-and-forth, with arrow keys and with clicking.
Tried zooming the page in/out, and resizing, while watching console.log

output. We calculated infrequently.