The new chunk highlighter had two regressions. First, if the width of
the page shrunk, there would be a horizontal scrollbar, due to the
highlighter width not being reset to the natural width of the container.

Second, the code assumed that there were chunks to display, but for
loading diffs, this wouldn't be the case. We now check to make sure we
can get the position of the chunk before doing any calculations based
off of it, mirroring the logic we had before.

No longer hit JavaScript errors due to pending diff loads.
Resized the window back and forth, and saw that the highlighter width

was correct.