Historically, the diff viewer table has used collapsed borders, meaning

that the browser would determine which of the borders being applied

in-between two cells would apply. This worked alright, but it meant we

were subject to the browser's implementation, and more recently it

impacted some work being done when rendering diff fragments

(specifically, that work involves transforming the position/size of

some rows, and doing this caused funky results with the border

positions).
This change moves us to a separated border model. In this model, two

neighboring cells can each have their own borders and they'll both

apply (doubling the border). Because of this, the border styles had to

be redone a bit. We now set the border-top of the first line of chunk

borders and leave the bottom borderless, allowing the following chunk to

use its border-top to complete the borders of the previous chunk.
The separate borders also allows us to have cleaner separators in the

diff fragments when expanding.
Hand-crafted precedence rules were written to choose the best borders

for each neighboring combination of chunks, helping to pick the

best-contrasting borders in each situation.
Presentation-wise, nothing has actually changed. Everything actually

renders as it did before, with the exception that some of the borders

pair up nicer. It will make some following work possible, though.

Checked all combinations of adjacent chunk types, and inspected all

borders everywhere in the diff viewer. Verified that everything looked

the same (or subtly better).
Expanded diff fragments in reviews and saw that the border lines were

still showing up.