One common request we've had over the years is to be able to know what

file is currently being viewed when scrolling through the diffviewer.

It's easy to lose track of the current file in the middle of a long

diff.
This change makes it so the file list docks into the unified banner once

the user scrolls past it. The view there is filtered to show only the

entries corresponding to any diff file blocks visible in the current

viewport. There's a 50px buffer area at the top and bottom in which we

calculate a partial vertical offset and height so it looks like the file

list is scrolling along with the page.
This does not yet allow users to pop open the entire file list in order

to quickly scroll to a specific file, but that's coming in a future

change.

Spent a ton of time scrolling around a large diff and verifying that the

docked file list looked correct. Paid particular attention to the

boundary at which we convert the file list from undocked -> docked and

vice-versa, and the boundaries where files were scrolling on or off the

page at both the top and bottom.