Draggable, scrollable, wrappable diffs

Description including problem, use cases, benefits, and/or goals

When reviewing code in parallel view, I (and I assume others) are often focused on one side (old version of the file or new version of the file). One of these sides of the diff is much more likely to be longer that the other, unless they only show diffs for a few characters per line here and there. Especially long lines can end up wrapping which as a no-soft-wrap-developer is particularly confusing.

I see the previous discussion on this from a few months ago gitlab-ce#15382, would the problems with the comments box be easier to solve with gitlab-ce!4776 merged? It was highlighted that scrolling and wrapping was a user preference.

Proposal

• Look into scrollable diff panels and profile its viability alongside gitlab-ce!4776.
• If scrollable diff panels is viable, add a user setting for the default behaviour of overflowing code lines.
• Look into draggable diff panels where a user can drag the center of a parallel diff view (highlighted in the screenshot below) in order to expand the trade-off width of the old and new panels. This isn't as good as scrollable due to case where expanding one side of the diff panel as far is it can go does not help clear all of the wrapped lines.