Web: scrolling is broken on compare view

Created by: lguychard

Steps to reproduce:

1. Visit https://sourcegraph.com/github.com/sourcegraph/sourcegraph/-/compare/c8a94201b0273a86a5f91631c915963bd648c55a...a0d0a97754e5963b0bcc4e003eec9534642f2b42#diff-8fafc9dac6c701a020eafaa59db7d806

Expected behavior:

You can scroll to the bottom of the diff.

Actual behavior:

No scrolling, the bottom of the diff is clipped.