1st file in GitHub PR does not have Sourcegraph actions/hovers initially

[sqs]

Repro:

1. Go to https://github.com/sourcegraph/go-diff/pull/31/files#diff-1d80fd3313706b7349ac29bdc684e129R513
2. Try to hover over things in the first file (diff/diff_test.go); nothing happens. Also notice that there is no View file diff button on the first file's header.
3. Scroll down all the way on the page
4. Scroll back up
5. Now hovers work on the first file and the View file diff button appears

I am not able to repro this on other PRs such as https://github.com/gorilla/mux/pull/463/files. I don't know why. But this is an important bug to fix because this we intend to use this go-diff PR as a demo at dotGo tomorrow.

[felixfbecker]

Confirmed that this is not reproducible with #2909