Hover tooltips on sourcegraph.com broken and page jitter

Created by: vanesa

• Sourcegraph version: Sourcegraph.com

Steps to reproduce:

1. Go to any code view on Sourcegraph.com e.g. https://sourcegraph.com/github.com/sourcegraph/sourcegraph/-/blob/web/src/repo/RepoHeader.tsx
2. Hover over the nav links on the right. Notice that no hover tooltips appear even though the DOM shows the rendering of the tooltip div.
3. Also notice the additional and redundant scrollbar that appears when hovering over the links, making the page jitter.

Expected behavior:

• No additional scrollbar and visible tooltips

Actual behavior:

• No tooltips, page shakes due to additional scrollbar