Regression: File pane scrolls up when item is clicked

Created by: pdubroy

• Sourcegraph version: 8469a10
• Platform information: macOS, Chrome 90.0.4430.93

Steps to reproduce:

1. Go to https://k8s.sgdev.org/ghe.sgdev.org/sourcegraph/sourcegraph-sourcegraph/-/blob/web/src/site-admin/SiteAdminAddExternalServicePage.tsx
2. Click on __snapshots__ in file tree.

Expected behavior:

1. File pane does not scroll up

Actual behavior:

1. File pane scrolls up, so that the tabs are no longer in view.

I was also able to reproduce this use the new local development server, and when I reverted fab61d100c293df8f944d0169dba720a5e3afe4b I couldn't reproduce the problem anymore.