Janky Monaco editor CSS/behavior

Created by: chrismwendt

• Sourcegraph version: Sourcegraph.com
• Platform information: Chrome and Firefox

Steps to reproduce:

1. Visit https://sourcegraph.com/site-admin/global-settings (or any settings page)
2. Try selecting text

Expected behavior:

The selection behaves like a normal text box.

Actual behavior:

The selection is erratic and the cursor shows up in the wrong place: