Environment detail page cuts off content with a scrollbar

Summary

Steps to reproduce

1. Create an environment with a single deployment and navigate to the environment's detail page
2. Click the "Deploy to..." button and scroll

What is the current bug behavior?

A scroll bar is shown and the dropdown content is cut off.

What is the expected correct behavior?

The scroll bar should not appear and the dropdown content should not be cut off.

See dropdown guidelines in Pajamas, and the component UI specs.

Output of checks

This bug happens on GitLab.com.