Changed files dropdown on commit page renders underneath diff headers

Summary

Steps to reproduce

1. Go here, or any commit with lots of changed files.
2. Under the Changes tab, open the X changed files
3. Be alarmed at how unusable it is because it is being rendered underneath other elements, blocking important information.

Among the elements that are hidden is a search box that should filter through changed files.

What is the current bug behavior?

See screenshot.

What is the expected correct behavior?

Dropdown should render above everything.

Output of checks

This bug happens on GitLab.com. Version 12.0.0-pre 8104b83f.

Possible fixes

The offending CSS:

• .diff-files-changed (the dropdown) has z-index: 200.
• .diff-file .file-title, .diff-file .file-title-flex-parent (the diff header) has z-index: 220.