Double scroll bar in file template dropdown in Web IDE

Summary

The file templates dropdown in the Web IDE shows two scroll bars:

Branches/MR selector:

This happens in Chrome and Firefox on Mac (with scrollbars on Always in system preferences), Linux and Windows.

What is the current bug behavior?

Two scroll bars are shown.

What is the expected correct behavior?

Only one scroll bar should be shown.

Output of checks

This bug happens on GitLab.com.