MR diffs control bar is positioned incorrectly in IE11

Summary

The MR diffs control bar is offset so that it overlays the tree and list in IE11.

Steps to reproduce

1. Create an MR that has file changes
2. Visit the MR diffs view in IE11

Example Project

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31730/diffs

What is the current bug behavior?

The control bar is positioned over the tree and list areas, blocking some interaction with them.

What is the expected correct behavior?

The control bar should be positioned above the tree and list areas.

Output of checks

This bug happens on GitLab.com

Possible fixes

This seems like the exact same problem that causes gitlab-ce#65966