Include a Scroll bar to scroll the files list on a PR

Issue Summary

Currently, the Pull Request page affords one scroll bar to roll the diff part, and there is no scroll bar to scroll the files list on the right side of the page.

Steps to Reproduce

1. Open a Pull request;
2. Check that there is just one scroll bar. 

Expected Results

The customer expects that we include a scroll bar to turn better visible from the computer view and the tablet view. 

Actual Results

The file list still being scrollable even without the scroll bar. 

Workaround

Use the mouse, touchpad, or  arrow-keys to scroll