Mentioning users using "@" gives the user list far below the expected place

Issue Summary

On the commits page, while mentioning users using "@" in a comment, the user list pop up shows far below the place where we typed the @ symbol. 

This is reproducible on Data Center: (yes) 

Steps to Reproduce

1. Goto to the commits page of a repository
2. Click on a commit
3. On the right panel where the contents of the files are shown, click on the small add a comment box
4. Type @ symbol to mention a user

Expected Results

The user list pop right under the dialog box

Actual Results

The list gets populated far below the dialog box. Please find the screenshot.

Workaround

Currently there is no known workaround for this behavior. A workaround will be added here when available