I have not been able to duplicate this issue on my own, but the user did provide some screenshots. Hopefully this is enough for anyone of you to point me in the right place to fix the problem.
If it's only happening for one user, tell them to hard refresh their browser - it could be due to outdated/cached content.
Other than that, we would need some more context: when does it happen, can it be triggered every time, does it happen in a default style with all add-ons disabled?
