Hi @Ralfhauser, thanks for taking the time to report this and welcome to Wikimedia Phabricator!

Unfortunately this report lacks information. Where to see a "responsive design"? Which software is this about? If you have time and can still reproduce the problem: Please add a more complete description to this report.
That can be a clear list of specific steps to reproduce the situation, as little details sometimes matter, so that nobody needs to guess how you performed each step, describing actual results and expected results after performing the steps to reproduce, providing a link to a public website where the issue can be seen, if you see the same behavior when you are not logged in, information about your web browser(s), MediaWiki version information (if it is not on a Wikimedia wiki), etc.
You can edit the task description by clicking Edit Task.
Ideally, exact and clear steps to reproduce should allow any other person to follow these steps (without having to interpret those steps) and see the same results. Problems that others can reliably reproduce can get fixed faster. Thanks!