This isn’t in Drupal core yet, and may never be in this exact design, but I think it’s a pretty decent example of how to handle complex forms on the limited screen size on mobile devices and could be great inspiration.
This looks like an excellent, accessible starting point for the priority navigation pattern:
Or the priority navigation pattern, or progressively collapsing navigation menu. We can name it in at least three ways.
There are multiple UX solutions for tabs and menus and each of them have their own advantages over another, you just need to pick the best for the case you are trying to solve. At design and development agency Kollegorna we were debating on the most appropriate UX technique for tabs for our client’s website…
We agreed it should be a one-liner because the amount of tab items is unknown and narrowed our options down to two: horizontal scroll and adaptive with “more” button. Firstly, the problem with the former one is that horizontal scroll as a feature is not always visually obvious for users (especially for narrow elements like tabs) whereas what else can be more obvious than a button (“more”), right? Secondly, scrolling horizontally using a mouse-controlled device isn’t a very comfortable thing to do, so we might need to make our UI more complex with additional arrow buttons. All considered, we ended up choosing the later option[.]
The quest for easy to implement concave corners on boxes is still ongoing, although there are some options open to us as the linked article explores.