@rene A bit of that, but I'd call it poor testing, poor CSS design, and a system architecture which cripples how they can actual handle being responsive, which is ironic, because one of their reasons to switch to "Stacks" was to enable "responsiveness". Well, OK, this one can probably be chalked up to just SE doing only limited testing prior to pushing massive changes out to everyone. The rest is true, but lack of testing is the real culprit in this case.
I expect there to be bugs and not catching everything in testing, as the testing matrix is huge. I'm just extremely frustrated at the consistently low level of testing which SE chooses to do prior to pushing such major changes to production.
As for this, yes, it's a bug. There are a few easy ways to fix it. Probably the most appropriate is adding a flex-grow on the container.
@Makyen I never really understood why they had to reinvent the wheel. I mean; When I used stacks I often looked at Bootstrap first to understand what the intent of a component was and then tried to fit the Stacks approach in my stuff. Sometimes i needed a hammer and a chisel to make it fit
I expect they still don't have a proper test team with dedicated testers. I hope they do code-reviews for a checkin and I do hope reviewing the proper use of CSS and markup is part of that but I doubt a reviewer will painstakingly verify all UI edge cases.
Another option might be to have the UX designer actually do the testing of the "finished" feature
