It's set to notification only and it's hard to reproduce. First of all mentions itself are rare, because you normally get direct replies to your messages in topics you have posted in, which don't count as mentions. In case someone mentions you outside of this context and you don't get a notification and you don't read the topic by chance, you cannot know.
When we tested this at the start of the thread, I first didn't get mention notifications and in the end I did get one. It was kinda random.
@joelyoung Whenever new subforums are created, you have to visit them and set them to "ignore" manually. Do it and you are done, nodeBB will hopefully fix this at some point. Doesn't happen all too often by the way.
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@pesala Yes, there's still plenty of space in the header before they disappear, but this is not the trigger for the mobile layout. The mobile layout comes in "one piece", so once the screen gets too narrow, it just kicks in regardless of how much space was left there for some of the page elements. I think it's measuring the screen width for the posts area, not the headers.
EDIT: Just to make it clear - I'm talking about "screen", but I'm actually thinking of the viewport because that's what actually matters to the browser...