On the event page, if you click on the word 'Participants' in the section header, it collapses the section. This behavior should be made a little more obvious. It seems kinda like an Easter egg right now.
This will also apply to the 'Categories' section we introduce in T194703: Add support for categories on the frontend and possibly also for wiki-family section, if we choose to do T200487: [Design] Allow for wiki-family sections on the event page to be collapsed.
There's a few different ways to do this - either have ⊕/⊖ symbols on the header which toggle between the two depending on whether the section is open or closed. Or have a simple 🔽 at the far end of the header to indicate it's collapsible. Let's do whatever is simplest.