It turned out that
overscroll-behavior – the CSS feature that dictates how scrollable elements behave relative to their parent element – does not prevent chain-scrolling if the element itself is not overflowing (i.e. if there's enough space on the screen to fit all contents).
Chain-scrolling is what occurs when you try scrolling an element, reach this element's top or bottom, and scroll further: parent element now scrolls instead. This is confusing behavior: you apply effort to one thing and move another one instead.
overscroll-behavior: contain is supposed to prevent this, but apparently no scrolling happens if the element is not overflowing. It's difficult to blame the property: it does precisely what it advertises, which is dictate overscroll behavior, rather than control how scrolling in general is treated.
This may be an issue with my layout. If so, I'd need to rewrite a few basic things – and then hustle about fixing all the little details broken when said basics had shifted. I'd rather avoid doing this right now, but it may be a good thing to experiment over and adopt whatever positive results may come out of it.
Other than that, I've added pages for Tools (both the category and the specific items) and Games, and worked on refining Mythos' classification further.