Just posting this so it's documented somewhere. I think the onus is on apple to fix it, but I haven't seen this mentioned elsewhere.
When a Dictionary.app entry's content grows taller after the entry has already loaded via <details>/<summary>, a CSS-only checkbox/:checked toggle, or JavaScript toggling display/class, the WebView's internal layout correctly reflows to the new height, but the surrounding native scroll view's content size does not update to match. The added content is visible but expanded below the window and cannot be scrolled down to.
Resizing the window or the sidebar within the dictionary app enables scrolling as expected, but then collapsing the content, without resizing you can still scroll to the prior (now empty) extent of the then-expanded section.
Environment: macOS 26.5.1 Dictionary.app 2.3.0 Not sure how to find the DDK version but I'm pretty sure that doesn't matter