It's like the Prosemirror component doesn't expand the height of it's container while editing. It's fine after the edit, i.e. the rendered content doesn't overflow the container once the content is saved and the editor closed.
Looking at the prosemirror npm repo it looks like it has been discontinued. What includes prosemirror anyway? Is that a foundry thing. Am I asking this in the wrong server? I'll ask in Foundry too just in case.
I think this probably is a TJS thing as I'm using this:
import { TJSProseMirror } from "@typhonjs-fvtt/svelte-standard/component";
import { TJSProseMirror } from "@typhonjs-fvtt/svelte-standard/component";
I get similar behaviour with TinyMCE btw except that the overflow-y seems to be handled with a scroll, although the wrapping element still does not expand, so the layout is awkward.