The introduction stays inside the screen.
A <dialog> has no height limit of its own, so a tall one simply ran past the
bottom of the viewport with nothing to scroll to. The technical view in German
was the case that showed it: it took the relay checkbox, "do not show again"
and the close button with it.
It is bounded now, and only the middle scrolls — the head and the foot stay
put, because that is where the close button and both checkboxes live. A dialog
whose way out scrolls away is the same bug one layer down.
Nothing new to call. From #167.