• appending to a column
• find on surface

For find we can dehydrate each pane after we're done with it, because we know there'll be a plan_draw that comes along to rehydrate when needed.

For append, however, we're hydrating inside height(pane), and that's called all over the place.

This strategy fits in with our overall plan: hydrate each pane the first time we compute its height. Otherwise hydrate only what's in view.