By the way, I think the “Observable is reactive” section in that notebook would benefit from warning people about a gotcha that seems to hit every new Observable user, which is writing cells that are too large and depend on too many other cells. See for example this question I answered a while ago:
It took me a while myself to figure out that refactoring cells into smaller cells is an important habit to have to avoid excessive recomputation or “resetting” of cells.