The thing about this calculator app is that it can display any number of digits just by scrolling the display field. The UX is "any number of digits the user wants" not some predetermined fixed number of digits.
As a developer, "infinite scroll to get more digits" sounds really cool. It sounds conceptually similar to lazily-evaluated sequences in languages like Clojure and Haskell (where you can have a 'virtually-infinite' list or array -- basically a function -- and can access arbitrarily large indices).
As a user, it sounds like an annoying interface. On the rare case I want to compute e^(-10000), I do not want to scroll for 3 minutes through screens filled with 0s to find the significant digits.
Furthermore, it's not very usable. A key question in this scenario would be: how many zeroes were there?
It's basically impossible to tell with this UI. A better approach is simply to switch to scientific notation for very large or very small numbers, and leave decimal expansion as an extra option for users who need it. (Roughly similar to what Wolfram Alpha gives you for certain expressions.)
