Per https://github.com/oppia/oppia/wiki/Google-Summer-of-Code-2021#implement-lightweight-checkpointing:

Oppia's lessons can require between 15 minutes and an hour of time depending on a number of factors from learners: understanding of the material, literacy capabilities, and general focus. One potential behavior in the app is that we don't save the user's progress if they navigate away from an exploration (lesson) which, given how long lessons can take, is expected to be a frustrating experience. To mitigate this, we'd like to introduce support for a lightweight checkpointing system wherein users' saved state is fully retained if they navigate away from a lesson & back.

Owner: @MaskedCarrot

Mentors: @aggarwalpulkit596 (primary), @rt4914