idea of a feature: mark a lesson as "studied"

Nadie ha tomado este issue todavía.

Since you already sell EPUBs, I'd like to propose a paid feature.

Background:
Because this book is so good, I really want to read it from start to finish, and I believe many people feel the same. However in reality, that never happens, because I can never find such a big chunk of time. What typically happens is when I need to learn something, I come to read on that specific thing.

Proposal:
As a signed-in user, I wish to be able to mark a lesson as "studied" or "have read", which serves two purposes: 1. it's a motivation for me to read all the lessons and "check them all off" 2. help me organize which lessons have been read, especially when I jump to different topics without following any orders.

It could be implemented in multiple ways, for example:
- add a button for user to "mark this lesson as read"
- mark a lesson as read when user clicks "next lesson"
- mark a lesson as read when user's browser tab has focused on this tab for more than x minutes

These behaviors could also be configurable as user preference.

Depending on the implementation, a lesson could even be read multiple times, for example, a ✔ is shown on the page to indicate it's been read, and ✔✔✔ shows if it's been read for 3 times. Clicking or hovering the symbol could show the date it happened, and the duration of the session, etc.

No se indica ningún archivo, prueba ni punto de entrada. Lee primero la propuesta y la discusión existente en los comentarios; después identifica el comportamiento seleccionado y sus criterios de aceptación antes de localizar los flujos relevantes de lecciones y cuentas. Done debe definirse como una única implementación acordada que registre y muestre el estado de estudio de una lección.

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