[rposborne]

There are a few tools out there that use a similar paradigm.

RescueTime, uses the same principal (active focused window) https://www.rescuetime.com/

Wakatime, which is similar, but closer to your "goals" I think. Which issues "heartbeats" of open files / websites / w/e you want that can be related to a given project programatically. https://wakatime.com

Though generally just love this, I constantly strive to automate timekeeping for the trivial stuff.