Event driven code crashes often

The design of the event code (buttons, reader events) is pretty fragile in two ways: 1) could be better designed, 2) browser reader events (if I remember correctly) can fail.

To reproduce one bug: go to a page with a lot of text. Click Play. Then click "Pause" "Resume" "Pause" "Resume" until it fails.

I tried a number of different ways of coding/detecting events. The various browser's implementations of the readers is (if I remember correctly,) very wonky. Tried to be defensive, but needs a new hard look if this is to ship to people-who-happen-to-not-be-developers.

Start by reproducing the Play/Pause/Resume sequence on a page with a lot of text, then inspect the button and browser reader event handling. Done means repeated pause and resume cycles no longer crash, including when reader events behave inconsistently across browsers.

Written by the indexing model from the issue text.