Per new lifecycle https://developers.google.com/web/updates/2018/07/page-lifecycle-api
On Android and latest Chrome when an app transitions from "frozen" to
active the new "resume" event fires with no accompanying "visibilitychange"
event.
This means that often background tabs may be stuck thinking that discourse
has no focus when, indeed, it has.
This leads to cases where no posts are marked read anymore.
Robin Ward
Joffrey JAFFEUX
Sam Saffron