Firefox fires the blur event on both document and window. Chrome
only fires it on window, so on chrome we were not seeing the window
being un-focused. window seems to be the standard so just use that.
This isn't the end of the story though since wer can get mousemove
events without the window ever having gained focus, in which case
we'll continue to think the user is active for another 2 mins when
in fact all they did was pass their cursor over the window.
https://github.com/vector-im/riot-web/issues/9023
Before, UserActivitity emitting actions meant that the user had very recently interaction with their hardware.
Now it means they are likely looking at the app.
You can attach a timer that is aborted when we think the user
stops looking at the page
(or hasn't touched their hardware for 2 minutes).
This works better than the previous approach for larger timeouts,
like the 30s we're about to implement for out-of-view RMs
We considered the user active if there had been a user_activity event within
the last 500ms, but those events were only raised every 500ms, so it was
possible that we would be considered inactive immediately. Use
UserActivity.userCurrentlyActive() instead, which fixes this.
Also increase CURRENTLY_ACTIVE_THRESHOLD_MS to 2 seconds.
