- unifies polling timeouts with everything else,
based on the client time instead of the server time;
- prepares polling tasks for cancellation token
behavior.
Unfortunately, RerunnableTask had to be rewritten almost
entirely.
While checking for hittarget, we first bubble from a target element
up to find the first element without `pointer-events: none` style.
This bubbling does not make much sense: we risk desperately clicking
"body" element, when we were actually asked to click some deeply-nested
"span".
Additionally, in many cases the original intent is to click a button. In this
case, we should use the enclosing "button" as a hit target directly.
Fixes#2175
