Delay re-scans until idle-time

[AshleyWright]

Is your feature request related to a problem? Please describe.
Even though resize handler is throttled, due to computation time scaling with number of DOM nodes, jank can still be an issue.

Describe the solution you'd like
Since during a resize the user is not actively reading, delaying the resize handler to idle time would likely be an acceptable solution using requestIdleCallback

Describe alternatives you've considered
If CSS Properties and Values offers a way of watching for property changes (currently looking like level 2 of P&V) re-scans will prove unnecessary.