Adds window.requestAnimationFrame polyfill to $.extra.
Changes $.rte scroll handler to use requestAnimationFrame for smoother experience.
Improves scroll performance by avoiding expensive calls in RTE when it's invisible.
Fixes wrong uses of :hidden selector to improve UI rendering performance.
Improves $.expandable rendering performance by grouping all getBoundingClientRect calls into a tight loop to minimize forced layouts.
Changes $.code and $.rte to wait to initialize until they're visible for better UI performance.