Question 1
window.onscroll = function() {
console.log('scrolling');
};
or if your targeted browsers support addEventListener
:
window.addEventListener('scroll', function() {
console.log('scrolling');
});
Question 2
In my opinion, if you're just scrolling from one section to a another section of your page, and not having some sort of constantly running scrolling movement, you're fine doing this without using requestAnimationFrame.
You can find good implementations of scrolling to a particular part of the window in pure javascript, I suggest checking out their source(or even using them).
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