[Performance] Improve scrolling performance

[HEIGE-PCloud]

The scrolling is super slow...

DoIt/src/js/theme.js

Lines 912 to 942 in f7386ce

	function handleScrollEvent () {
	window.newScrollTop = getScrollTop()
	const scroll = window.newScrollTop - window.oldScrollTop
	const isMobile = isMobileWindow()
	forEach($headers, $header => {
	if (scroll > ACCURACY) {
	$header.classList.remove('animate__fadeInDown')
	animateCSS($header, ['animate__fadeOutUp', 'animate__faster'], true)
	} else if (scroll < -ACCURACY || window.newScrollTop <= 20) {
	$header.classList.remove('animate__fadeOutUp')
	animateCSS($header, ['animate__fadeInDown', 'animate__faster'], true)
	}
	})
	if (window.newScrollTop > MINIMUM) {
	if (isMobile && scroll > ACCURACY) {
	$fixedButtons.classList.remove('animate__fadeIn')
	animateCSS($fixedButtons, ['animate__fadeOut', 'animate__faster'], true)
	} else if (!isMobile || scroll < -ACCURACY) {
	$fixedButtons.style.display = 'block'
	$fixedButtons.classList.remove('animate__fadeOut')
	animateCSS($fixedButtons, ['animate__fadeIn', 'animate__faster'], true)
	}
	} else {
	if (!isMobile) {
	$fixedButtons.classList.remove('animate__fadeIn')
	animateCSS($fixedButtons, ['animate__fadeOut', 'animate__faster'], true)
	}
	$fixedButtons.style.display = 'none'
	}
	for (const event of window.scrollEventSet) event()
	window.oldScrollTop = window.newScrollTop

#368

[HEIGE-PCloud]

The main cost comes from getBoundingClientRect. The usage of this function needs to be audited and use constants to cache its result if possible.