hornwitser.no/web/assets/scripts/viewport.js

let heightOffset = 0;
let lastHeight = window.innerHeight;
function getWindowHeight(event) {

	/*
		When scrolling such that the browser toolbar is pushed out of view or
		pulled into view on a mobile device the height of the viewport changes.
		This causes the perspective to shift noticeably when you let go of your
		finger after scrolling. To avoid this, I store the last change of the
		window height if this was previously 0 and apply that to the calculated
		height of the window when the size changes.

		This causes the heightOffset to toggle between 0 and the value that
		negates the perspective shift when you switch between having the browser
		toolbar visible and not visible on mobile, but stay as 0 in most other
		situations involving window resize.
	*/
	if (event?.type === 'resize') {
		if (heightOffset === 0 && lastHeight !== 0) {
			const newOffset = lastHeight - window.innerHeight;
			// Expect the browser toolbar resize to be between 20 and 100 pixels.
			if (Math.abs(newOffset) > 20 && Math.abs(newOffset) < 100) {
				heightOffset = newOffset;
			} else {
				heightOffset = 0;
			}
		} else {
			heightOffset = 0;
		}
		lastHeight = window.innerHeight;
	}
	return window.innerHeight + heightOffset;
}

/**
	Returns x rescaled such that the values in the range a-b are mapped to the range c-d.
*/
function rescaleRange(x, a, b, c, d){
	return c + (x - a) * (d - c) / (b - a);
}

/**
	If value is not undefined return it parsed as a float, otherwise returns undefined.
*/
function maybeFloat(value) {
	if (value === undefined) {
		return undefined;
	}
	return Number.parseFloat(value);
}

function setViewportOffset(viewport, windowHeight) {

	/*
		Ignoring updates if the the bounding height of the viewport element is
		not inside of the visible client area on the page to optimise for when
		it not visible due to being off screen.
	*/
	const viewportRect = viewport.getBoundingClientRect();
	if (viewportRect.top > window.innerHeight || viewportRect.bottom < 0) {
		return;
	}

	/*
		Calculate the distance from the top of the viewport to the center of the
		screen.  This is an accurate rendering of perspective shift as you
		scroll.

		If the the bounds of a viewport element is close to the top or bottom
		of the page the perspective origin is adjusted so that it is still
		contained inside of the bounds of the element to not look out of place.
		endOffset controls how much into the bounds the perspective origin should be
		pushed when the screen in scrolled all the way to the top/bottom.
	*/
	const documentRect = document.documentElement.getBoundingClientRect();
	const endOffset = viewportRect.height * 0.25;
	let origin = Math.min(
		(viewportRect.bottom - documentRect.top) - endOffset,
		Math.max(
			windowHeight / 2,
			windowHeight - (documentRect.bottom - viewportRect.top) + endOffset,
		)
	);

	/*
		In some cases it's undesirable to let the perspective origin move too far
		away in one or both directions as it causes elements inside the 3d view to
		become obscured by the top/bottom of the viewport. To aid with this
		optionally clamp how far the perspective origin can move relative to the top
		and bottom.

		This is done by calculating how far the origin can go un-clamped while the
		viewport is visible on the screen and then rescaling the range of the
		perspective origin from it's un-clamped range to its clamped range.
	*/
	const clampTop = maybeFloat(viewport.dataset.clampTop);
	const clampBottom = maybeFloat(viewport.dataset.clampBottom);
	if (clampBottom !== undefined || clampTop !== undefined) {
		const rootFontSize = Number.parseFloat(getComputedStyle(document.documentElement).fontSize);
		const topOrigin = Math.min(
			viewportRect.bottom - endOffset,
			Math.max(viewportRect.top, documentRect.top + windowHeight) - windowHeight / 2,
		)
		const topClampPosition = rootFontSize * (clampTop ?? -Infinity) + viewportRect.top;
		const topLimit = Math.max(topOrigin, topClampPosition);

		const bottomOrigin = Math.max(
			viewportRect.top + endOffset,
			Math.min(viewportRect.bottom, documentRect.bottom - windowHeight) + windowHeight / 2,
		)
		const bottomClampPosition = rootFontSize * (clampBottom ?? +Infinity) + viewportRect.bottom;
		const bottomLimit = Math.min(bottomOrigin, bottomClampPosition);

		origin = rescaleRange(origin, topOrigin, bottomOrigin, topLimit, bottomLimit);
	}

	const yOffset = origin - viewportRect.top;
	viewport.style.setProperty('perspective-origin', `50% ${yOffset}px`);
}

const viewports = document.querySelectorAll(".viewport");
function updateViewports(event) {
	const windowHeight = getWindowHeight(event);
	for (const viewport of viewports) {
		setViewportOffset(viewport, windowHeight);
	}
}
window.addEventListener("scroll", updateViewports, { passive: true });
window.addEventListener("resize", updateViewports, { passive: true });
updateViewports();