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fix: Change highlight logic from intersector to scroll detection #302

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Merged
lenderom merged 1 commit into from
Sep 12, 2025
Merged

fix: Change highlight logic from intersector to scroll detection #302

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90 changes: 32 additions & 58 deletions assets/js/highlightHeadline.js
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Original file line number Diff line number Diff line change
Expand Up @@ -15,74 +15,48 @@ function initHighlightHeadline() {
const markTocItemInactive = (a) => {return a.removeAttribute('data-current');};
const getTocLinkFromHeading = (h) => {return document.querySelector(`.o-aside__toc a[href="${windowPath}#${encodeURIComponent(h.id).replace(/%\w\w/g, match => match.toLowerCase())}"]`);}

const getDocHeight = () => Math.floor(document.body.clientHeight);
let scrollDebounce;

const visibleHeadings = new Set();
let resizeDebounce;
let currentObserver;
let height = getDocHeight();

function beginObservation(docHeight) {
const observer = new IntersectionObserver(
(entries) => {
if (!isAsideActive()) {
return;
}
function updateActiveHeading() {
if (!isAsideActive()) {
return;
}

entries.forEach((entry) => {
if (entry.isIntersecting) {
visibleHeadings.add(entry.target);
} else {
visibleHeadings.delete(entry.target);
}
});
let currentHeading = null;

// Sort visible (intersecting) headings by inverted order of appearance, then grab the first item (i.e. last visible heading)
const lastVisible = Array.from(visibleHeadings.values()).sort((a, b) => headings.indexOf(b) - headings.indexOf(a))[0];
if (!lastVisible) {
return;
}
for (let i = 0; i < headings.length; i++) {
const heading = headings[i];

headings.forEach((heading) => {
// If it's the last visible item, mark it to make it stand out, else, revert to the default style
// Find the link in the TOC list matching the heading in this list of heading elements
const tocLink = getTocLinkFromHeading(heading);
if (heading === lastVisible) {
if(tocLink){
markTocItemActive(tocLink);
}
} else {
if(tocLink){
markTocItemInactive(tocLink);
}
}
});
},
{
rootMargin: `${docHeight}px 0px -90% 0px`,
threshold: 1, // Only considered intersecting if all the pixels are inside the intersection area
// Add the scroll-padding-top defined in styles.scss + 1px to trigger the detection
if (heading.offsetTop <= window.pageYOffset + 81) {
currentHeading = heading;
} else {
break;
}
);

headings.forEach((heading) => observer.observe(heading));
}

return observer;
headings.forEach((heading) => {
const tocLink = getTocLinkFromHeading(heading);
if (tocLink) {
if (heading === currentHeading) {
markTocItemActive(tocLink);
} else {
markTocItemInactive(tocLink);
}
}
});
}

currentObserver = beginObservation(height); // Start the intersection observer
updateActiveHeading();

window.addEventListener('scroll', () => {
clearTimeout(scrollDebounce);
scrollDebounce = setTimeout(updateActiveHeading, 5);
});

// On resize, replace the observer with a new one matching the updated body height, if different
window.addEventListener('resize', () => {
clearTimeout(resizeDebounce);
resizeDebounce = setTimeout(() => {
const heightAfterResize = getDocHeight();
if (height !== heightAfterResize) {
if (currentObserver) {
currentObserver.disconnect();
}
currentObserver = beginObservation(heightAfterResize);
}
}, 200);
clearTimeout(scrollDebounce);
scrollDebounce = setTimeout(updateActiveHeading, 5);
});
}

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