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feat: flow the TOC rail across visible sections #7

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Merged
MentorFilou merged 1 commit into from
Jul 19, 2026
Merged

feat: flow the TOC rail across visible sections #7

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107 changes: 86 additions & 21 deletions src/scripts/site.js
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Original file line number Diff line number Diff line change
Expand Up @@ -107,29 +107,94 @@
pre.appendChild(btn);
});

// --- right-rail TOC scrollspy ---
// --- right-rail TOC flow indicator ---
// A single rail marker whose top/height track the projection of the visible
// document window onto the TOC entries, so the highlight "flows" across the
// sections currently on screen instead of snapping to one heading.
var tocLinks = document.querySelectorAll('.toc a[href^="#"]');
if (tocLinks.length && 'IntersectionObserver' in window) {
var byId = {};
tocLinks.forEach(function (a) {
byId[a.getAttribute('href').slice(1)] = a;
var docContent = document.querySelector('.doc-content');
var tocList = document.querySelector('.toc ul');
if (tocLinks.length && docContent && tocList) {
var sections = [];
tocLinks.forEach(function (link) {
var heading = document.getElementById(link.getAttribute('href').slice(1));
if (heading) sections.push({ link: link, li: link.parentNode, heading: heading });
});
var current = null;
var observer = new IntersectionObserver(
function (entries) {
entries.forEach(function (entry) {
if (!entry.isIntersecting) return;
var link = byId[entry.target.id];
if (!link) return;
if (current) current.classList.remove('is-active');
link.classList.add('is-active');
current = link;

if (sections.length) {
var marker = document.createElement('span');
marker.className = 'toc-flow';
tocList.appendChild(marker);

var topbarH =
parseFloat(getComputedStyle(document.documentElement).getPropertyValue('--topbar-h')) || 56;

function updateFlow() {
var scrollY = window.pageYOffset;
var viewTop = scrollY + topbarH; // top reading line, below the sticky bar
var viewBottom = scrollY + window.innerHeight;
var readingH = Math.max(window.innerHeight - topbarH, 1);
var ulTop = tocList.getBoundingClientRect().top + scrollY;
var contentBottom = docContent.getBoundingClientRect().bottom + scrollY;

var barTop = Infinity;
var barBottom = -Infinity;

for (var i = 0; i < sections.length; i++) {
var s = sections[i];
var secTop = s.heading.getBoundingClientRect().top + scrollY;
var secBottom =
i + 1 < sections.length
? sections[i + 1].heading.getBoundingClientRect().top + scrollY
: contentBottom;
var secH = Math.max(secBottom - secTop, 1);

var clipTop = Math.max(viewTop, secTop);
var clipBottom = Math.min(viewBottom, secBottom);
var visible = clipBottom - clipTop;

if (visible <= 0) {
s.link.classList.remove('is-active');
continue;
}

// Project this section's visible slice onto its TOC entry's height.
var liRect = s.li.getBoundingClientRect();
var eTop = liRect.top + scrollY;
var segTop = eTop + ((clipTop - secTop) / secH) * liRect.height;
var segBottom = eTop + ((clipBottom - secTop) / secH) * liRect.height;
if (segTop < barTop) barTop = segTop;
if (segBottom > barBottom) barBottom = segBottom;

// Binary text highlight: lit while the section holds the top reading
// line or occupies a meaningful share of the viewport.
var containsTop = viewTop >= secTop && viewTop < secBottom;
var frac = visible / Math.min(readingH, secH);
s.link.classList.toggle('is-active', containsTop || frac >= 0.4);
}

if (barBottom > barTop) {
marker.style.top = barTop - ulTop + 'px';
marker.style.height = barBottom - barTop + 'px';
marker.style.opacity = '1';
} else {
marker.style.opacity = '0';
}
}

var ticking = false;
function onScroll() {
if (ticking) return;
ticking = true;
requestAnimationFrame(function () {
ticking = false;
updateFlow();
});
},
{ rootMargin: '-56px 0px -70% 0px', threshold: 0 },
);
document.querySelectorAll('.doc-content h2[id], .doc-content h3[id]').forEach(function (h) {
observer.observe(h);
});
}

window.addEventListener('scroll', onScroll, { passive: true });
window.addEventListener('resize', onScroll);
updateFlow();
}
}
})();
16 changes: 15 additions & 1 deletion src/styles/site.css
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Original file line number Diff line number Diff line change
Expand Up @@ -976,12 +976,27 @@ h3:hover .heading-anchor,
}

.toc ul {
position: relative;
list-style: none;
margin: 0;
padding: 0;
border-left: 1px solid var(--border);
}

/* Rail marker that flows to span the sections currently on screen. */
.toc-flow {
position: absolute;
left: -1px;
width: 2px;
top: 0;
height: 0;
background: var(--accent);
border-radius: 1px;
opacity: 0;
transition: opacity 0.2s ease;
pointer-events: none;
}

.toc li a {
display: block;
padding: 0.2rem 0 0.2rem 0.75rem;
Expand All @@ -1001,7 +1016,6 @@ h3:hover .heading-anchor,

.toc li a.is-active {
color: var(--accent-strong);
border-left-color: var(--accent);
}

/* ---------- legal notice ---------- */
Expand Down