diff --git a/changelogs/internal/newsfragments/1991.clarification b/changelogs/internal/newsfragments/1991.clarification
new file mode 100644
index 00000000..5f641bff
--- /dev/null
+++ b/changelogs/internal/newsfragments/1991.clarification
@@ -0,0 +1 @@
+Improve the JS script to highlight the current ToC entry.
diff --git a/layouts/partials/hooks/body-end.html b/layouts/partials/hooks/body-end.html
index 377470d9..f975eddf 100644
--- a/layouts/partials/hooks/body-end.html
+++ b/layouts/partials/hooks/body-end.html
@@ -2,16 +2,9 @@
This template is included at the end of each page's ``.
- We're using it here to:
-
- 1) include the JS that generates the table of contents. It would be better
- to generate the table of contents as part of the Hugo build process, but
- that doesn't work nicely with the way we want to author client-server modules
- as separate files.
-
- 2) highlight and scroll the ToC in the sidebar to match the place we are at
- in the document.
+ We're using it here to highlight and scroll the ToC in the sidebar to match
+ the place we are at in the document.
*/}}
-
+
diff --git a/static/js/toc.js b/static/js/toc.js
index 786ec6c6..af8682b3 100644
--- a/static/js/toc.js
+++ b/static/js/toc.js
@@ -15,137 +15,150 @@ limitations under the License.
*/
/*
-Set a new ToC entry.
-Clear any previously highlighted ToC items, set the new one,
-and adjust the ToC scroll position.
+ Only call the given function once every 250 milliseconds to avoid impacting
+ the performance of the browser.
+ Source: https://remysharp.com/2010/07/21/throttling-function-calls
*/
-function setTocEntry(newEntry) {
- const activeEntries = document.querySelectorAll("#toc a.active");
- for (const activeEntry of activeEntries) {
- activeEntry.classList.remove('active');
- }
-
- newEntry.classList.add('active');
- // don't scroll the sidebar nav if the main content is not scrolled
- const nav = document.querySelector("#td-section-nav");
- const content = document.querySelector("html");
- if (content.scrollTop !== 0) {
- nav.scrollTop = newEntry.offsetTop - 100;
- } else {
- nav.scrollTop = 0;
+function throttle(fn) {
+ const threshold = 250;
+ let last = null;
+ let deferTimer = null;
+
+ return function (...args) {
+ const now = new Date();
+
+ if (last && now < last + threshold) {
+ // Hold on to it.
+ clearTimeout(deferTimer);
+ deferTimer = setTimeout(() => {
+ last = now;
+ fn.apply(this, args);
+ }, threshold);
+ } else {
+ last = now;
+ fn.apply(this, args);
+ }
}
}
/*
-Test whether a node is in the viewport
+ Get the list of headings that appear in the ToC.
+ This is not as simple as querying all the headings in the content, because
+ some headings are not rendered in the ToC (e.g. in the endpoint definitions).
*/
-function isInViewport(node) {
- const rect = node.getBoundingClientRect();
- return (
- rect.top >= 0 &&
- rect.left >= 0 &&
- rect.bottom <= (window.innerHeight || document.documentElement.clientHeight) &&
- rect.right <= (window.innerWidth || document.documentElement.clientWidth)
- );
+function getHeadings() {
+ let headings = [];
+
+ // First get the anchors in the ToC.
+ const toc_anchors = document.querySelectorAll("#toc nav a");
+
+ for (const anchor of toc_anchors) {
+ // Then get the heading from its selector in the anchor's href.
+ const selector = anchor.getAttribute("href");
+ if (!selector) {
+ console.error("Got ToC anchor without href");
+ continue;
+ }
+
+ const heading = document.querySelector(selector);
+ if (!heading) {
+ console.error("Heading not found for selector:", selector);
+ continue;
+ }
+
+ headings.push(heading);
+ }
+
+ return headings;
}
/*
-The callback we pass to the IntersectionObserver constructor.
-
-Called when any of our observed nodes starts or stops intersecting
-with the viewport.
+ Get the heading of the text visible at the top of the viewport.
+ This is the first heading above or at the top of the viewport.
*/
-function handleIntersectionUpdate(entries) {
-
- /*
- Special case: If the current URL hash matches a ToC entry, and
- the corresponding heading is visible in the viewport, then that is
- made the current ToC entry, and we don't even look at the intersection
- observer data.
- This means that if the user has clicked on a ToC entry,
- we won't unselect it through the intersection observer.
- */
- const hash = document.location.hash;
- if (hash) {
- let tocEntryForHash = document.querySelector(`nav li a[href="${hash}"]`);
- // if the hash isn't a direct match for a ToC item, check the data attributes
- if (!tocEntryForHash) {
- const fragment = hash.substring(1);
- tocEntryForHash = document.querySelector(`nav li a[data-${fragment}]`);
+function getCurrentHeading(headings, headerOffset) {
+ const scrollTop = document.documentElement.scrollTop;
+ let prevHeading = null;
+ let currentHeading = null;
+ let index = 0;
+
+ for (const heading of headings) {
+ // Compute the position compared to the viewport.
+ const rect = heading.getBoundingClientRect();
+
+ if (rect.top >= headerOffset && rect.top <= headerOffset + 30) {
+ // This heading is at the top of the viewport, this is the current heading.
+ currentHeading = heading;
+ break;
}
- if (tocEntryForHash) {
- const headingForHash = document.querySelector(hash);
- if (headingForHash && isInViewport(headingForHash)) {
- setTocEntry(tocEntryForHash);
- return;
+ if (rect.top >= headerOffset) {
+ // This is in or below the viewport, the current heading should be the
+ // previous one.
+ if (prevHeading) {
+ currentHeading = prevHeading;
+ } else {
+ // The first heading does not have a prevHeading.
+ currentHeading = heading;
}
+ break;
}
+
+ prevHeading = heading;
+ index += 1;
}
- let newEntry = null;
-
- for (const entry of entries) {
- if (entry.intersectionRatio > 0) {
- const heading = entry.target;
- /*
- This sidebar nav consists of two sections:
- * at the top, a sitenav containing links to other pages
- * under that, the ToC for the current page
-
- Since the sidebar scrolls to match the document position,
- the sitenav will tend to scroll off the screen.
-
- If the user has scrolled up to (or near) the top of the page,
- we want to show the sitenav so.
-
- So: if the H1 (title) for the current page has started
- intersecting, then always scroll the sidebar back to the top.
- */
- if (heading.tagName === "H1" && heading.parentNode.tagName === "DIV") {
- const nav = document.querySelector("#td-section-nav");
- nav.scrollTop = 0;
- return;
- }
- /*
- Otherwise, get the ToC entry for the first entry that
- entered the viewport, if there was one.
- */
- const id = entry.target.getAttribute('id');
- let tocEntry = document.querySelector(`nav li a[href="#${id}"]`);
- // if the id isn't a direct match for a ToC item,
- // check the ToC entry's `data-*` attributes
- if (!tocEntry) {
- tocEntry = document.querySelector(`nav li a[data-${id}]`);
- }
- if (tocEntry && !newEntry) {
- newEntry = tocEntry;
- }
- }
+ return currentHeading;
+}
+
+/*
+ Select the ToC entry that points to the given ID.
+ Clear any previously highlighted ToC items, select the new one,
+ and adjust the ToC scroll position.
+*/
+function selectTocEntry(id) {
+ // Deselect previously selected entries.
+ const activeEntries = document.querySelectorAll("#toc nav a.active");
+ for (const activeEntry of activeEntries) {
+ activeEntry.classList.remove('active');
}
- if (newEntry) {
- setTocEntry(newEntry);
+ // Find the new entry and select it.
+ const newEntry = document.querySelector(`#toc nav a[href="#${id}"]`);
+ if (!newEntry) {
+ console.error("ToC entry not found for ID:", id);
return;
}
+ newEntry.classList.add('active');
+
+ // Don't scroll the sidebar nav if the main content is not scrolled
+ const nav = document.querySelector("#td-section-nav");
+ const content = document.querySelector("html");
+ if (content.scrollTop !== 0) {
+ nav.scrollTop = newEntry.offsetTop - 100;
+ } else {
+ nav.scrollTop = 0;
+ }
}
/*
-Track when headings enter the viewport, and use this to update the highlight
-for the corresponding ToC entry.
+ Track when the view is scrolled, and use this to update the highlight for the
+ corresponding ToC entry.
*/
window.addEventListener('DOMContentLoaded', () => {
-
- const toc = document.querySelector("#toc");
- toc.addEventListener("click", event => {
- if (event.target.tagName === "A") {
- setTocEntry(event.target);
- }
+ // Part of the viewport is below the header so we should take it into account.
+ const headerOffset = document.querySelector("body > header > nav").clientHeight;
+ const headings = getHeadings();
+
+ const onScroll = throttle((_e) => {
+ // Update the ToC.
+ let heading = getCurrentHeading(headings, headerOffset);
+ selectTocEntry(heading.id);
});
- const observer = new IntersectionObserver(handleIntersectionUpdate);
-
- document.querySelectorAll("h1, h2, h3, h4, h5, h6").forEach((section) => {
- observer.observe(section);
- });
+ // Initialize the state of the ToC.
+ onScroll();
+ // Listen to scroll and resizing changes.
+ document.addEventListener('scroll', onScroll, false);
+ document.addEventListener('resize', onScroll, false);
});