feat(article): Scrollspy for the table of contents (#425)

* Add first try at scrollspy (broken right now)

* Scrollspy actually works now

* Fix VS Code errors by setting JS version

* Recompute offsets when window size changes

* Improve list compatibility for toc active selection

Support up to 6 levels of indentation, properly support <ol>

* Remove debug string

* Add more docs in smoothAnchors

* Use a map to match ids to navigation elements

6 files changed, 209 insertions, 5 deletions

diff --git a/.gitignore b/.gitignore
index 9ebefdf..9ff142d 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,4 @@
 public
 resources
-assets/jsconfig.json
\ No newline at end of file
+assets/jsconfig.json
+.hugo_build.lock
\ No newline at end of file
diff --git a/assets/scss/partials/base.scss b/assets/scss/partials/base.scss
index f02dcbd..408629e 100644
--- a/assets/scss/partials/base.scss
+++ b/assets/scss/partials/base.scss
@@ -1,7 +1,6 @@
 html {
     font-size: 62.5%;
     overflow-y: scroll;
-    scroll-behavior: smooth;
 }
 
 * {
diff --git a/assets/scss/partials/layout/article.scss b/assets/scss/partials/layout/article.scss
index b421cfc..78ed81c 100644
--- a/assets/scss/partials/layout/article.scss
+++ b/assets/scss/partials/layout/article.scss
@@ -123,7 +123,6 @@
 }
 
 .article-page.has-toc {
-    scroll-behavior: smooth;
 
     .left-sidebar {
         display: none;
@@ -194,6 +193,10 @@
     color: var(--card-text-color-main);
     overflow: hidden;
 
+    ::-webkit-scrollbar-thumb {
+        background-color: var(--card-separator-color);
+    }
+
     #TableOfContents {
         overflow-x: auto;
         max-height: 75vh;
@@ -208,7 +211,7 @@
             list-style-type: none;
             counter-reset: item;
 
-            li:before {
+            li a::before {
                 counter-increment: item;
                 content: counters(item, ".") ". ";
                 font-weight: bold;
@@ -221,7 +224,7 @@
         }
 
         li {
-            margin: 15px 20px;
+            margin: 15px 0 15px 20px;
             padding: 5px;
 
             & > ol,
@@ -235,6 +238,38 @@
                 }
             }
         }
+        li.active-class > a {
+            border-left: var(--heading-border-size) solid var(--accent-color);
+            font-weight: bold;
+        }
+
+        ul li.active-class > a {
+            display: block;
+        }
+
+        @function repeat($str, $n) {
+            $result: "";
+            @for $_ from 0 to $n {
+                $result: $result + $str;
+            }
+            @return $result;
+        }
+
+        // Support up to 6 levels of indentation for lists in ToCs
+        @for $i from 0 to 5 {
+            & > ul #{repeat("> li > ul", $i)} > li.active-class > a {
+                $n: 25 + $i * 35;
+                margin-left: calc(-#{$n}px - 1em);
+                padding-left: calc(#{$n}px + 1em - var(--heading-border-size));
+            }
+
+            & > ol #{repeat("> li > ol", $i)} > li.active-class > a {
+                $n: 9 + $i * 35;
+                margin-left: calc(-#{$n}px - 1em);
+                padding-left: calc(#{$n}px + 1em - var(--heading-border-size));
+                display: block;
+            }
+        }
     }
 }
 
diff --git a/assets/ts/main.ts b/assets/ts/main.ts
index 625bbbc..20de18c 100644
--- a/assets/ts/main.ts
+++ b/assets/ts/main.ts
@@ -10,6 +10,8 @@ import { getColor } from 'ts/color';
 import menu from 'ts/menu';
 import createElement from 'ts/createElement';
 import StackColorScheme from 'ts/colorScheme';
+import { setupScrollspy } from 'ts/scrollspy';
+import { setupSmoothAnchors } from "ts/smoothAnchors";
 
 let Stack = {
     init: () => {
@@ -21,6 +23,8 @@ let Stack = {
         const articleContent = document.querySelector('.article-content') as HTMLElement;
         if (articleContent) {
             new StackGallery(articleContent);
+            setupSmoothAnchors();
+            setupScrollspy();
         }
 
         /**
diff --git a/assets/ts/scrollspy.ts b/assets/ts/scrollspy.ts
new file mode 100644
index 0000000..8a14085
--- /dev/null
+++ b/assets/ts/scrollspy.ts
@@ -0,0 +1,131 @@
+// Implements a scroll spy system for the ToC, displaying the current section with an indicator and scrolling to it when needed.
+
+// Inspired from https://gomakethings.com/debouncing-your-javascript-events/
+function debounced(func: Function) {
+    let timeout;
+    return () => {
+        if (timeout) {
+            window.cancelAnimationFrame(timeout);
+        }
+
+        timeout = window.requestAnimationFrame(() => func());
+    }
+}
+
+const headersQuery = ".article-content h1[id], .article-content h2[id], .article-content h3[id], .article-content h4[id], .article-content h5[id], .article-content h6[id]";
+const tocQuery = "#TableOfContents";
+const navigationQuery = "#TableOfContents li";
+const activeClass = "active-class";
+
+function scrollToTocElement(tocElement: HTMLElement, scrollableNavigation: HTMLElement) {
+    let textHeight = tocElement.querySelector("a").offsetHeight;
+    let scrollTop = tocElement.offsetTop - scrollableNavigation.offsetHeight / 2 + textHeight / 2 - scrollableNavigation.offsetTop;
+    if (scrollTop < 0) {
+        scrollTop = 0;
+    }
+    scrollableNavigation.scrollTo({ top: scrollTop, behavior: "smooth" });
+}
+
+type IdToElementMap = { [key: string]: HTMLElement };
+
+function buildIdToNavigationElementMap(navigation: NodeListOf<Element>): IdToElementMap {
+    const sectionLinkRef: IdToElementMap = {};
+    navigation.forEach((navigationElement: HTMLElement) => {
+        const link = navigationElement.querySelector("a");
+        const href = link.getAttribute("href");
+        if (href.startsWith("#")) {
+            sectionLinkRef[href.slice(1)] = navigationElement;
+        }
+    });
+
+    return sectionLinkRef;
+}
+
+function computeOffsets(headers: NodeListOf<Element>) {
+    let sectionsOffsets = [];
+    headers.forEach((header: HTMLElement) => { sectionsOffsets.push({ id: header.id, offset: header.offsetTop }) });
+    sectionsOffsets.sort((a, b) => a.offset - b.offset);
+    return sectionsOffsets;
+}
+
+function setupScrollspy() {
+    let headers = document.querySelectorAll(headersQuery);
+    if (!headers) {
+        console.warn("No header matched query", headers);
+        return;
+    }
+
+    let scrollableNavigation = document.querySelector(tocQuery) as HTMLElement | undefined;
+    if (!scrollableNavigation) {
+        console.warn("No toc matched query", tocQuery);
+        return;
+    }
+
+    let navigation = document.querySelectorAll(navigationQuery);
+    if (!navigation) {
+        console.warn("No navigation matched query", navigationQuery);
+        return;
+    }
+
+    let sectionsOffsets = computeOffsets(headers);
+
+    // We need to avoid scrolling when the user is actively interacting with the ToC. Otherwise, if the user clicks on a link in the ToC,
+    // we would scroll their view, which is not optimal usability-wise.
+    let tocHovered: boolean = false;
+    scrollableNavigation.addEventListener("mouseenter", debounced(() => tocHovered = true));
+    scrollableNavigation.addEventListener("mouseleave", debounced(() => tocHovered = false));
+
+    let activeSectionLink: Element;
+
+    let idToNavigationElement: IdToElementMap = buildIdToNavigationElementMap(navigation);
+
+    function scrollHandler() {
+        let scrollPosition = document.documentElement.scrollTop || document.body.scrollTop;
+
+        let newActiveSection: HTMLElement | undefined;
+
+        // Find the section that is currently active.
+        // It is possible for no section to be active, so newActiveSection may be undefined.
+        sectionsOffsets.forEach((section) => {
+            if (scrollPosition >= section.offset - 20) {
+                newActiveSection = document.getElementById(section.id);
+            }
+        });
+
+        // Find the link for the active section. Once again, there are a few edge cases:
+        // - No active section = no link => undefined
+        // - No active section but the link does not exist in toc (e.g. because it is outside of the applicable ToC levels) => undefined
+        let newActiveSectionLink: HTMLElement | undefined
+        if (newActiveSection) {
+            newActiveSectionLink = idToNavigationElement[newActiveSection.id];
+        }
+
+        if (newActiveSection && !newActiveSectionLink) {
+            // The active section does not have a link in the ToC, so we can't scroll to it.
+            console.debug("No link found for section", newActiveSection);
+        } else if (newActiveSectionLink !== activeSectionLink) {
+            if (activeSectionLink)
+                activeSectionLink.classList.remove(activeClass);
+            if (newActiveSectionLink) {
+                newActiveSectionLink.classList.add(activeClass);
+                if (!tocHovered) {
+                    // Scroll so that newActiveSectionLink is in the middle of scrollableNavigation, except when it's from a manual click (hence the tocHovered check)
+                    scrollToTocElement(newActiveSectionLink, scrollableNavigation);
+                }
+            }
+            activeSectionLink = newActiveSectionLink;
+        }
+    }
+
+    window.addEventListener("scroll", debounced(scrollHandler));
+    
+    // Resizing may cause the offset values to change: recompute them.
+    function resizeHandler() {
+        sectionsOffsets = computeOffsets(headers);
+        scrollHandler();
+    }
+
+    window.addEventListener("resize", debounced(resizeHandler));
+}
+
+export { setupScrollspy };
\ No newline at end of file
diff --git a/assets/ts/smoothAnchors.ts b/assets/ts/smoothAnchors.ts
new file mode 100644
index 0000000..0718bf5
--- /dev/null
+++ b/assets/ts/smoothAnchors.ts
@@ -0,0 +1,34 @@
+// Implements smooth scrolling when clicking on an anchor link.
+// This is required instead of using modern CSS because Chromium does not currently support scrolling
+// one element with scrollTo while another element is scrolled because of a click on a link. This would
+// thus not work with the ToC scrollspy and e.g. footnotes.
+
+// Here are additional links about this issue:
+// - https://stackoverflow.com/questions/49318497/google-chrome-simultaneously-smooth-scrollintoview-with-more-elements-doesn
+// - https://stackoverflow.com/questions/57214373/scrollintoview-using-smooth-function-on-multiple-elements-in-chrome
+// - https://bugs.chromium.org/p/chromium/issues/detail?id=833617
+// - https://bugs.chromium.org/p/chromium/issues/detail?id=1043933
+// - https://bugs.chromium.org/p/chromium/issues/detail?id=1121151
+
+const anchorLinksQuery = "a[href]";
+
+function setupSmoothAnchors() {
+    document.querySelectorAll(anchorLinksQuery).forEach(aElement => {
+        let href = aElement.getAttribute("href");
+        if (!href.startsWith("#")) {
+            return;
+        }
+        aElement.addEventListener("click", clickEvent => {
+            clickEvent.preventDefault();
+
+            let targetId = aElement.getAttribute("href").substring(1);
+            // The replace done on ':' is here for footnotes, as this character would otherwise interfere when used as a CSS selector.
+            let target = document.querySelector(`#${targetId.replace(":", "\\:")}`) as HTMLElement;
+
+            window.history.pushState({}, "", aElement.getAttribute("href"));
+            scrollTo({ top: target.offsetTop, behavior: "smooth" });
+        });
+    });
+}
+
+export { setupSmoothAnchors };
\ No newline at end of file