fix: toc does not highlight clicked anchor + use scroll-margin-top (#5425)

* fix toc highlighting anchorTopOffset issues

* fix comment

* use ternary

* revert to previous offset

packages/docusaurus-theme-classic/src/theme/Heading/index.tsx

@@ -49,7 +49,8 @@ const createAnchorHeading = (
           aria-hidden="true"
           tabIndex={-1}
           className={clsx('anchor', `anchor__${Tag}`, {
-            [styles.enhancedAnchor]: !hideOnScroll,
+            [styles.anchorWithHideOnScrollNavbar]: hideOnScroll,
+            [styles.anchorWithStickyNavbar]: !hideOnScroll,
           })}
           id={id}
         />

packages/docusaurus-theme-classic/src/theme/Heading/styles.module.css

@@ -5,6 +5,16 @@
  * LICENSE file in the root directory of this source tree.
  */
 
-.enhancedAnchor {
-  top: calc(var(--ifm-navbar-height) * -1 - 0.5rem);
+/*
+When the navbar is sticky, ensure that on anchor click,
+the browser does not scroll that anchor behind the navbar
+See https://twitter.com/JoshWComeau/status/1332015868725891076
+ */
+.anchorWithStickyNavbar {
+  scroll-margin-top: calc(var(--ifm-navbar-height) + 0.5rem);
+  /*   top: calc(var(--ifm-navbar-height) * -1 - 0.5rem); */
+}
+
+.anchorWithHideOnScrollNavbar {
+  scroll-margin-top: 0.5rem;
 }

packages/docusaurus-theme-classic/src/theme/hooks/useTOCHighlight.ts

@@ -7,6 +7,7 @@
 
 import {Params} from '@theme/hooks/useTOCHighlight';
 import {useEffect, useRef} from 'react';
+import {useThemeConfig} from '@docusaurus/theme-common';
 
 // If the anchor has no height and is just a "marker" in the dom; we'll use the parent (normally the link text) rect boundaries instead
 function getVisibleBoundingClientRect(element: HTMLElement): DOMRect {
@@ -30,11 +31,13 @@ function getAnchors() {
   return Array.from(document.querySelectorAll(selector)) as HTMLElement[];
 }
 
-function getActiveAnchor(): Element | null {
+function getActiveAnchor({
+  anchorTopOffset,
+}: {
+  anchorTopOffset: number;
+}): Element | null {
   const anchors = getAnchors();
 
-  const anchorTopOffset = 100; // Skip anchors that are too close to the viewport top
-
   // Naming is hard
   // The "nextVisibleAnchor" is the first anchor that appear under the viewport top boundary
   // Note: it does not mean this anchor is visible yet, but if user continues scrolling down, it will be the first to become visible
@@ -74,9 +77,30 @@ function getLinks(linkClassName: string) {
   ) as HTMLAnchorElement[];
 }
 
+function getNavbarHeight(): number {
+  // Not ideal to obtain actual height this way
+  // Using TS ! (not ?) because otherwise a bad selector would be un-noticed
+  return document.querySelector('.navbar')!.clientHeight;
+}
+
+function useAnchorTopOffsetRef() {
+  const anchorTopOffsetRef = useRef<number>(0);
+  const {
+    navbar: {hideOnScroll},
+  } = useThemeConfig();
+
+  useEffect(() => {
+    anchorTopOffsetRef.current = hideOnScroll ? 0 : getNavbarHeight();
+  }, [hideOnScroll]);
+
+  return anchorTopOffsetRef;
+}
+
 function useTOCHighlight(params: Params): void {
   const lastActiveLinkRef = useRef<HTMLAnchorElement | undefined>(undefined);
 
+  const anchorTopOffsetRef = useAnchorTopOffsetRef();
+
   useEffect(() => {
     const {linkClassName, linkActiveClassName} = params;
 
@@ -94,7 +118,9 @@ function useTOCHighlight(params: Params): void {
 
     function updateActiveLink() {
       const links = getLinks(linkClassName);
-      const activeAnchor = getActiveAnchor();
+      const activeAnchor = getActiveAnchor({
+        anchorTopOffset: anchorTopOffsetRef.current,
+      });
       const activeLink = links.find(
         (link) => activeAnchor && activeAnchor.id === getLinkAnchorValue(link),
       );
@@ -113,7 +139,7 @@ function useTOCHighlight(params: Params): void {
       document.removeEventListener('scroll', updateActiveLink);
       document.removeEventListener('resize', updateActiveLink);
     };
-  }, [params]);
+  }, [params, anchorTopOffsetRef]);
 }
 
 export default useTOCHighlight;