From 5eba8786570f987cc5d2c12c5d04cd84bcc46720 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Greg=20Berg=C3=A9?= <berge.greg@gmail.com>
Date: Sun, 13 Apr 2025 10:02:36 +0200
Subject: [PATCH] wip

---
 .../PageAside/ScrollSectionsList.tsx          |  24 ++--
 .../gitbook/src/components/hooks/useHash.ts   |  38 +++---
 .../components/hooks/usePageActiveSections.ts | 128 ++++++++++++++++++
 .../src/components/hooks/useScrollListener.ts |   2 +-
 .../src/components/hooks/useScrollPage.ts     |  34 ++---
 5 files changed, 181 insertions(+), 45 deletions(-)
 create mode 100644 packages/gitbook/src/components/hooks/usePageActiveSections.ts

diff --git a/packages/gitbook/src/components/PageAside/ScrollSectionsList.tsx b/packages/gitbook/src/components/PageAside/ScrollSectionsList.tsx
index a2adf315e..82688786e 100644
--- a/packages/gitbook/src/components/PageAside/ScrollSectionsList.tsx
+++ b/packages/gitbook/src/components/PageAside/ScrollSectionsList.tsx
@@ -1,18 +1,19 @@
 'use client';
 import { motion } from 'framer-motion';
-import React from 'react';
+import React, { useRef } from 'react';
 
-import { useScrollActiveId } from '@/components/hooks';
+// import { useScrollActiveId } from '@/components/hooks';
 import type { DocumentSection } from '@/lib/document-sections';
 import { tcls } from '@/lib/tailwind';
 
-import { HEADER_HEIGHT_DESKTOP } from '../layout';
+import { usePageActiveSections } from '../hooks/usePageActiveSections';
+// import { HEADER_HEIGHT_DESKTOP } from '../layout';
 import { AsideSectionHighlight } from './AsideSectionHighlight';
 
 /**
  * The threshold at which we consider a section as intersecting the viewport.
  */
-const SECTION_INTERSECTING_THRESHOLD = 0.9;
+// const SECTION_INTERSECTING_THRESHOLD = 0.9;
 
 const springCurve = {
     type: 'spring',
@@ -30,10 +31,13 @@ export function ScrollSectionsList(props: { sections: DocumentSection[] }) {
         });
     }, [sections]);
 
-    const activeId = useScrollActiveId(ids, {
-        rootMargin: `-${HEADER_HEIGHT_DESKTOP}px 0px -40% 0px`,
-        threshold: SECTION_INTERSECTING_THRESHOLD,
-    });
+    const windowRef = useRef(typeof window === 'undefined' ? null : window);
+    const activeIds = usePageActiveSections(ids, windowRef);
+
+    // const activeId = useScrollActiveId(ids, {
+    //     rootMargin: `-${HEADER_HEIGHT_DESKTOP}px 0px -40% 0px`,
+    //     threshold: SECTION_INTERSECTING_THRESHOLD,
+    // });
 
     return (
         <ul className={tcls('sidebar-list-line:border-l', 'border-tint-subtle')}>
@@ -49,7 +53,7 @@ export function ScrollSectionsList(props: { sections: DocumentSection[] }) {
                         section.depth > 1 && ['ml-3', 'my-0', 'sidebar-list-line:ml-0']
                     )}
                 >
-                    {activeId === section.id ? (
+                    {activeIds.includes(section.id) ? (
                         <AsideSectionHighlight
                             transition={springCurve}
                             className={tcls(
@@ -102,7 +106,7 @@ export function ScrollSectionsList(props: { sections: DocumentSection[] }) {
                                 'sidebar-list-default:border-tint',
                             ],
 
-                            activeId === section.id && [
+                            activeIds.includes(section.id) && [
                                 'text-primary-subtle',
                                 'hover:text-primary',
                                 'contrast-more:text-primary',
diff --git a/packages/gitbook/src/components/hooks/useHash.ts b/packages/gitbook/src/components/hooks/useHash.ts
index 2c1ce20df..a1cc4f53e 100644
--- a/packages/gitbook/src/components/hooks/useHash.ts
+++ b/packages/gitbook/src/components/hooks/useHash.ts
@@ -1,24 +1,26 @@
 'use client';
 
-import { useParams } from 'next/navigation';
-import React from 'react';
+import { useCallback, useEffect, useState } from 'react';
 
-function getHash(): string | null {
-    if (typeof window === 'undefined') {
-        return null;
-    }
-    return window.location.hash.slice(1);
-}
-
-/**
- * Hook to get the current hash from the URL.
- * @see https://github.com/vercel/next.js/discussions/49465
- */
+// How do I get the pathname with hash.
+// source: https://github.com/vercel/next.js/discussions/49465
 export function useHash() {
-    const params = useParams();
-    const [hash, setHash] = React.useState<string | null>(getHash);
-    React.useEffect(() => {
-        setHash(getHash());
-    }, [params]);
+    const getCurrentHash = useCallback(
+        () => (typeof window !== 'undefined' ? window.location.hash.replace(/^#!?/, '') : ''),
+        []
+    );
+    const [hash, setHash] = useState<string>(getCurrentHash());
+
+    useEffect(() => {
+        const handleHashChange = () => {
+            setHash(getCurrentHash());
+        };
+        window.addEventListener('hashchange', handleHashChange);
+
+        return () => {
+            window.removeEventListener('hashchange', handleHashChange);
+        };
+    }, [getCurrentHash]);
+
     return hash;
 }
diff --git a/packages/gitbook/src/components/hooks/usePageActiveSections.ts b/packages/gitbook/src/components/hooks/usePageActiveSections.ts
new file mode 100644
index 000000000..8e7be6c22
--- /dev/null
+++ b/packages/gitbook/src/components/hooks/usePageActiveSections.ts
@@ -0,0 +1,128 @@
+'use client';
+import * as React from 'react';
+import { useEventCallback } from 'usehooks-ts';
+import { useScrollListener } from './useScrollListener';
+
+/**
+ * Hook that returns the active section key of the page.
+ * The active section is the section that is currently visible on the viewport.
+ *
+ * Here's a diagram: https://user-images.githubusercontent.com/8937991/206921462-14e5b8b7-5a19-4c8b-912d-ed1f17710bc4.png
+ */
+export function usePageActiveSections(
+    ids: string[],
+    scrollRef: React.RefObject<HTMLElement | Window | null>
+): string[] {
+    const [activeSections, setActiveSections] = React.useState<string[]>(noActiveSections);
+
+    const update = useEventCallback(() => {
+        const element = scrollRef.current;
+        if (element) {
+            const rect =
+                element instanceof Window
+                    ? new DOMRect(0, 0, window.innerWidth, window.innerHeight)
+                    : element.getBoundingClientRect();
+            const activeSections = findActiveSectionIds({
+                ids,
+                containerRect: rect,
+            });
+            setActiveSections((previous) => {
+                if (activeSections.length !== previous.length) {
+                    return activeSections;
+                }
+                if (activeSections.some((key, i) => key !== previous[i])) {
+                    return activeSections;
+                }
+                return previous;
+            });
+        }
+    });
+
+    useScrollListener(update, scrollRef);
+
+    React.useEffect(() => {
+        update();
+    }, [update]);
+
+    return activeSections;
+}
+
+const noActiveSections: string[] = [];
+
+// We consider anything in [-MARGINpx; +MARGINpx] to be at the boundary
+const MARGIN = 24;
+
+/**
+ * findActiveSectionIndex finds the section that is in the window's view
+ * it uses binary-search to efficiently (minimize DOM queries)
+ * find the DOM node whose .top is closest to but below `0` (the window's top)
+ */
+function findActiveSectionIndex(ids: string[], scrollTop: number): number {
+    const N = ids.length;
+    // Number of max iterations to find the element (so our search is bounded)
+    // in theory, at most floor(log(n)+1) steps to find the specific element
+    const maxIterations = Math.floor(Math.log2(N) + 1);
+    // Our binary search variables
+    let start = 0;
+    let end = N - 1;
+    let mid = Math.ceil((start + end) / 2);
+    let idx = -1;
+    let y: number | null = null;
+    for (let i = 0; i < maxIterations; i++) {
+        const id = ids[mid];
+        // Double check that we have a section, else exit
+        if (!id) {
+            return -1;
+        }
+        const element = document.getElementById(id);
+        const sy = element ? element.getBoundingClientRect().top - scrollTop : null;
+        if (sy === null || sy > MARGIN) {
+            // Go left
+            end = mid;
+            mid = Math.floor((start + end) / 2);
+        } else if (sy >= -MARGIN && sy <= MARGIN) {
+            // Hit
+            return mid;
+        } else if (sy < MARGIN) {
+            // If bigger than previous (but still smaller than 0)
+            // that becomes our new active index
+            if (y === null || sy > y) {
+                y = sy;
+                idx = mid;
+            }
+            // Go right
+            start = mid;
+            mid = Math.ceil((start + end) / 2);
+        }
+    }
+    return idx;
+}
+
+/**
+ * Find the active sections in the viewport.
+ */
+function findActiveSectionIds(input: {
+    ids: string[];
+    containerRect: DOMRect;
+}): string[] {
+    const activeIndex = findActiveSectionIndex(input.ids, input.containerRect.top);
+
+    // Check if the next sections are visible, stop when they are not.
+    const activeSections: string[] = input.ids[activeIndex] ? [input.ids[activeIndex]] : [];
+
+    for (let i = activeIndex + 1; i < input.ids.length; i++) {
+        const id = input.ids[i];
+        const element = document.getElementById(id);
+        if (!element) {
+            continue;
+        }
+
+        const rect = element.getBoundingClientRect();
+        if (rect.bottom + MARGIN > input.containerRect.bottom) {
+            break;
+        }
+
+        activeSections.push(id);
+    }
+    return activeSections;
+}
diff --git a/packages/gitbook/src/components/hooks/useScrollListener.ts b/packages/gitbook/src/components/hooks/useScrollListener.ts
index 639465691..c623b5bb5 100644
--- a/packages/gitbook/src/components/hooks/useScrollListener.ts
+++ b/packages/gitbook/src/components/hooks/useScrollListener.ts
@@ -17,7 +17,7 @@ import { type RefObject, useEffect, useLayoutEffect, useRef } from 'react';
  */
 export function useScrollListener(
     listener: (event: Event) => void,
-    elementRef: RefObject<HTMLElement | Window> | null
+    elementRef: RefObject<HTMLElement | Window | null> | null
 ) {
     const listenerRef = useRef(listener);
     useLayoutEffect(() => {
diff --git a/packages/gitbook/src/components/hooks/useScrollPage.ts b/packages/gitbook/src/components/hooks/useScrollPage.ts
index 0b3c7cb1b..f421fe9d1 100644
--- a/packages/gitbook/src/components/hooks/useScrollPage.ts
+++ b/packages/gitbook/src/components/hooks/useScrollPage.ts
@@ -4,6 +4,7 @@ import { usePathname } from 'next/navigation';
 import React from 'react';
 
 import { useHash } from './useHash';
+import { usePrevious } from './usePrevious';
 
 /**
  * Scroll the page to an anchor point or
@@ -13,28 +14,29 @@ import { useHash } from './useHash';
 export function useScrollPage(props: { scrollMarginTop?: number }) {
     const hash = useHash();
     const pathname = usePathname();
+    const prevPathname = usePrevious(pathname);
+    const scrollMarginTopRef = React.useRef(props.scrollMarginTop);
     React.useLayoutEffect(() => {
         if (hash) {
             const element = document.getElementById(hash);
             if (element) {
-                if (props.scrollMarginTop) {
-                    element.style.scrollMarginTop = `${props.scrollMarginTop}px`;
+                const originalScrollMarginTop = element.style.scrollMarginTop;
+                if (scrollMarginTopRef.current) {
+                    element.style.scrollMarginTop = `${scrollMarginTopRef.current}px`;
                 }
-                element.scrollIntoView({
-                    block: 'start',
-                    behavior: 'smooth',
-                });
+                element.scrollIntoView({ block: 'start', behavior: 'smooth' });
+                return () => {
+                    const element = document.getElementById(hash);
+                    if (element) {
+                        element.style.scrollMarginTop = originalScrollMarginTop;
+                    }
+                };
             }
-        } else {
+            return;
+        }
+
+        if (prevPathname && pathname !== prevPathname) {
             window.scrollTo(0, 0);
         }
-        return () => {
-            if (hash) {
-                const element = document.getElementById(hash);
-                if (element) {
-                    element.style.scrollMarginTop = '';
-                }
-            }
-        };
-    }, [hash, pathname, props.scrollMarginTop]);
+    }, [hash, pathname, prevPathname]);
 }