From ac069b2776f177c239b6b8b0b98838bf6fe5ae7d Mon Sep 17 00:00:00 2001
From: Ilya Y <75578537+ilya-bkv@users.noreply.github.com>
Date: Tue, 17 Oct 2023 13:29:30 +0300
Subject: [PATCH] Table of contents - detect active item (#267)

Table of contents - detect active item
---
 package-lock.json                             | 11 +++++
 package.json                                  |  1 +
 .../TableOfContents.module.scss               |  4 ++
 .../TableOfContents/TableOfContents.tsx       | 45 ++++++++++++-------
 4 files changed, 46 insertions(+), 15 deletions(-)

diff --git a/package-lock.json b/package-lock.json
index 06509ac4..4553ab3a 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -13,6 +13,7 @@
         "i18next": "22.4.15",
         "i18next-icu": "2.3.0",
         "intl-messageformat": "10.5.3",
+        "just-throttle": "4.2.0",
         "mailgun.js": "8.2.1",
         "node-fetch": "3.3.1"
       },
@@ -13168,6 +13169,11 @@
         "node": ">=4.0"
       }
     },
+    "node_modules/just-throttle": {
+      "version": "4.2.0",
+      "resolved": "https://registry.npmjs.org/just-throttle/-/just-throttle-4.2.0.tgz",
+      "integrity": "sha512-/iAZv1953JcExpvsywaPKjSzfTiCLqeguUTE6+VmK15mOcwxBx7/FHrVvS4WEErMR03TRazH8kcBSHqMagYIYg=="
+    },
     "node_modules/kebab-case": {
       "version": "1.0.2",
       "resolved": "https://registry.npmjs.org/kebab-case/-/kebab-case-1.0.2.tgz",
@@ -28106,6 +28112,11 @@
         "object.values": "^1.1.6"
       }
     },
+    "just-throttle": {
+      "version": "4.2.0",
+      "resolved": "https://registry.npmjs.org/just-throttle/-/just-throttle-4.2.0.tgz",
+      "integrity": "sha512-/iAZv1953JcExpvsywaPKjSzfTiCLqeguUTE6+VmK15mOcwxBx7/FHrVvS4WEErMR03TRazH8kcBSHqMagYIYg=="
+    },
     "kebab-case": {
       "version": "1.0.2",
       "resolved": "https://registry.npmjs.org/kebab-case/-/kebab-case-1.0.2.tgz",
diff --git a/package.json b/package.json
index e6d3f2bb..7de87825 100644
--- a/package.json
+++ b/package.json
@@ -33,6 +33,7 @@
     "i18next": "22.4.15",
     "i18next-icu": "2.3.0",
     "intl-messageformat": "10.5.3",
+    "just-throttle": "4.2.0",
     "mailgun.js": "8.2.1",
     "node-fetch": "3.3.1"
   },
diff --git a/src/components/TableOfContents/TableOfContents.module.scss b/src/components/TableOfContents/TableOfContents.module.scss
index a743a105..ba191338 100644
--- a/src/components/TableOfContents/TableOfContents.module.scss
+++ b/src/components/TableOfContents/TableOfContents.module.scss
@@ -164,6 +164,10 @@
   &:hover {
     color: rgb(0 0 0 / 50%);
   }
+
+  &.active {
+    font-weight: 700 !important;
+  }
 }
 
 .TableOfContentsHeadingsItemH3,
diff --git a/src/components/TableOfContents/TableOfContents.tsx b/src/components/TableOfContents/TableOfContents.tsx
index 30146f9d..7f018eb6 100644
--- a/src/components/TableOfContents/TableOfContents.tsx
+++ b/src/components/TableOfContents/TableOfContents.tsx
@@ -1,16 +1,12 @@
-import { For, Show, createSignal, createEffect, on } from 'solid-js'
+import { For, Show, createSignal, createEffect, on, onMount, onCleanup } from 'solid-js'
 import { clsx } from 'clsx'
-
 import { DEFAULT_HEADER_OFFSET } from '../../stores/router'
-
 import { useLocalize } from '../../context/localize'
-
 import debounce from 'debounce'
-
 import { Icon } from '../_shared/Icon'
-
 import styles from './TableOfContents.module.scss'
 import { isDesktop } from '../../utils/media-query'
+import throttle from 'just-throttle'
 
 interface Props {
   variant: 'article' | 'editor'
@@ -18,6 +14,15 @@ interface Props {
   body: string
 }
 
+const isInViewport = (el: Element): boolean => {
+  const rect = el.getBoundingClientRect()
+  return (
+    rect.top >= 0 &&
+    rect.left >= 0 &&
+    rect.bottom <= (window.innerHeight || document.documentElement.clientHeight) &&
+    rect.right <= (window.innerWidth || document.documentElement.clientWidth)
+  )
+}
 const scrollToHeader = (element) => {
   window.scrollTo({
     behavior: 'smooth',
@@ -31,9 +36,9 @@ const scrollToHeader = (element) => {
 export const TableOfContents = (props: Props) => {
   const { t } = useLocalize()
 
-  const [headings, setHeadings] = createSignal<Element[]>([])
+  const [headings, setHeadings] = createSignal<HTMLElement[]>([])
   const [areHeadingsLoaded, setAreHeadingsLoaded] = createSignal<boolean>(false)
-
+  const [activeHeaderIndex, setActiveHeaderIndex] = createSignal<number>(-1)
   const [isVisible, setIsVisible] = createSignal<boolean>(props.variant === 'article')
   const toggleIsVisible = () => {
     setIsVisible((visible) => !visible)
@@ -42,15 +47,20 @@ export const TableOfContents = (props: Props) => {
   setIsVisible(isDesktop())
 
   const updateHeadings = () => {
-    const { parentSelector } = props
-
-    // eslint-disable-next-line unicorn/prefer-spread
-    setHeadings(Array.from(document.querySelector(parentSelector).querySelectorAll('h2, h3, h4')))
+    setHeadings(
+      // eslint-disable-next-line unicorn/prefer-spread
+      Array.from(document.querySelector(props.parentSelector).querySelectorAll<HTMLElement>('h2, h3, h4'))
+    )
     setAreHeadingsLoaded(true)
   }
 
   const debouncedUpdateHeadings = debounce(updateHeadings, 500)
 
+  const updateActiveHeader = throttle(() => {
+    const newActiveIndex = headings().findIndex((heading) => isInViewport(heading))
+    setActiveHeaderIndex(newActiveIndex)
+  }, 50)
+
   createEffect(
     on(
       () => props.body,
@@ -58,6 +68,11 @@ export const TableOfContents = (props: Props) => {
     )
   )
 
+  onMount(() => {
+    window.addEventListener('scroll', updateActiveHeader)
+    onCleanup(() => window.removeEventListener('scroll', updateActiveHeader))
+  })
+
   return (
     <Show
       when={
@@ -77,17 +92,17 @@ export const TableOfContents = (props: Props) => {
               </div>
               <ul class={styles.TableOfContentsHeadingsList}>
                 <For each={headings()}>
-                  {(h) => (
+                  {(h, index) => (
                     <li>
                       <button
                         class={clsx(styles.TableOfContentsHeadingsItem, {
                           [styles.TableOfContentsHeadingsItemH3]: h.nodeName === 'H3',
-                          [styles.TableOfContentsHeadingsItemH4]: h.nodeName === 'H4'
+                          [styles.TableOfContentsHeadingsItemH4]: h.nodeName === 'H4',
+                          [styles.active]: index() === activeHeaderIndex()
                         })}
                         innerHTML={h.textContent}
                         onClick={(e) => {
                           e.preventDefault()
-
                           scrollToHeader(h)
                         }}
                       />