refactor(theme): improve robustness and readability of outline compon…

…ent (#3368)

src/client/theme-default/composables/outline.ts

@@ -4,10 +4,11 @@ import type { Header } from '../../shared'
 import { useAside } from './aside'
 import { throttleAndDebounce } from '../support/utils'
 
-// magic number to avoid repeated retrieval
-const PAGE_OFFSET = 71
+// cached list of anchor elements from resolveHeaders
+const resolvedHeaders: { element: HTMLHeadElement; link: string }[] = []
 
 export type MenuItem = Omit<Header, 'slug' | 'children'> & {
+  element: HTMLHeadElement
   children?: MenuItem[]
 }
 
@@ -29,6 +30,7 @@ export function getHeaders(range: DefaultTheme.Config['outline']) {
     .map((el) => {
       const level = Number(el.tagName[1])
       return {
+        element: el as HTMLHeadElement,
         title: serializeHeader(el),
         link: '#' + el.id,
         level
@@ -78,6 +80,12 @@ export function resolveHeaders(
         : levelsRange
 
   headers = headers.filter((h) => h.level >= high && h.level <= low)
+  // clear previous caches
+  resolvedHeaders.length = 0
+  // update global header list for active link rendering
+  for (const { element, link } of headers) {
+    resolvedHeaders.push({ element, link })
+  }
 
   const ret: MenuItem[] = []
   outer: for (let i = 0; i < headers.length; i++) {
@@ -128,40 +136,55 @@ export function useActiveAnchor(
       return
     }
 
-    const links = [].slice.call(
-      container.value.querySelectorAll('.outline-link')
-    ) as HTMLAnchorElement[]
-
-    const anchors = [].slice
-      .call(document.querySelectorAll('.content .header-anchor'))
-      .filter((anchor: HTMLAnchorElement) => {
-        return links.some((link) => {
-          return link.hash === anchor.hash && anchor.offsetParent !== null
-        })
-      }) as HTMLAnchorElement[]
+    // pixel offset, start of main content
+    const offsetDocTop = (() => {
+      const container =
+        document.querySelector('#VPContent .VPDoc')?.firstElementChild
+      if (container) return getAbsoluteTop(container as HTMLElement)
+      else return 78
+    })()
 
     const scrollY = window.scrollY
     const innerHeight = window.innerHeight
     const offsetHeight = document.body.offsetHeight
     const isBottom = Math.abs(scrollY + innerHeight - offsetHeight) < 1
 
-    // page bottom - highlight last one
-    if (anchors.length && isBottom) {
-      activateLink(anchors[anchors.length - 1].hash)
+    // resolvedHeaders may be repositioned, hidden or fix positioned
+    const headers = resolvedHeaders
+      .map(({ element, link }) => ({
+        link,
+        top: getAbsoluteTop(element)
+      }))
+      .filter(({ top }) => !Number.isNaN(top))
+      .sort((a, b) => a.top - b.top)
+
+    // no headers available for active link
+    if (!headers.length) {
+      activateLink(null)
       return
     }
 
-    for (let i = 0; i < anchors.length; i++) {
-      const anchor = anchors[i]
-      const nextAnchor = anchors[i + 1]
+    // page top
+    if (scrollY < 1) {
+      activateLink(null)
+      return
+    }
 
-      const [isActive, hash] = isAnchorActive(i, anchor, nextAnchor)
+    // page bottom - highlight last link
+    if (isBottom) {
+      activateLink(headers[headers.length - 1].link)
+      return
+    }
 
-      if (isActive) {
-        activateLink(hash)
-        return
+    // find the last header above the top of viewport
+    let activeLink: string | null = null
+    for (const { link, top } of headers) {
+      if (top > scrollY + offsetDocTop) {
+        break
       }
+      activeLink = link
     }
+    activateLink(activeLink)
   }
 
   function activateLink(hash: string | null) {
@@ -190,28 +213,18 @@ export function useActiveAnchor(
   }
 }
 
-function getAnchorTop(anchor: HTMLAnchorElement): number {
-  return anchor.parentElement!.offsetTop - PAGE_OFFSET
-}
-
-function isAnchorActive(
-  index: number,
-  anchor: HTMLAnchorElement,
-  nextAnchor: HTMLAnchorElement | undefined
-): [boolean, string | null] {
-  const scrollTop = window.scrollY
-
-  if (index === 0 && scrollTop === 0) {
-    return [true, null]
-  }
-
-  if (scrollTop < getAnchorTop(anchor)) {
-    return [false, null]
-  }
-
-  if (!nextAnchor || scrollTop < getAnchorTop(nextAnchor)) {
-    return [true, anchor.hash]
+function getAbsoluteTop(element: HTMLElement): number {
+  let offsetTop = 0
+  while (element !== document.body) {
+    if (element === null) {
+      // child element is:
+      // - not attached to the DOM (display: none)
+      // - set to fixed position (not scrollable)
+      // - body or html element (null offsetParent)
+      return NaN
+    }
+    offsetTop += element.offsetTop
+    element = element.offsetParent as HTMLElement
   }
-
-  return [false, null]
+  return offsetTop
 }