fix(a11y)!: use github-style header anchor generation

BREAKING CHANGES:
- `--vp-header-anchor-symbol` is removed
- The default permalink function of markdown-it-anchor is changed to `linkAfterHeader`. This will need updates in your custom themes.

This brings the original proposed behavior of #2039, #2040 which wasn't introduced to avoid breaking changes.

fixes #2980
closes #2982

src/client/theme-default/styles/components/vp-doc.css

@@ -2,6 +2,10 @@
  * Headings
  * -------------------------------------------------------------------------- */
 
+.vp-doc .header-wrapper {
+  position: relative;
+}
+
 .vp-doc h1,
 .vp-doc h2,
 .vp-doc h3,
@@ -15,22 +19,45 @@
 
 .vp-doc h1 {
   letter-spacing: -0.02em;
+}
+
+.vp-doc h1,
+.vp-doc h1 + .header-anchor {
   line-height: 40px;
   font-size: 28px;
 }
 
+@media (min-width: 768px) {
+  .vp-doc h1,
+  .vp-doc h1 + .header-anchor {
+    font-size: 32px;
+  }
+}
+
 .vp-doc h2 {
   margin: 48px 0 16px;
   border-top: 1px solid var(--vp-c-divider);
   padding-top: 24px;
   letter-spacing: -0.02em;
+}
+
+.vp-doc h2,
+.vp-doc h2 + .header-anchor {
   line-height: 32px;
   font-size: 24px;
 }
 
+.vp-doc h2 + .header-anchor {
+  top: 24px;
+}
+
 .vp-doc h3 {
   margin: 32px 0 0;
   letter-spacing: -0.01em;
+}
+
+.vp-doc h3,
+.vp-doc h3 + .header-anchor {
   line-height: 28px;
   font-size: 20px;
 }
@@ -38,6 +65,10 @@
 .vp-doc h4 {
   margin: 24px 0 0;
   letter-spacing: -0.01em;
+}
+
+.vp-doc h4,
+.vp-doc h4 + .header-anchor {
   line-height: 24px;
   font-size: 18px;
 }
@@ -47,6 +78,7 @@
   top: 0;
   left: 0;
   margin-left: -0.87em;
+  padding-right: 0.3em;
   font-weight: 500;
   user-select: none;
   opacity: 0;
@@ -56,37 +88,11 @@
     opacity 0.25s;
 }
 
-.vp-doc .header-anchor:before {
-  content: var(--vp-header-anchor-symbol);
-}
-
-.vp-doc h1:hover .header-anchor,
-.vp-doc h1 .header-anchor:focus,
-.vp-doc h2:hover .header-anchor,
-.vp-doc h2 .header-anchor:focus,
-.vp-doc h3:hover .header-anchor,
-.vp-doc h3 .header-anchor:focus,
-.vp-doc h4:hover .header-anchor,
-.vp-doc h4 .header-anchor:focus,
-.vp-doc h5:hover .header-anchor,
-.vp-doc h5 .header-anchor:focus,
-.vp-doc h6:hover .header-anchor,
-.vp-doc h6 .header-anchor:focus {
+.vp-doc .header-wrapper:hover .header-anchor,
+.vp-doc .header-wrapper .header-anchor:focus {
   opacity: 1;
 }
 
-@media (min-width: 768px) {
-  .vp-doc h1 {
-    letter-spacing: -0.02em;
-    line-height: 40px;
-    font-size: 32px;
-  }
-}
-
-.vp-doc h2 .header-anchor {
-  top: 24px;
-}
-
 /**
  * Paragraph and inline elements
  * -------------------------------------------------------------------------- */

src/client/theme-default/styles/utils.css

@@ -1,9 +1,11 @@
 .visually-hidden {
   position: absolute;
+  overflow: hidden;
+  clip: rect(0, 0, 0, 0);
   width: 1px;
   height: 1px;
+  margin: -1px;
+  padding: 0;
+  border-width: 0;
   white-space: nowrap;
-  clip: rect(0 0 0 0);
-  clip-path: inset(50%);
-  overflow: hidden;
 }

src/client/theme-default/styles/vars.css

@@ -316,14 +316,6 @@
   --vp-layout-max-width: 1440px;
 }
 
-/**
- * Component: Header Anchor
- * -------------------------------------------------------------------------- */
-
-:root {
-  --vp-header-anchor-symbol: '#';
-}
-
 /**
  * Component: Code
  * -------------------------------------------------------------------------- */

src/node/markdown/markdown.ts

@@ -274,31 +274,11 @@ export async function createMarkdownRenderer(
         .map((t) => t.content)
         .join('')
     },
-    permalink: (slug, _, state, idx) => {
-      const title =
-        state.tokens[idx + 1]?.children
-          ?.filter((token) => ['text', 'code_inline'].includes(token.type))
-          .reduce((acc, t) => acc + t.content, '')
-          .trim() || ''
-
-      const linkTokens = [
-        Object.assign(new state.Token('text', '', 0), { content: ' ' }),
-        Object.assign(new state.Token('link_open', 'a', 1), {
-          attrs: [
-            ['class', 'header-anchor'],
-            ['href', `#${slug}`],
-            ['aria-label', `Permalink to “${title}”`]
-          ]
-        }),
-        Object.assign(new state.Token('html_inline', '', 0), {
-          content: '​',
-          meta: { isPermalinkSymbol: true }
-        }),
-        new state.Token('link_close', 'a', -1)
-      ]
-
-      state.tokens[idx + 1].children?.push(...linkTokens)
-    },
+    permalink: anchorPlugin.permalink.linkAfterHeader({
+      assistiveText: (title) => `Permalink to “${title}”`,
+      visuallyHiddenClass: 'visually-hidden',
+      wrapper: ['<div class="header-wrapper">', '</div>']
+    }),
     ...options.anchor
   } as anchorPlugin.AnchorOptions).use(frontmatterPlugin, {
     ...options.frontmatter