fix(client,a11y): improve focus handling and scrolling behavior in router (#4943)

src/client/app/router.ts

@@ -54,7 +54,7 @@ export interface Router {
 export const RouterSymbol: InjectionKey<Router> = Symbol()
 
 // we are just using URL to parse the pathname and hash - the base doesn't
-// matter and is only passed to support same-host hrefs.
+// matter and is only passed to support same-host hrefs
 const fakeHost = 'http://a.com'
 
 const getDefaultRoute = (): Route => ({
@@ -261,35 +261,57 @@ export function scrollTo(hash: string, smooth = false, scrollPosition = 0) {
     return
   }
 
-  let target: Element | null = null
+  let target: HTMLElement | null = null
-
   try {
     target = document.getElementById(decodeURIComponent(hash).slice(1))
   } catch (e) {
     console.warn(e)
   }
-
+  if (!target) return
-  if (target) {
-    const targetPadding = parseInt(
-      window.getComputedStyle(target).paddingTop,
-      10
-    )
 
   const targetTop =
     window.scrollY +
       target.getBoundingClientRect().top -
       getScrollOffset() +
-      targetPadding
+      Number.parseInt(window.getComputedStyle(target).paddingTop, 10) || 0
+
+  const behavior = window.matchMedia('(prefers-reduced-motion)').matches
+    ? 'instant'
+    : // only smooth scroll if distance is smaller than screen height
+      smooth && Math.abs(targetTop - window.scrollY) <= window.innerHeight
+      ? 'smooth'
+      : 'auto'
+
+  const scrollToTarget = () => {
+    window.scrollTo({ left: 0, top: targetTop, behavior })
+
+    // focus the target element for better accessibility
+    target.focus({ preventScroll: true })
+
+    // return if focus worked
+    if (document.activeElement === target) return
 
-    function scrollToTarget() {
+    // element has tabindex already, likely not focusable
-      // only smooth scroll if distance is smaller than screen height.
+    // because of some other reason, bail out
-      if (!smooth || Math.abs(targetTop - window.scrollY) > window.innerHeight)
+    if (target.hasAttribute('tabindex')) return
-        window.scrollTo(0, targetTop)
+
-      else window.scrollTo({ left: 0, top: targetTop, behavior: 'smooth' })
+    const restoreTabindex = () => {
+      target.removeAttribute('tabindex')
+      target.removeEventListener('blur', restoreTabindex)
     }
 
-    requestAnimationFrame(scrollToTarget)
+    // temporarily make the target element focusable
+    target.setAttribute('tabindex', '-1')
+    target.addEventListener('blur', restoreTabindex)
+
+    // try to focus again
+    target.focus({ preventScroll: true })
+
+    // remove tabindex and event listener if focus still not worked
+    if (document.activeElement !== target) restoreTabindex()
   }
+
+  requestAnimationFrame(scrollToTarget)
 }
 
 function handleHMR(route: Route): void {
@@ -313,7 +335,7 @@ function shouldHotReload(payload: PageDataPayload): boolean {
 function normalizeHref(href: string): string {
   const url = new URL(href, fakeHost)
   url.pathname = url.pathname.replace(/(^|\/)index(\.html)?$/, '$1')
-  // ensure correct deep link so page refresh lands on correct files.
+  // ensure correct deep link so page refresh lands on correct files
   if (siteDataRef.value.cleanUrls) {
     url.pathname = url.pathname.replace(/\.html$/, '')
   } else if (!url.pathname.endsWith('/') && !url.pathname.endsWith('.html')) {

src/client/theme-default/components/VPDocOutlineItem.vue

@@ -5,18 +5,12 @@ defineProps<{
   headers: DefaultTheme.OutlineItem[]
   root?: boolean
 }>()
-
-function onClick({ target: el }: Event) {
-  const id = (el as HTMLAnchorElement).href!.split('#')[1]
-  const heading = document.getElementById(decodeURIComponent(id))
-  heading?.focus({ preventScroll: true })
-}
 </script>
 
 <template>
   <ul class="VPDocOutlineItem" :class="root ? 'root' : 'nested'">
     <li v-for="{ children, link, title } in headers">
-      <a class="outline-link" :href="link" @click="onClick" :title>
+      <a class="outline-link" :href="link" :title>
         {{ title }}
       </a>
       <template v-if="children?.length">

src/client/theme-default/components/VPSkipLink.vue

@@ -8,39 +8,18 @@ const route = useRoute()
 const backToTop = ref()
 
 watch(() => route.path, () => backToTop.value.focus())
-
-function focusOnTargetAnchor({ target }: Event) {
-  const el = document.getElementById(
-    decodeURIComponent((target as HTMLAnchorElement).hash).slice(1)
-  )
-
-  if (el) {
-    const removeTabIndex = () => {
-      el.removeAttribute('tabindex')
-      el.removeEventListener('blur', removeTabIndex)
-    }
-
-    el.setAttribute('tabindex', '-1')
-    el.addEventListener('blur', removeTabIndex)
-    el.focus()
-    window.scrollTo(0, 0)
-  }
-}
 </script>
 
 <template>
   <span ref="backToTop" tabindex="-1" />
-  <a
+  <a href="#VPContent" class="VPSkipLink visually-hidden">
-    href="#VPContent"
-    class="VPSkipLink visually-hidden"
-    @click="focusOnTargetAnchor"
-  >
     {{ theme.skipToContentLabel || 'Skip to content' }}
   </a>
 </template>
 
 <style scoped>
 .VPSkipLink {
+  position: fixed;
   top: 8px;
   left: 8px;
   padding: 8px 16px;