fix(theme): prevent layout shifts when locking page scroll

fixes #5386

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

@@ -1,10 +1,11 @@
 <script setup lang="ts">
-import { onKeyStroke, useScrollLock } from '@vueuse/core'
+import { onKeyStroke } from '@vueuse/core'
-import { inBrowser, onContentUpdated } from 'vitepress'
+import { onContentUpdated } from 'vitepress'
 import type { DefaultTheme } from 'vitepress/theme'
 import { nextTick, ref, watch } from 'vue'
 import { useData } from '../composables/data'
 import { resolveTitle } from '../composables/outline'
+import { useBodyScrollLock } from '../composables/scroll-lock'
 import VPDocOutlineItem from './VPDocOutlineItem.vue'
 
 const props = defineProps<{
@@ -19,7 +20,7 @@ const main = ref<HTMLDivElement>()
 const items = ref<HTMLDivElement>()
 
 // lock body scroll while the dropdown is open to prevent scroll chaining
-const isLocked = useScrollLock(inBrowser ? document.body : null)
+const isLocked = useBodyScrollLock()
 
 function closeOnClickOutside(e: Event) {
   if (!main.value?.contains(e.target as Node)) {

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

@@ -6,13 +6,12 @@ import {
   onKeyStroke,
   useEventListener,
   useLocalStorage,
-  useScrollLock,
   useSessionStorage
 } from '@vueuse/core'
 import { useFocusTrap } from '@vueuse/integrations/useFocusTrap'
 import Mark from 'mark.js/src/vanilla.js'
 import MiniSearch, { type SearchResult } from 'minisearch'
-import { dataSymbol, inBrowser, useRouter } from 'vitepress'
+import { dataSymbol, useRouter } from 'vitepress'
 import {
   computed,
   createApp,
@@ -30,6 +29,7 @@ import type { LocalSearchTranslations } from '../../../../types/local-search'
 import { pathToFile } from '../../app/utils'
 import { escapeRegExp } from '../../shared'
 import { useData } from '../composables/data'
+import { useBodyScrollLock } from '../composables/scroll-lock'
 import { LRUCache } from '../support/lru'
 import { createSearchTranslate } from '../support/translation'
 
@@ -409,7 +409,7 @@ useEventListener('popstate', (event) => {
 
 /** Lock body */
 
-const isLocked = useScrollLock(inBrowser ? document.body : null)
+const isLocked = useBodyScrollLock()
 
 onMounted(() => {
   nextTick(() => {

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

@@ -1,6 +1,5 @@
 <script setup lang="ts">
-import { useScrollLock } from '@vueuse/core'
+import { useBodyScrollLock } from '../composables/scroll-lock'
-import { inBrowser } from 'vitepress'
 import VPNavScreenAppearance from './VPNavScreenAppearance.vue'
 import VPNavScreenMenu from './VPNavScreenMenu.vue'
 import VPNavScreenSocialLinks from './VPNavScreenSocialLinks.vue'
@@ -10,7 +9,7 @@ defineProps<{
   open: boolean
 }>()
 
-const isLocked = useScrollLock(inBrowser ? document.body : null)
+const isLocked = useBodyScrollLock()
 </script>
 
 <template>
@@ -45,6 +44,7 @@ const isLocked = useScrollLock(inBrowser ? document.body : null)
   width: 100%;
   background-color: var(--vp-nav-screen-bg-color);
   overflow-y: auto;
+  overscroll-behavior: contain;
   transition: background-color 0.25s;
   pointer-events: auto;
 }

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

@@ -1,8 +1,7 @@
 <script lang="ts" setup>
-import { useScrollLock } from '@vueuse/core'
-import { inBrowser } from 'vitepress'
 import { ref, watch } from 'vue'
 import { useLayout } from '../composables/layout'
+import { useBodyScrollLock } from '../composables/scroll-lock'
 import VPSidebarGroup from './VPSidebarGroup.vue'
 
 const { sidebarGroups, hasSidebar } = useLayout()
@@ -13,7 +12,7 @@ const props = defineProps<{
 
 // a11y: focus Nav element when menu has opened
 const navEl = ref<HTMLElement | null>(null)
-const isLocked = useScrollLock(inBrowser ? document.body : null)
+const isLocked = useBodyScrollLock()
 
 watch(
   [props, navEl],

src/client/theme-default/composables/scroll-lock.ts

@@ -0,0 +1,141 @@
+import { inBrowser } from 'vitepress'
+import {
+  computed,
+  onScopeDispose,
+  shallowRef,
+  type WritableComputedRef
+} from 'vue'
+
+const isIOS =
+  inBrowser &&
+  (/iP(?:ad|hone|od)/.test(navigator.userAgent) ||
+    (navigator.platform === 'MacIntel' && navigator.maxTouchPoints > 1))
+
+const scrollKeys = new Set([
+  ' ',
+  'PageUp',
+  'PageDown',
+  'Home',
+  'End',
+  'ArrowUp',
+  'ArrowDown',
+  'ArrowLeft',
+  'ArrowRight'
+])
+
+const listenerOptions = { capture: true, passive: false } as const
+
+function isScrollable(target: EventTarget | null): boolean {
+  let el = target instanceof Element ? target : null
+  while (el && el !== document.body) {
+    const { overflowX, overflowY } = getComputedStyle(el)
+    if (
+      ((overflowY === 'auto' || overflowY === 'scroll') &&
+        el.scrollHeight > el.clientHeight) ||
+      ((overflowX === 'auto' || overflowX === 'scroll') &&
+        el.scrollWidth > el.clientWidth)
+    ) {
+      return true
+    }
+    el = el.parentElement
+  }
+  return false
+}
+
+function blockScroll(e: Event) {
+  if ('touches' in e && (e as TouchEvent).touches.length > 1) return
+  if (!isScrollable(e.target)) e.preventDefault()
+}
+
+function blockScrollKeys(e: KeyboardEvent) {
+  if (e.metaKey || e.ctrlKey || e.altKey || !scrollKeys.has(e.key)) return
+  const el = e.target
+  if (
+    el instanceof HTMLElement &&
+    (el.isContentEditable || el.matches('input, textarea, select'))
+  ) {
+    return
+  }
+  blockScroll(e)
+}
+
+// all instances lock the same element, so the style juggling is refcounted
+let overflowLockCount = 0
+let eventLockCount = 0
+let initialOverflow: string | undefined
+let initialGutter: string | undefined
+
+function lockOverflow() {
+  if (++overflowLockCount > 1) return
+  const html = document.documentElement
+  if (!getComputedStyle(html).scrollbarGutter.includes('stable')) {
+    initialGutter = html.style.scrollbarGutter
+    html.style.scrollbarGutter = 'stable'
+  }
+  initialOverflow = document.body.style.overflow
+  document.body.style.overflow = 'hidden'
+  if (isIOS) {
+    document.addEventListener('touchmove', blockScroll, listenerOptions)
+  }
+}
+
+function unlockOverflow() {
+  if (--overflowLockCount > 0) return
+  if (isIOS) {
+    document.removeEventListener('touchmove', blockScroll, listenerOptions)
+  }
+  document.body.style.overflow = initialOverflow ?? ''
+  if (initialGutter !== undefined) {
+    document.documentElement.style.scrollbarGutter = initialGutter
+  }
+  initialOverflow = initialGutter = undefined
+}
+
+function lockEvents() {
+  if (++eventLockCount > 1) return
+  document.addEventListener('wheel', blockScroll, listenerOptions)
+  document.addEventListener('touchmove', blockScroll, listenerOptions)
+  document.addEventListener('keydown', blockScrollKeys, listenerOptions)
+}
+
+function unlockEvents() {
+  if (--eventLockCount > 0) return
+  document.removeEventListener('wheel', blockScroll, listenerOptions)
+  document.removeEventListener('touchmove', blockScroll, listenerOptions)
+  document.removeEventListener('keydown', blockScrollKeys, listenerOptions)
+}
+
+/**
+ * Locks page scrolling behind an overlay.
+ *
+ * Prefer `scrollbar-gutter: stable` + `overflow: hidden` so layout width
+ * stays stable when the scrollbar is hidden. If unsupported, fall back to
+ * blocking scroll events while still allowing events inside scrollable
+ * elements.
+ */
+export function useBodyScrollLock(): WritableComputedRef<boolean> {
+  const isLocked = shallowRef(false)
+  let useEvents = false
+
+  function lock() {
+    if (isLocked.value) return
+    useEvents =
+      window.innerWidth > document.documentElement.clientWidth &&
+      !CSS.supports('scrollbar-gutter', 'stable')
+    useEvents ? lockEvents() : lockOverflow()
+    isLocked.value = true
+  }
+
+  function unlock() {
+    if (!isLocked.value) return
+    useEvents ? unlockEvents() : unlockOverflow()
+    isLocked.value = false
+  }
+
+  onScopeDispose(unlock)
+
+  return computed({
+    get: () => isLocked.value,
+    set: (value) => (value ? lock() : unlock())
+  })
+}