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fix: eliminate CLS 0.975 via static page shell in index.html (#432) #433

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rdmueller merged 2 commits into from
Apr 14, 2026
+68 −21
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fix: eliminate CLS 0.975 via static page shell in index.html (#432) #433

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7062bfc
fix: eliminate CLS 0.975 via static page shell in index.html (#432)
raifdmueller Apr 14, 2026
2770703
fix: remove stale eslint-disable directive for unconfigured rule
raifdmueller Apr 14, 2026
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36 changes: 21 additions & 15 deletions scripts/prerender-routes.js
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Original file line number Diff line number Diff line change
Expand Up @@ -132,18 +132,16 @@ function escapeHtml(str) {
}

/**
* Build the pre-populated markup that goes inside <div id="app">.
* Mirrors the layout produced at runtime by renderHeader() + renderDocPage()
* + renderFooter() in website/src/main.js, but statically — so crawlers see
* real content in the initial HTML response.
* Wrap the doc fragment in the structure that website/src/components/doc-page.js
* produces at runtime: a centered article container with a #doc-content div.
* The content is injected into the shell's #page-content div so crawlers and
* non-JS fetchers see real content in the initial HTML response.
*/
function buildAppMarkup(fragmentHtml) {
function buildDocContentMarkup(fragmentHtml) {
return `
<main class="flex-1">
<article class="mx-auto max-w-4xl px-4 py-8 sm:px-6 lg:px-8">
<div id="doc-content" class="asciidoc-content">${fragmentHtml}</div>
</article>
</main>
<article class="mx-auto max-w-4xl px-4 py-8 sm:px-6 lg:px-8">
<div id="doc-content" class="asciidoc-content">${fragmentHtml}</div>
</article>
`
}

Expand Down Expand Up @@ -187,11 +185,19 @@ function prerenderRoute(shell, route) {
`<link rel="canonical" href="${canonicalUrl}" />`
)

// Inject pre-rendered content into #app
html = html.replace(
/<div\s+id="app"\s*>\s*<\/div>/,
`<div id="app">${buildAppMarkup(fragment)}</div>`
)
// Inject pre-rendered content into the static shell's #page-content div.
// The shell (set up in website/index.html and preserved through vite build)
// contains:
// <div id="page-content" ... style="..."></div>
// We match that empty div by id and fill it with the doc content so crawlers
// receive real HTML while JS users still get the SPA hydration on top.
const pageContentRegex = /(<div\s+id="page-content"[^>]*>)\s*(<\/div>)/
if (!pageContentRegex.test(html)) {
throw new Error(
`Shell #page-content div not found in dist/index.html. Did website/index.html lose the skeleton structure?`
)
}
html = html.replace(pageContentRegex, `$1${buildDocContentMarkup(fragment)}$2`)

const outDir = path.join(DIST, route.path)
const outFile = path.join(outDir, 'index.html')
Expand Down
15 changes: 14 additions & 1 deletion website/index.html
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Original file line number Diff line number Diff line change
Expand Up @@ -67,7 +67,20 @@
</script>
</head>
<body>
<div id="app"></div>
<!--
Skeleton shell: reserves viewport-height layout space so first paint
already has the expected header/content/footer footprint. Without this
the SPA boot caused CLS ~0.975 (empty #app → populated #app). main.js
detects the skeleton via #shell-header / #shell-footer and replaces the
header + footer in place, leaving #page-content untouched. See #432.
-->
<div id="app">
<div id="shell-root" class="flex flex-col" style="min-height: 100vh;">
<div id="shell-header" class="border-b border-[var(--color-border)]" style="min-height: 10.5rem;" aria-hidden="true"></div>
<div id="page-content" class="flex-1" style="min-height: calc(100vh - 16.5rem);"></div>
<div id="shell-footer" class="border-t border-[var(--color-border)]" style="min-height: 6rem;" aria-hidden="true"></div>
</div>
</div>
<script type="module" src="/src/main.js"></script>
</body>
</html>
38 changes: 33 additions & 5 deletions website/src/main.js
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Original file line number Diff line number Diff line change
Expand Up @@ -97,6 +97,38 @@ function triggerSearchIndexBuild() {
})
}

/**
* Hydrate the #app element on boot.
*
* The production build ships website/index.html with a static skeleton
* inside #app: a #shell-header placeholder, a real #page-content div, and
* a #shell-footer placeholder. Reserving those boxes at first paint
* eliminates the empty-→-populated layout shift that used to dominate CLS
* on the homepage (see #432).
*
* If the skeleton is present, replace just the header + footer placeholders
* in place — #page-content remains untouched so the route handlers can
* populate it normally.
*
* If the skeleton is absent (dev server / non-prerendered load) we still
* need the shell, so fall back to the previous full rewrite of #app.
*/
function hydrateShell(app) {
const shellHeader = document.getElementById('shell-header')
const shellFooter = document.getElementById('shell-footer')
if (shellHeader && shellFooter) {
shellHeader.outerHTML = renderHeader()
shellFooter.outerHTML = renderFooter(APP_VERSION)
return
}
const shellHtml = `
${renderHeader()}
<div id="page-content"></div>
${renderFooter(APP_VERSION)}
`
app.innerHTML = shellHtml
}

function initApp() {
i18n.init()
initTheme()
Expand All @@ -117,11 +149,7 @@ function initApp() {
const app = document.querySelector('#app')
if (!app) return

app.innerHTML = `
${renderHeader()}
<div id="page-content"></div>
${renderFooter(APP_VERSION)}
`
hydrateShell(app)

applyTranslations()
updateThemeIcon()
Expand Down
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