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fix: eliminate dark mode Flash of Unstyled Content (FOUC) #8180

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fix: eliminate dark mode Flash of Unstyled Content (FOUC) #8180

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397fe99
fix: eliminate dark mode Flash of Unstyled Content (FOUC)
ryzrr Apr 9, 2026
1056ecc
chore: format index.html with prettier
ryzrr Apr 9, 2026
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24 changes: 23 additions & 1 deletion src/index.html
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Original file line number Diff line number Diff line change
@@ -1,6 +1,28 @@
<!doctype html>
<html lang="en">
<head> </head>
<head>
<script>
(function () {
try {
var d = document.documentElement;
var s = localStorage.getItem("theme");
var isDark = false;
if (s) {
isDark = s === '"dark"' || s === "dark";
} else {
isDark = window.matchMedia("(prefers-color-scheme: dark)").matches;
if (isDark) {
localStorage.setItem("theme", '"dark"');
}
}
if (isDark) {
d.setAttribute("data-theme", "dark");
d.classList.add("dark");
}
} catch (e) {}
})();
</script>
</head>
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@alexander-akait alexander-akait Apr 10, 2026

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Can we fix it without such code?

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@ryzrr ryzrr Apr 10, 2026

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I think the inline script is the most reliable way to avoid flash 'cause it handles both the system theme and the users saved preference. BUT, another option is using CSS with prefers-color-scheme, which covers the OS setting without any script.

The trade off is that if the user has chosen a different theme than the OS, there might be a brief flash until React loads.

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@alexander-akait alexander-akait Apr 10, 2026

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I think the inline script is the most reliable way to avoid flash 'cause it handles both the system theme and the users saved preference. BUT, another option is using CSS with prefers-color-scheme, which covers the OS setting without any script.

I think it is a good choose

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@ryzrr ryzrr Apr 10, 2026

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I implemented the CSS-only route , but it creates a reversed flash for users who override their OS theme preference.

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@ryzrr ryzrr Apr 10, 2026

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This , CSS-only approach works for most users, but if someone's OS is set to dark but they've manually picked light mode on the site (or vice versa), they still see a brief flash. So we've just moved the problem. Let me know how you'd like me to proceed.

<body>
<div id="root"></div>
</body>
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6 changes: 5 additions & 1 deletion src/server.jsx
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Expand Up @@ -42,5 +42,9 @@ export default (locals) => {
const hoistedTags = headTagsMatch ? headTagsMatch[1] : "";
const bodyHtml = renderedHtml.slice(hoistedTags.length);

return `<!DOCTYPE html><html><head>${css}${hoistedTags}</head><body><div id="root">${bodyHtml}</div>${scripts}</body></html>`;
// Blocking inline script to prevent dark mode FOUC
const themeScript =
"<script>(function(){try{var d=document.documentElement;var s=localStorage.getItem('theme');var isDark=false;if(s){isDark=s==='\"dark\"'||s==='dark'}else{isDark=window.matchMedia('(prefers-color-scheme: dark)').matches;if(isDark){localStorage.setItem('theme','\"dark\"')}}if(isDark){d.setAttribute('data-theme','dark');d.classList.add('dark')}}catch(e){}})();</script>";

return `<!DOCTYPE html><html lang="en"><head>${themeScript}${css}${hoistedTags}</head><body><div id="root">${bodyHtml}</div>${scripts}</body></html>`;
};
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