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fix: resolve flash of unstyled content in dark mode #766

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fix: resolve flash of unstyled content in dark mode #766

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2 changes: 1 addition & 1 deletion src/generators/web/template.html
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Original file line number Diff line number Diff line change
Expand Up @@ -19,7 +19,7 @@
href="https://fonts.googleapis.com/css2?family=IBM+Plex+Mono&family=Open+Sans:ital,wght@0,300..800;1,300..800" />

<!-- Apply theme before paint to avoid Flash of Unstyled Content -->
<script>document.documentElement.setAttribute("data-theme", document.documentElement.style.colorScheme = localStorage.getItem("theme") || (matchMedia("(prefers-color-scheme: dark)").matches ? "dark" : "light"));</script>
<script>${themeScript}</script>
<script type="importmap">${importMap}</script>
<script type="speculationrules">${speculationRules}</script>
</head>
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25 changes: 25 additions & 0 deletions src/generators/web/ui/theme-script.mjs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,25 @@
'use strict';

/**
* This script is designed to be inlined in the <head> of the HTML template.
* It must execute BEFORE the body renders to prevent a Flash of Unstyled Content (FOUC).
*/

export default `(function initializeTheme() {
const THEME_STORAGE_KEY = 'theme';
const THEME_DATA_ATTRIBUTE = 'data-theme';
const DARK_QUERY = '(prefers-color-scheme: dark)';

const savedUserPreference = localStorage.getItem(THEME_STORAGE_KEY);
const systemSupportsDarkMode = window.matchMedia(DARK_QUERY).matches;

const shouldApplyDark =
savedUserPreference === 'dark' ||
(savedUserPreference === 'system' && systemSupportsDarkMode) ||
(!savedUserPreference && systemSupportsDarkMode);

const themeToApply = shouldApplyDark ? 'dark' : 'light';

document.documentElement.setAttribute(THEME_DATA_ATTRIBUTE, themeToApply);
document.documentElement.style.colorScheme = themeToApply;
})();`.trim();
2 changes: 2 additions & 0 deletions src/generators/web/utils/processing.mjs
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Expand Up @@ -13,6 +13,7 @@ import getConfig from '../../../utils/configuration/index.mjs';
import { populate } from '../../../utils/configuration/templates.mjs';
import { minifyHTML } from '../../../utils/html-minifier.mjs';
import { SPECULATION_RULES } from '../constants.mjs';
import themeScript from '../ui/theme-script.mjs';

/**
* Populates a template string by evaluating it as a JavaScript template literal,
Expand Down Expand Up @@ -137,6 +138,7 @@ export async function processJSXEntries(entries, template) {
importMap: clientBundle.importMap?.replaceAll('/', root) ?? '',
entrypoint: `${data.api}.js?${randomUUID()}`,
speculationRules: SPECULATION_RULES,
themeScript,
root,
metadata: data,
config,
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