feat(core): warn in dev when SSR styles are rehydrated more than once

[vashek]

renderToStyleElements returns the renderer's entire accumulated CSS on every call. In the Next.js App Router, useServerInsertedHTML runs once per streaming flush, so a setup that does not clear the renderer between flushes re-emits the full stylesheet and the copies are streamed into . Those stale copies override styles inserted after a client-side navigation, silently breaking makeStyles overrides (they lose to their makeResetStyles base). The failure looks correct on a hard reload and only appears after a soft navigation, which makes it very hard to diagnose.

rehydrateRendererCache scans every server-rendered <style> element, so it is the natural place to detect this: in development, count CSS rules that appear in more than one element (duplicate rule text, not merely a repeated bucket — a bucket legitimately recurs across flushes with different rules) and emit one console.error pointing at the renderer.stylesheets reset fix. Zero production overhead. Also DRYs the previously thrice-repeated devtools guard into a cacheRule helper.

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[vashek]

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[layershifter]

Suggested change

	'Clear the renderer after each flush:',
	'\n\n',
	'useServerInsertedHTML(() => {\n const styles = renderToStyleElements(renderer);\n renderer.stylesheets = {};\n return styles;\n});',
	'\n\n',

Let's remove the actual recommendation from code.

[layershifter]

Suggested change

	const seenRules: Set<string> | undefined = process.env.NODE_ENV !== 'production' ? new Set() : undefined;
	let duplicateRuleCount = 0;
	const cacheRule = (cssRule: string, bucketName: StyleBucketName) => {
	if (seenRules) {
	if (seenRules.has(cssRule)) {
	duplicateRuleCount++;
	} else {
	seenRules.add(cssRule);
	}
	}
	const seenRules: Set<string> | undefined = new Set();
	let duplicateRuleCount = 0;
	const cacheRule = (cssRule: string, bucketName: StyleBucketName) => {
	if (process.env.NODE_ENV !== 'production') {
	if (seenRules.has(cssRule)) {
	duplicateRuleCount++;
	} else {
	seenRules.add(cssRule);
	}
	}

nit: let's write it this way. While Terser is able to DCE the original code, I would prefer to keep instructions gated in more solid way. Thx!

[layershifter]

Suggested change

	"comment": "Warn in development when duplicate CSS rules are found during rehydration — a signal that server-rendered styles were flushed into the HTML more than once (e.g. the Next.js App Router calling renderToStyleElements on every flush without clearing the renderer)",
	"comment": "chore: add development warnings for hydration issues in `rehydrateRendererCache`",

[layershifter]

@vashek Can you please check comments? Thx!