diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index fd24429..a1b4036 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -145,6 +145,30 @@ jobs:
       - if: always()
         run: npx supabase stop
 
+  pokedex-performance:
+    runs-on: ubuntu-latest
+    timeout-minutes: 20
+    env:
+      PUBLIC_USE_LOCAL_POKEMON_SPRITE_FOLDER: 'true'
+    steps:
+      - uses: actions/checkout@v5
+      - uses: actions/setup-node@v5
+        with:
+          node-version: 24
+          cache: npm
+      - run: npm ci
+      - run: npx playwright install --with-deps chromium
+      - run: npx supabase start
+      - run: npm run test:performance
+      - uses: actions/upload-artifact@v6
+        if: always()
+        with:
+          name: pokedex-performance
+          path: test-results/performance/
+          if-no-files-found: ignore
+      - if: always()
+        run: npx supabase stop
+
   cloudflare-preview-build:
     runs-on: ubuntu-latest
     timeout-minutes: 15
diff --git a/package.json b/package.json
index e2a3f10..a44e3b3 100644
--- a/package.json
+++ b/package.json
@@ -36,7 +36,7 @@
 		"test:build:inject-static": "npm run build-inject-manifest && vitest run --config vitest.build.config.mts",
 		"test:build:inject-node": "npm run build-inject-manifest-node && NODE_ADAPTER=true vitest run --config vitest.build.config.mts",
 		"test:build": "npm run test:build:generate-static && npm run test:build:generate-node && npm run test:build:inject-static && npm run test:build:inject-node",
-		"test:ci": "npm run test:fast && npm run test:integration && npm run test:build && npm run test:bdd",
+		"test:ci": "npm run test:fast && npm run test:integration && npm run test:build && npm run test:bdd && npm run test:performance",
 		"test": "npm run test:ci",
 		"supabase:start": "supabase start",
 		"supabase:stop": "supabase stop",
@@ -48,7 +48,8 @@
 		"sprites:grid": "node scripts/grid-thumbnails.mjs",
 		"build:cloudflare": "DEPLOY_TARGET=cloudflare npm run build",
 		"preview:cloudflare": "wrangler dev",
-		"check:cloudflare": "wrangler deploy --dry-run"
+		"check:cloudflare": "wrangler deploy --dry-run",
+		"test:performance": "node scripts/run-with-local-supabase.mjs node scripts/performance/run-local.mjs"
 	},
 	"devDependencies": {
 		"@lhci/cli": "^0.15.1",
diff --git a/scripts/performance/benchmark.mjs b/scripts/performance/benchmark.mjs
new file mode 100644
index 0000000..c235e04
--- /dev/null
+++ b/scripts/performance/benchmark.mjs
@@ -0,0 +1,181 @@
+import { chromium } from '@playwright/test';
+import { readFile, writeFile } from 'node:fs/promises';
+
+// Configuration holds session-file paths, never passwords. Keep it outside the repository.
+const [configPath, outputPath] = process.argv.slice(2);
+if (!configPath || !outputPath)
+	throw new Error('Usage: node scripts/performance/benchmark.mjs CONFIG.json RESULTS.json');
+const config = JSON.parse(await readFile(configPath, 'utf8'));
+const samples = config.samples ?? 30;
+if (samples < 30)
+	throw new Error('Host decisions require at least 30 warm samples per fixture/navigation');
+if (!config.revision || !config.databaseLabel || !config.clientLocation)
+	throw new Error('Record revision, databaseLabel and clientLocation for comparable results');
+const browser = await chromium.launch();
+const results = [];
+try {
+	for (const host of config.hosts) {
+		for (const fixture of host.fixtures) {
+			for (const navigation of ['direct', 'client']) {
+				const context = await browser.newContext({
+					baseURL: host.url,
+					storageState: host.storageState,
+					viewport: { width: 1350, height: 940 },
+					deviceScaleFactor: 1
+				});
+				await context.addInitScript(() => {
+					window.__perfShifts = [];
+					new PerformanceObserver((list) => {
+						for (const entry of list.getEntries())
+							if (!entry.hadRecentInput)
+								window.__perfShifts.push({ value: entry.value, time: entry.startTime });
+					}).observe({ type: 'layout-shift', buffered: true });
+					window.__watchGrid = () => {
+						window.__perfVisible = null;
+						function check() {
+							const cell = document.querySelector('[data-entry-index]');
+							if (
+								location.pathname.startsWith('/pokedex/') &&
+								cell &&
+								cell.getBoundingClientRect().top < innerHeight &&
+								cell.getBoundingClientRect().height > 0
+							) {
+								window.__perfVisible = performance.now();
+								performance.mark('pokedex:first-visible');
+							} else requestAnimationFrame(check);
+						}
+						requestAnimationFrame(check);
+					};
+					window.__watchGrid();
+				});
+				const page = await context.newPage();
+				for (let index = 0; index <= samples; index++) {
+					if (navigation === 'client') {
+						await page.goto('/my-pokedexes');
+						await page.getByText(fixture.name, { exact: true }).first().waitFor();
+					} else if (index > 0) await page.goto('about:blank');
+					const bodies = [];
+					const requests = { grid: 0, details: 0, snapshot: 0, backup: 0 };
+					const onRequest = (request) => {
+						const path = new URL(request.url()).pathname;
+						if (/\/pokedexes\/[^/]+\/(grid|combined-data)$/.test(path)) requests.grid++;
+						if (/\/pokedexes\/[^/]+\/entries\//.test(path)) requests.details++;
+						if (path === '/api/offline-snapshot') requests.snapshot++;
+						if (path === '/api/export-integrations') requests.backup++;
+					};
+					const onResponse = (response) => {
+						const path = new URL(response.url()).pathname;
+						if (!path.startsWith(`/pokedex/${fixture.id}`)) return;
+						bodies.push(
+							(async () => {
+								// Chromium may not expose bodies routed through a service worker.
+								const body = await response.body().catch(() => null);
+								return {
+									url: response.url(),
+									bytes: body?.length ?? null,
+									status: response.status(),
+									fromServiceWorker: response.fromServiceWorker(),
+									timings: response.request().timing(),
+									serverTiming: response.headers()['server-timing'] ?? null
+								};
+							})()
+						);
+					};
+					page.on('request', onRequest);
+					page.on('response', onResponse);
+					let started = 0;
+					if (navigation === 'direct')
+						await page.goto(`/pokedex/${fixture.id}`, { waitUntil: 'domcontentloaded' });
+					else {
+						started = await page.evaluate(() => {
+							performance.clearMarks('pokedex:first-interactive');
+							window.__watchGrid();
+							return performance.now();
+						});
+						await page
+							.locator('.card')
+							.filter({ hasText: fixture.name })
+							.first()
+							.getByRole('button', { name: 'View', exact: true })
+							.click();
+					}
+					await page.waitForSelector('[data-grid-interactive]');
+					await page.waitForFunction(
+						() =>
+							window.__perfVisible !== null &&
+							performance.getEntriesByName('pokedex:first-interactive').length > 0
+					);
+					await page.waitForTimeout(1500);
+					const browserData = await page.evaluate(
+						(start) => ({
+							visibleMs: window.__perfVisible - start,
+							interactiveMs:
+								performance.getEntriesByName('pokedex:first-interactive').at(-1).startTime - start,
+							cells: document.querySelectorAll('[data-entry-index]').length,
+							dom: document.querySelectorAll('*').length,
+							cls: window.__perfShifts
+								.filter((entry) => entry.time >= start)
+								.reduce((total, entry) => total + entry.value, 0)
+						}),
+						started
+					);
+					page.off('request', onRequest);
+					page.off('response', onResponse);
+					const resources = await page.evaluate(() =>
+						performance.getEntriesByType('resource').map((entry) => ({
+							url: entry.name,
+							decodedBytes: entry.decodedBodySize,
+							encodedBytes: entry.encodedBodySize,
+							transferBytes: entry.transferSize
+						}))
+					);
+					const responses = (await Promise.all(bodies)).map(({ url, ...response }) => {
+						const resource = resources.findLast((entry) => entry.url === url);
+						return {
+							...response,
+							bytes: response.bytes ?? (resource?.decodedBytes || null),
+							byteSource: response.bytes !== null ? 'response-body' : 'resource-timing',
+							encodedBytes: resource?.encodedBytes ?? null,
+							transferBytes: resource?.transferBytes ?? null
+						};
+					});
+					results.push({
+						host: host.label,
+						fixture: fixture.label,
+						navigation,
+						run: index === 0 ? 'first-observed' : 'warm',
+						...browserData,
+						requests,
+						responses
+					});
+				}
+				await context.close();
+				console.log(
+					`Completed ${host.label}/${fixture.label}/${navigation}: ${samples} warm samples.`
+				);
+			}
+		}
+	}
+	await writeFile(
+		outputPath,
+		JSON.stringify(
+			{
+				version: 1,
+				capturedAt: new Date().toISOString(),
+				revision: config.revision,
+				databaseLabel: config.databaseLabel,
+				clientLocation: config.clientLocation,
+				environment: config.environment ?? 'deployed',
+				compatibilityPassed: config.compatibilityPassed ?? false,
+				results
+			},
+			null,
+			2
+		)
+	);
+	console.log(
+		`Recorded ${results.length} samples; first-observed runs are excluded from warm statistics.`
+	);
+} finally {
+	await browser.close();
+}
diff --git a/scripts/performance/compare.mjs b/scripts/performance/compare.mjs
new file mode 100644
index 0000000..829ccb6
--- /dev/null
+++ b/scripts/performance/compare.mjs
@@ -0,0 +1,46 @@
+import { readFile } from 'node:fs/promises';
+const data = JSON.parse(await readFile(process.argv[2], 'utf8'));
+const warm = data.results.filter((row) => row.run === 'warm');
+const p75 = (values) => [...values].sort((a, b) => a - b)[Math.ceil(values.length * 0.75) - 1];
+const groups = [...new Set(warm.map((row) => `${row.fixture}/${row.navigation}`))];
+const paired = groups.map((group) => {
+	const rows = (host) =>
+		warm.filter((row) => `${row.fixture}/${row.navigation}` === group && row.host === host);
+	for (const host of ['netlify', 'cloudflare'])
+		if (rows(host).length < 30) throw new Error(`Insufficient ${host} warm samples for ${group}`);
+	return {
+		group,
+		netlify: p75(rows('netlify').map((row) => row.interactiveMs)),
+		cloudflare: p75(rows('cloudflare').map((row) => row.interactiveMs))
+	};
+});
+if (groups.length !== 4)
+	throw new Error(
+		'Expected national and scoped-form fixtures, each with direct and client navigation'
+	);
+const netlify = p75(warm.filter((row) => row.host === 'netlify').map((row) => row.interactiveMs));
+const cloudflare = p75(
+	warm.filter((row) => row.host === 'cloudflare').map((row) => row.interactiveMs)
+);
+const improvement = netlify - cloudflare;
+const passes =
+	improvement >= 200 &&
+	improvement / netlify >= 0.2 &&
+	paired.every((row) => row.cloudflare <= row.netlify * 1.1);
+console.log(
+	JSON.stringify(
+		{
+			overallP75: { netlify, cloudflare },
+			improvementMs: improvement,
+			improvementPercent: (100 * improvement) / netlify,
+			groups: paired,
+			performanceGatePassed: passes,
+			recommendation:
+				passes && data.compatibilityPassed && data.environment === 'deployed'
+					? 'Cloudflare qualifies for a migration proposal; review operating cost before cutover.'
+					: 'Retain Netlify: performance, deployed evidence or compatibility gate is unmet.'
+		},
+		null,
+		2
+	)
+);
diff --git a/scripts/performance/fixture.mjs b/scripts/performance/fixture.mjs
new file mode 100644
index 0000000..4032b07
--- /dev/null
+++ b/scripts/performance/fixture.mjs
@@ -0,0 +1,113 @@
+import { createClient } from '@supabase/supabase-js';
+import { createServerClient } from '@supabase/ssr';
+import { mkdir, readFile, writeFile, unlink } from 'node:fs/promises';
+import { randomUUID } from 'node:crypto';
+
+const directory = process.env.PERF_FIXTURE_DIR ?? '/tmp/livingdex-grid-performance';
+const url = process.env.PUBLIC_SUPABASE_URL;
+if (!url || !['localhost', '127.0.0.1', '[::1]'].includes(new URL(url).hostname))
+	throw new Error('Fixtures require the local Supabase wrapper');
+const admin = createClient(url, process.env.SUPABASE_SERVICE_ROLE_KEY, {
+	auth: { persistSession: false }
+});
+const check = ({ data, error }) => {
+	if (error) throw error;
+	return data;
+};
+await mkdir(directory, { recursive: true, mode: 0o700 });
+const fixturePath = `${directory}/fixture.json`;
+if (process.argv.includes('--cleanup')) {
+	const fixture = JSON.parse(await readFile(fixturePath, 'utf8'));
+	const { user } = check(await admin.auth.admin.getUserById(fixture.userId));
+	if (user.email !== fixture.email || !user.email.startsWith('grid-performance-'))
+		throw new Error('Refusing to remove a non-fixture account');
+	check(await admin.auth.admin.deleteUser(fixture.userId));
+	await unlink(fixturePath);
+	await unlink(`${directory}/storage-state.json`).catch(() => {});
+	console.log('Removed disposable performance account and session.');
+	process.exit(0);
+}
+try {
+	await readFile(fixturePath);
+	throw new Error('An existing fixture needs cleanup first');
+} catch (error) {
+	if (error.code !== 'ENOENT') throw error;
+}
+const email = `grid-performance-${randomUUID()}@example.test`;
+const password = randomUUID() + 'aA1!';
+const { user } = check(await admin.auth.admin.createUser({ email, password, email_confirm: true }));
+const fixture = { userId: user.id, email, password, dexes: [] };
+await writeFile(fixturePath, JSON.stringify(fixture), { mode: 0o600 });
+fixture.dexes = check(
+	await admin
+		.from('pokedexes')
+		.insert([
+			{ userId: user.id, name: 'Performance National', isLivingDex: true, isFormDex: false },
+			{
+				userId: user.id,
+				name: 'Performance Scarlet Forms',
+				isLivingDex: true,
+				isFormDex: true,
+				gameScope: 'Scarlet'
+			}
+		])
+		.select()
+);
+const game = fixture.dexes.find((dex) => dex.gameScope);
+check(
+	await admin.from('pokedex_dex_scopes').insert({ pokedexId: game.id, dexId: 'scarlet-paldea' })
+);
+const entries = [];
+for (let from = 0; ; from += 1000) {
+	const page = check(
+		await admin
+			.from('pokemon')
+			.select('id,isDefaultForm')
+			.order('id')
+			.range(from, from + 999)
+	);
+	entries.push(...page);
+	if (page.length < 1000) break;
+}
+for (const dex of fixture.dexes) {
+	const selected = entries.filter((entry) => dex.isFormDex || entry.isDefaultForm);
+	for (let from = 0; from < selected.length; from += 500) {
+		check(
+			await admin.from('catch_records').insert(
+				selected.slice(from, from + 500).map((entry, index) => ({
+					userId: user.id,
+					pokedexId: dex.id,
+					pokemonId: entry.id,
+					caught: (from + index) % 3 === 0,
+					haveToEvolve: (from + index) % 3 === 1,
+					inHome: (from + index) % 7 === 0,
+					personalNotes: index === 0 ? 'Performance fixture note: preserve me' : ''
+				}))
+			)
+		);
+	}
+}
+await writeFile(fixturePath, JSON.stringify(fixture), { mode: 0o600 });
+const cookies = [];
+const client = createServerClient(url, process.env.PUBLIC_SUPABASE_ANON_KEY, {
+	cookies: { getAll: () => [], setAll: (values) => cookies.push(...values) }
+});
+check(await client.auth.signInWithPassword({ email, password }));
+const base = new URL(process.env.PERF_BASE_URL ?? 'http://127.0.0.1:4173');
+await writeFile(
+	`${directory}/storage-state.json`,
+	JSON.stringify({
+		cookies: cookies.map(({ name, value }) => ({
+			name,
+			value,
+			domain: base.hostname,
+			path: '/',
+			httpOnly: false,
+			secure: base.protocol === 'https:',
+			sameSite: 'Lax'
+		})),
+		origins: []
+	}),
+	{ mode: 0o600 }
+);
+console.log('Prepared national and scoped-form fixtures and a private browser session.');
diff --git a/scripts/performance/run-local.mjs b/scripts/performance/run-local.mjs
new file mode 100644
index 0000000..eff5e0c
--- /dev/null
+++ b/scripts/performance/run-local.mjs
@@ -0,0 +1,105 @@
+import { spawn } from 'node:child_process';
+import { mkdtemp, mkdir, cp, rm, readFile, writeFile } from 'node:fs/promises';
+import os from 'node:os';
+import path from 'node:path';
+
+const directory = await mkdtemp(path.join(os.tmpdir(), 'livingdex-grid-tests-'));
+const port = process.env.PERF_PORT ?? '4185';
+const env = {
+	...process.env,
+	NODE_ADAPTER: 'true',
+	PUBLIC_USE_LOCAL_POKEMON_SPRITE_FOLDER: 'true',
+	POKEDEX_PERFORMANCE: 'true',
+	PERF_FIXTURE_DIR: directory,
+	PERF_BASE_URL: `http://127.0.0.1:${port}`,
+	PORT: port,
+	HOST: '127.0.0.1'
+};
+function run(command, args) {
+	return new Promise((resolve, reject) => {
+		const process = spawn(command, args, { env, stdio: 'inherit' });
+		process.once('error', reject);
+		process.once('exit', (code) =>
+			code === 0 ? resolve() : reject(new Error(`${command} failed (${code})`))
+		);
+	});
+}
+let server;
+let fixture = false;
+try {
+	await run('npm', ['run', 'build-inject-manifest-node']);
+	fixture = true;
+	await run('node', ['scripts/performance/fixture.mjs']);
+	server = spawn('node', ['build'], { env, stdio: 'inherit' });
+	await new Promise((resolve, reject) => {
+		const timer = setInterval(async () => {
+			try {
+				if ((await fetch(env.PERF_BASE_URL)).ok) {
+					clearInterval(timer);
+					clearTimeout(timeout);
+					resolve();
+				}
+			} catch {}
+		}, 200);
+		const timeout = setTimeout(() => {
+			clearInterval(timer);
+			reject(new Error('Local production server did not start'));
+		}, 15000);
+		server.once('error', (error) => {
+			clearInterval(timer);
+			clearTimeout(timeout);
+			reject(error);
+		});
+		server.once('exit', (code) => {
+			clearInterval(timer);
+			clearTimeout(timeout);
+			reject(new Error(`Local server exited (${code})`));
+		});
+	});
+	if (process.env.PERF_BENCHMARK === 'true') {
+		const data = JSON.parse(await readFile(path.join(directory, 'fixture.json'), 'utf8'));
+		await writeFile(
+			path.join(directory, 'benchmark-config.json'),
+			JSON.stringify({
+				samples: 30,
+				revision: process.env.PERF_REVISION ?? 'local-working-tree',
+				databaseLabel: 'local-seeded-supabase',
+				clientLocation: 'local-loopback',
+				environment: 'local-node',
+				hosts: [
+					{
+						label: 'node',
+						url: env.PERF_BASE_URL,
+						storageState: path.join(directory, 'storage-state.json'),
+						fixtures: data.dexes.map((dex) => ({
+							id: dex.id,
+							name: dex.name,
+							label: dex.gameScope ? 'scoped-forms' : 'national'
+						}))
+					}
+				]
+			})
+		);
+		await run('node', [
+			'scripts/performance/benchmark.mjs',
+			path.join(directory, 'benchmark-config.json'),
+			path.join(directory, 'benchmark.json')
+		]);
+		await mkdir('test-results/performance', { recursive: true });
+		await cp(path.join(directory, 'benchmark.json'), 'test-results/performance/benchmark.json');
+	}
+	await run('node', ['scripts/performance/verify.mjs']);
+	await run('node', ['scripts/performance/verify-behavior.mjs']);
+	await mkdir('test-results/performance', { recursive: true });
+	for (const file of ['verification.json', 'behavior.json', 'national.png', 'scoped.png'])
+		await cp(path.join(directory, file), path.join('test-results/performance', file));
+} finally {
+	server?.kill('SIGTERM');
+	if (fixture)
+		await run('node', ['scripts/performance/fixture.mjs', '--cleanup']).catch((error) => {
+			throw new Error(`Fixture cleanup failed; retained recovery files in ${directory}`, {
+				cause: error
+			});
+		});
+	await rm(directory, { recursive: true, force: true });
+}
diff --git a/scripts/performance/verify-behavior.mjs b/scripts/performance/verify-behavior.mjs
new file mode 100644
index 0000000..1b476df
--- /dev/null
+++ b/scripts/performance/verify-behavior.mjs
@@ -0,0 +1,165 @@
+import { chromium } from '@playwright/test';
+import { readFile, writeFile } from 'node:fs/promises';
+import assert from 'node:assert/strict';
+const dir = process.env.PERF_FIXTURE_DIR ?? '/tmp/livingdex-grid-performance';
+const fixture = JSON.parse(await readFile(`${dir}/fixture.json`, 'utf8'));
+const browser = await chromium.launch();
+const context = await browser.newContext({
+	baseURL: process.env.PERF_BASE_URL ?? 'http://127.0.0.1:4173',
+	storageState: `${dir}/storage-state.json`,
+	viewport: { width: 1350, height: 940 }
+});
+await context.addInitScript(() => {
+	window.__cls = 0;
+	new PerformanceObserver((list) => {
+		for (const entry of list.getEntries()) if (!entry.hadRecentInput) window.__cls += entry.value;
+	}).observe({ type: 'layout-shift', buffered: true });
+});
+const page = await context.newPage();
+const errors = [];
+page.on('pageerror', (error) => errors.push(error.message));
+const national = fixture.dexes.find((dex) => !dex.gameScope);
+const scoped = fixture.dexes.find((dex) => dex.gameScope);
+const results = [];
+try {
+	await page.goto(`/pokedex/${national.id}`);
+	await page.waitForSelector('[data-grid-interactive]');
+	for (const width of [1350, 390]) {
+		await page.setViewportSize({ width, height: 940 });
+		for (const density of ['comfortable', 'compact', 'ultra']) {
+			await page.getByLabel('Choose box view layout density').selectOption(density);
+			await page.reload();
+			await page.waitForSelector('[data-grid-interactive]');
+			await page.waitForTimeout(350);
+			assert.equal(await page.getByLabel('Choose box view layout density').inputValue(), density);
+			const geometry = await page.evaluate(() => {
+				const cell = document.querySelector('[data-entry-index="0"]').getBoundingClientRect();
+				const shell = document.querySelector('[data-box-number="1"]').getBoundingClientRect();
+				return {
+					cellWidth: cell.width,
+					cellHeight: cell.height,
+					cls: window.__cls,
+					horizontalOverflow: document.documentElement.scrollWidth > innerWidth,
+					shellHeight: shell.height
+				};
+			});
+			assert.ok(Math.abs(geometry.cellWidth - geometry.cellHeight) < 1, JSON.stringify(geometry));
+			assert.ok(!geometry.horizontalOverflow, `Horizontal overflow at ${width}/${density}`);
+			assert.ok(geometry.cls <= 0.1, `CLS ${geometry.cls} at ${width}/${density}`);
+			results.push({ width, density, ...geometry });
+		}
+	}
+	await page.setViewportSize({ width: 1350, height: 940 });
+	await page.getByLabel('Choose box view layout density').selectOption('comfortable');
+	await page.getByLabel('Not caught', { exact: true }).check();
+	assert.equal(await page.locator('[data-entry-index="0"]').getAttribute('aria-disabled'), 'true');
+	assert.equal(await page.locator('[data-entry-index="0"]').getAttribute('data-entry-id'), '1');
+	await page.getByLabel('Not caught', { exact: true }).uncheck();
+	// Resizing across the mobile breakpoint keeps the same box at the scroll anchor.
+	await page.locator('[data-box-number="15"]').evaluate((node) => node.scrollIntoView());
+	await page.waitForTimeout(100);
+	const anchorTop = await page
+		.locator('[data-box-number="15"]')
+		.evaluate((node) => node.getBoundingClientRect().top);
+	await page.setViewportSize({ width: 390, height: 940 });
+	await page.waitForTimeout(200);
+	const resizedTop = await page
+		.locator('[data-box-number="15"]')
+		.evaluate((node) => node.getBoundingClientRect().top);
+	assert.ok(
+		Math.abs(anchorTop - resizedTop) < 2,
+		`Scroll anchor moved: ${anchorTop} to ${resizedTop}`
+	);
+	await page.setViewportSize({ width: 1350, height: 940 });
+	await page.evaluate(() => window.scrollTo(0, document.body.scrollHeight));
+	await page.waitForSelector('[data-entry-index="1024"]');
+	assert.ok((await page.locator('[data-entry-index]').count()) < 300);
+	await page.locator('[data-entry-index="1024"]').focus();
+	await page.keyboard.press('ArrowRight');
+	assert.equal(
+		await page.evaluate(() => document.activeElement.getAttribute('data-entry-index')),
+		'1024'
+	);
+	await page.evaluate(() => window.scrollTo(0, 0));
+	await page.waitForSelector('[data-entry-index="0"]');
+	// Closing an in-flight modal must prevent its response from replacing the next selection.
+	let release;
+	const held = new Promise((resolve) => {
+		release = resolve;
+	});
+	await page.route(`**/api/pokedexes/${national.id}/entries/1`, async (route) => {
+		await held;
+		await route.continue().catch(() => {});
+	});
+	await page.locator('[data-entry-index="0"]').click();
+	await page.getByRole('dialog').getByRole('button', { name: 'Close', exact: true }).click();
+	await page.locator('[data-entry-index="1"]').click();
+	release();
+	await page.getByRole('dialog').getByLabel('Notes:', { exact: true }).waitFor();
+	assert.equal(
+		await page.getByRole('dialog').getByRole('heading', { name: 'Ivysaur', exact: true }).count(),
+		1
+	);
+	assert.equal(
+		await page.getByRole('dialog').getByRole('heading', { name: 'Bulbasaur', exact: true }).count(),
+		0
+	);
+	await page.getByRole('dialog').getByRole('button', { name: 'Close', exact: true }).click();
+	await page.unrouteAll({ behavior: 'wait' });
+	// Navigate through actual list cards, preserving fixture-specific totals and positions.
+	await page.getByRole('link', { name: 'My Pokédexes' }).click();
+	await page
+		.locator('.card')
+		.filter({ hasText: scoped.name })
+		.first()
+		.getByRole('button', { name: 'View', exact: true })
+		.click();
+	await page.waitForSelector('[data-grid-interactive]');
+	assert.ok((await page.locator('body').innerText()).includes('Showing 439 of 439'));
+	await page.goBack();
+	await page
+		.locator('.card')
+		.filter({ hasText: national.name })
+		.first()
+		.getByRole('button', { name: 'View', exact: true })
+		.click();
+	await page.waitForSelector('[data-grid-interactive]');
+	// Wait for the existing worker's full snapshot, then read an uncached detail without a network.
+	await page.waitForFunction(
+		async () => {
+			const meta = await (
+				await caches.open('livingdex-offline-meta-v1')
+			).match('/__offline/current');
+			return !!(await meta?.json())?.dataCache;
+		},
+		{ timeout: 15000 }
+	);
+	await context.setOffline(true);
+	await page.locator('[data-entry-index="5"]').click();
+	await page.waitForFunction(() =>
+		document.querySelector('[role="dialog"]')?.textContent.includes('Where to catch:')
+	);
+	assert.equal(await page.getByRole('dialog').locator('textarea').count(), 0);
+	await page.getByRole('dialog').getByRole('button', { name: 'Close', exact: true }).click();
+	// A mismatched cache owner is never used as fallback.
+	await page.evaluate(async () => {
+		const cache = await caches.open('livingdex-offline-meta-v1');
+		const response = await cache.match('/__offline/current');
+		const meta = await response.json();
+		await cache.put(
+			'/__offline/current',
+			new Response(JSON.stringify({ ...meta, userId: 'other-account' }))
+		);
+	});
+	await page.locator('[data-entry-index="6"]').click();
+	await page.getByRole('alert').filter({ hasText: 'not saved for offline use' }).waitFor();
+	await page.getByRole('dialog').getByRole('button', { name: 'Close', exact: true }).click();
+	assert.deepEqual(errors, []);
+	await writeFile(`${dir}/behavior.json`, JSON.stringify(results, null, 2));
+	console.log(
+		'Passed density/mobile geometry, filtered placement, virtual boundaries, modal races, client navigation and isolated offline details.'
+	);
+	console.log(JSON.stringify(results, null, 2));
+} finally {
+	await browser.close();
+}
diff --git a/scripts/performance/verify.mjs b/scripts/performance/verify.mjs
new file mode 100644
index 0000000..f5261f8
--- /dev/null
+++ b/scripts/performance/verify.mjs
@@ -0,0 +1,129 @@
+import { chromium } from '@playwright/test';
+import { readFile, writeFile } from 'node:fs/promises';
+import assert from 'node:assert/strict';
+
+const directory = process.env.PERF_FIXTURE_DIR ?? '/tmp/livingdex-grid-performance';
+const fixture = JSON.parse(await readFile(`${directory}/fixture.json`, 'utf8'));
+const baseURL = process.env.PERF_BASE_URL ?? 'http://127.0.0.1:4173';
+const browser = await chromium.launch({ headless: true });
+const context = await browser.newContext({
+	baseURL,
+	storageState: `${directory}/storage-state.json`,
+	viewport: { width: 1350, height: 940 }
+});
+await context.addInitScript(() => {
+	window.__layoutShifts = [];
+	window.__firstVisible = null;
+	new PerformanceObserver((list) => {
+		for (const entry of list.getEntries())
+			if (!entry.hadRecentInput) window.__layoutShifts.push(entry.value);
+	}).observe({ type: 'layout-shift', buffered: true });
+	function visible() {
+		const cell = document.querySelector('[data-entry-index]');
+		if (
+			cell &&
+			cell.getBoundingClientRect().height > 0 &&
+			cell.getBoundingClientRect().top < innerHeight
+		)
+			window.__firstVisible ??= performance.now();
+		if (window.__firstVisible === null) requestAnimationFrame(visible);
+	}
+	requestAnimationFrame(visible);
+});
+const page = await context.newPage();
+const readJson = (path) =>
+	page.evaluate(async (path) => {
+		const response = await fetch(path);
+		if (!response.ok) throw new Error(`Fixture API failed: ${response.status}`);
+		return response.json();
+	}, path);
+const errors = [];
+page.on('pageerror', (error) => errors.push(error.message));
+const requests = [];
+page.on('request', (request) => requests.push(new URL(request.url()).pathname));
+const results = [];
+try {
+	for (const dex of fixture.dexes) {
+		requests.length = 0;
+		const response = await page.goto(`/pokedex/${dex.id}`);
+		assert.equal(response.status(), 200);
+		const html = await response.text();
+		assert.ok(/data-entry-index=["']?0["'\s>]/.test(html), 'Initial cells must be server rendered');
+		await page.waitForSelector('[data-grid-interactive]');
+		await page.waitForTimeout(1000);
+		const stats = await page.evaluate(() => ({
+			cells: document.querySelectorAll('[data-entry-index]').length,
+			dom: document.querySelectorAll('*').length,
+			cls: window.__layoutShifts.reduce((sum, value) => sum + value, 0),
+			firstVisible: window.__firstVisible,
+			interactive: performance.getEntriesByName('pokedex:first-interactive').at(-1)?.startTime
+		}));
+		assert.ok(stats.cells <= 180, `Mounted cells: ${stats.cells}`);
+		assert.ok(stats.dom < 2500, `DOM elements: ${stats.dom}`);
+		assert.ok(stats.cls <= 0.1, `CLS: ${stats.cls}`);
+		assert.equal(
+			requests.filter((path) => /\/api\/pokedexes\/[^/]+\/(grid|combined-data)$/.test(path)).length,
+			0
+		);
+		assert.equal(await page.locator('button button').count(), 0);
+		await page.screenshot({ path: `${directory}/${dex.gameScope ? 'scoped' : 'national'}.png` });
+		const first = page.locator('[data-entry-index="0"]');
+		const entryId = await first.getAttribute('data-entry-id');
+		const notes = await readJson(`/api/pokedexes/${dex.id}/entries/${entryId}`);
+		await first.click();
+		const modal = page.getByRole('dialog', { name: 'Pokémon details' });
+		await modal.getByLabel('Notes:', { exact: true }).waitFor();
+		assert.equal(
+			await modal.getByLabel('Notes:', { exact: true }).inputValue(),
+			notes.catchRecord.personalNotes
+		);
+		const image = modal.locator('img').first();
+		await image.waitFor();
+		await page.waitForFunction(
+			() => document.querySelector('[role="dialog"] img')?.naturalWidth > 0
+		);
+		assert.equal(await image.evaluate((node) => node.naturalWidth), 512);
+		assert.ok(!(await image.getAttribute('src')).includes('sprites-grid'));
+		await modal.getByRole('button', { name: 'Close', exact: true }).click();
+		assert.equal(await first.evaluate((node) => document.activeElement === node), true);
+		// Reopening uses cached details; bulk edits must retain the unloaded personal notes.
+		const detailCalls = () =>
+			requests.filter((path) => path.endsWith(`/entries/${entryId}`)).length;
+		const before = detailCalls();
+		await first.click();
+		await modal.getByLabel('Notes:', { exact: true }).waitFor();
+		assert.equal(detailCalls(), before);
+		await modal.getByRole('button', { name: 'Close', exact: true }).click();
+		await page.getByRole('button', { name: 'Open bulk actions menu' }).first().click();
+		await page.getByRole('button', { name: 'Mark box as In HOME', exact: true }).click();
+		await page.waitForResponse(
+			(r) =>
+				r.url().endsWith(`/pokedexes/${dex.id}/catch-records`) && r.request().method() === 'POST'
+		);
+		const after = await readJson(`/api/pokedexes/${dex.id}/entries/${entryId}`);
+		assert.equal(after.catchRecord.personalNotes, notes.catchRecord.personalNotes);
+		assert.equal(after.catchRecord.inHome, true);
+		// Focus navigation must reach entries that were not initially mounted.
+		await page.locator('[data-entry-index="29"]').focus();
+		for (let index = 0; index < 22; index++) await page.keyboard.press('ArrowDown');
+		assert.equal(
+			await page.evaluate(() => document.activeElement?.getAttribute('data-entry-index')),
+			'161'
+		);
+		await page.getByLabel('Render all boxes').check();
+		const grid = await readJson(`/api/pokedexes/${dex.id}/grid`);
+		assert.equal(await page.locator('[data-entry-index]').count(), grid.grid.length);
+		await page.getByLabel('Render all boxes').uncheck();
+		results.push({
+			fixture: dex.gameScope ? 'scoped-forms' : 'national',
+			...stats,
+			documentBytes: Buffer.byteLength(html),
+			gridBytes: Buffer.byteLength(JSON.stringify(grid))
+		});
+	}
+	assert.deepEqual(errors, [], 'Browser errors');
+	await writeFile(`${directory}/verification.json`, JSON.stringify(results, null, 2));
+	console.log(JSON.stringify(results, null, 2));
+} finally {
+	await browser.close();
+}