Add e2e test

test/e2e/specs/site-editor/preload.spec.js

@@ -0,0 +1,41 @@
+/**
+ * WordPress dependencies
+ */
+const { test, expect } = require( '@wordpress/e2e-test-utils-playwright' );
+
+test.describe( 'Preload', () => {
+	test.beforeAll( async ( { requestUtils } ) => {
+		await requestUtils.activateTheme( 'emptytheme' );
+	} );
+
+	test.afterAll( async ( { requestUtils } ) => {
+		await requestUtils.activateTheme( 'twentytwentyone' );
+	} );
+
+	test( 'Should make no requests before the iframe is loaded', async ( {
+		page,
+		admin,
+	} ) => {
+		// Do not use `visitSiteEditor` because it waits for the iframe to load.
+		await admin.visitAdminPage( 'site-editor.php' );
+
+		const requests = [];
+		let isLoaded = false;
+
+		page.on( 'request', ( request ) => {
+			if ( request.resourceType() === 'document' ) {
+				// The iframe also "requests" a blob document. This is the most
+				// reliable way to wait for the iframe to start loading.
+				// `waitForSelector` is always a bit delayed, and we don't want
+				// to detect requests after the iframe mounts.
+				isLoaded = true;
+			} else if ( ! isLoaded && request.resourceType() === 'fetch' ) {
+				requests.push( request.url() );
+			}
+		} );
+
+		await page.waitForFunction( ( _isLoaded ) => _isLoaded, [ isLoaded ] );
+
+		expect( requests ).toEqual( [] );
+	} );
+} );