Cover: Decode the oEmbed iframe source before parsing its query string

packages/block-library/CHANGELOG.md

@@ -15,6 +15,7 @@
 ### Bug Fixes
 
 -   Accordion Heading: Anchor notes to the selected title text instead of the whole block ([#84497](https://github.com/WordPress/gutenberg/pull/84497)).
+-   Cover: Decode the oEmbed iframe source before parsing it, so an embedded background video keeps the parameters the provider sent ([#83770](https://github.com/WordPress/gutenberg/pull/83770)).
 
 ## 11.2.0 (2026-10-07)
 

packages/block-library/src/cover/index.php

@@ -34,11 +34,13 @@ function render_block_core_cover( $attributes, $content ) {
 		$oembed_html = wp_oembed_get( $url );
 
 		if ( $oembed_html ) {
-			// Extract iframe src from the oEmbed HTML.
-			preg_match( '/src=["\']([^"\']+)["\']/', $oembed_html, $src_matches );
-			if ( ! empty( $src_matches[1] ) ) {
-				$iframe_src = $src_matches[1];
-
+			// Read the iframe src with the HTML API, which returns the attribute
+			// value decoded, so `&` in the query string becomes `&`.
+			$oembed_processor = new WP_HTML_Tag_Processor( $oembed_html );
+			$iframe_src       = $oembed_processor->next_tag( 'IFRAME' )
+				? $oembed_processor->get_attribute( 'src' )
+				: null;
+			if ( is_string( $iframe_src ) && '' !== $iframe_src ) {
 				// Detect provider from iframe src URL.
 				$lower_src = strtolower( $iframe_src );
 				$provider  = null;

phpunit/blocks/render-block-cover-test.php

@@ -128,4 +128,38 @@ public function test_gutenberg_render_block_core_cover_fixed_or_repeated_backgro
 		$this->assertStringNotContainsString( 'background-image', $rendered );
 		$this->assertStringNotContainsString( 'min-height', $rendered );
 	}
+
+	/**
+	 * Tests that an embed background reads the iframe src, decoded, from the
+	 * oEmbed HTML.
+	 *
+	 * @covers ::gutenberg_render_block_core_cover
+	 */
+	public function test_gutenberg_render_block_core_cover_embed_background_reads_decoded_iframe_src() {
+		$oembed_html = '<iframe data-src="https://example.com/placeholder" src="https://www.youtube.com/embed/abc123?feature=oembed&amp;si=xyz" title="Video"></iframe>';
+		$filter      = static function () use ( $oembed_html ) {
+			return $oembed_html;
+		};
+		add_filter( 'pre_oembed_result', $filter );
+
+		$attributes = array(
+			'backgroundType' => 'embed-video',
+			'url'            => 'https://www.youtube.com/watch?v=abc123',
+		);
+		$content    = '<div class="wp-block-cover"><figure class="wp-block-embed"><div class="wp-block-embed__wrapper">https://www.youtube.com/watch?v=abc123</div></figure></div>';
+		$rendered   = gutenberg_render_block_core_cover( $attributes, $content );
+
+		remove_filter( 'pre_oembed_result', $filter );
+
+		$processor = new WP_HTML_Tag_Processor( $rendered );
+		$this->assertTrue( $processor->next_tag( 'IFRAME' ), 'The rendered cover should contain an iframe.' );
+
+		$src = $processor->get_attribute( 'src' );
+		$this->assertSame( 'www.youtube.com', wp_parse_url( $src, PHP_URL_HOST ), 'The src should come from the src attribute, not data-src.' );
+
+		parse_str( wp_parse_url( $src, PHP_URL_QUERY ), $query_params );
+		$this->assertSame( 'oembed', $query_params['feature'] ?? null );
+		$this->assertSame( 'xyz', $query_params['si'] ?? null, 'Parameters after the first one should keep their names.' );
+		$this->assertArrayNotHasKey( 'amp;si', $query_params );
+	}
 }