Responsive images: do not generate `srcset` for GIFs that are inserted at full size. Prevents breaking animated GIFs.

Props joemcgill.
Fixes #34528.

git-svn-id: https://develop.svn.wordpress.org/trunk@35524 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/media.php

@@ -983,6 +983,13 @@ function wp_calculate_image_srcset( $image_src, $size_array, $image_meta, $attac
 		return false;
 	}
 
+	// Don't add srcset attributes to (animated) gifs that are inserted at full size.
+	if ( isset( $image_sizes['thumbnail']['mime-type'] ) && 'image/gif' === $image_sizes['thumbnail']['mime-type'] &&
+		false !== strpos( $image_src, $image_meta['file'] ) ) {
+
+		return false;
+	}
+
 	$image_basename = wp_basename( $image_meta['file'] );
 	$image_baseurl = _wp_upload_dir_baseurl();
 

tests/phpunit/tests/media.php

@@ -963,4 +963,43 @@ EOF;
 		// The content filter should return the image unchanged.
 		$this->assertSame( $image_html, wp_make_content_images_responsive( $image_html ) );
 	}
+
+	/**
+	 * @ticket 33641
+	 * @ticket 34528
+	 */
+	function test_wp_calculate_image_srcset_animated_gifs() {
+		// Mock meta for an animated gif.
+		$image_meta = array(
+			'width' => 1200,
+			'height' => 600,
+			'file' => 'animated.gif',
+			'sizes' => array(
+				'thumbnail' => array(
+					'file' => 'animated-150x150.gif',
+					'width' => 150,
+					'height' => 150,
+					'mime-type' => 'image/gif'
+				),
+				'medium' => array(
+					'file' => 'animated-300x150.gif',
+					'width' => 300,
+					'height' => 150,
+					'mime-type' => 'image/gif'
+				),
+				'large' => array(
+					'file' => 'animated-1024x512.gif',
+					'width' => 1024,
+					'height' => 512,
+					'mime-type' => 'image/gif'
+				),
+			)
+		);
+
+		$image_src = 'http://' . WP_TESTS_DOMAIN . '/wp-content/uploads/' . $image_meta['file'];
+		// Test with soft resized size array.
+		$size_array = array(900, 450);
+
+		$this->assertFalse( wp_calculate_image_srcset( $image_src, $size_array, $image_meta ) );
+	}
 }