Remove lookup for external image full dimensions

If a user uses an external image, then we only know
one set of dimensions, and we can use those dimensions
from the image in the content for the lightbox.

This commit adds logic to handle that scenario.

lib/block-supports/behaviors.php

@@ -97,9 +97,8 @@ function gutenberg_render_behaviors_support_lightbox( $block_content, $block ) {
 		$img_uploaded_srcset = wp_get_attachment_image_srcset( $block['attrs']['id'] );
 	} else {
 		$img_uploaded_src    = $z->get_attribute( 'src' );
-		$img_dimensions      = wp_getimagesize( $img_uploaded_src );
-		$img_width           = $img_dimensions[0];
-		$img_height          = $img_dimensions[1];
+		$img_width           = 'none';
+		$img_height          = 'none';
 		$img_uploaded_srcset = '';
 	}
 

packages/block-library/src/image/view-interactivity.js

@@ -202,8 +202,18 @@ store( {
 } );
 
 function setZoomStyles( imgDom, context, event ) {
-	let targetWidth = context.core.image.targetWidth;
-	let targetHeight = context.core.image.targetHeight;
+	// Typically, we use the image's full-sized dimensions. If those
+	// dimensions have not been set (i.e. an external image with only one size),
+	// the image's dimensions in the lightbox are the same
+	// as those of the image in the content.
+	let targetWidth =
+		context.core.image.targetWidth !== 'none'
+			? context.core.image.targetWidth
+			: event.target.nextElementSibling.naturalWidth;
+	let targetHeight =
+		context.core.image.targetHeight !== 'none'
+			? context.core.image.targetHeight
+			: event.target.nextElementSibling.naturalHeight;
 
 	// Since the lightbox image has `position:absolute`, it
 	// ignores its parent's padding, so we need to set padding here