Fix variation selectors using selectors API

lib/class-wp-theme-json-gutenberg.php

@@ -2874,8 +2874,14 @@ public function get_styles_for_block( $block_metadata ) {
 
 				// Combine selectors with style variation's selector and add to overall style variation declarations.
 				foreach ( $variation_declarations as $current_selector => $new_declarations ) {
-					// If current selector includes block classname, remove it but leave the whitespace in.
-					$shortened_selector = str_replace( $block_metadata['selector'] . ' ', ' ', $current_selector );
+					/*
+					 * Clean up any whitespace between comma separated selectors.
+					 * This prevents these spaces breaking compound selectors such as:
+					 * - `.wp-block-list:not(.wp-block-list .wp-block-list)`
+					 * - `.wp-block-image img, .wp-block-image.my-class img`
+					 */
+					$clean_current_selector = preg_replace( '/,\s+/', ',', $current_selector );
+					$shortened_selector     = str_replace( $block_metadata['selector'], '', $clean_current_selector );
 
 					// Prepend the variation selector to the current selector.
 					$split_selectors    = explode( ',', $shortened_selector );