Backport/55415 make duotone support compatible with enhanced pagination

src/wp-includes/class-wp-duotone.php

@@ -1074,7 +1074,7 @@ private static function get_all_global_style_block_names() {
 	 * @return string Filtered block content.
 	 */
 	public static function render_duotone_support( $block_content, $block, $wp_block ) {
-		if ( empty( $block_content ) || ! $block['blockName'] ) {
+		if ( ! $block['blockName'] ) {
 			return $block_content;
 		}
 		$duotone_selector = self::get_selector( $wp_block->block_type );

tests/phpunit/tests/block-supports/duotone.php

@@ -83,6 +83,28 @@ public function test_get_slug_from_attribute( $data_attr, $expected ) {
 		$this->assertSame( $expected, $reflection->invoke( null, $data_attr ) );
 	}
 
+	/**
+	 * Tests whether the CSS declarations are generated even if the block content is
+	 * empty. This is needed to make the CSS output stable across paginations for
+	 * features like the enhanced pagination of the Query block.
+	 *
+	 * @ticket 59694
+	 *
+	 * @covers ::render_duotone_support
+	 */
+	public function test_css_declarations_are_generated_even_with_empty_block_content() {
+		$block                           = array(
+			'blockName' => 'core/image',
+			'attrs'     => array( 'style' => array( 'color' => array( 'duotone' => 'var:preset|duotone|blue-orange' ) ) ),
+		);
+		$wp_block                        = new WP_Block( $block );
+		$block_css_declarations_property = new ReflectionProperty( 'WP_Duotone', 'block_css_declarations' );
+		$block_css_declarations_property->setAccessible( true );
+		$block_css_declarations_property->setValue( array() );
+		WP_Duotone::render_duotone_support( '', $block, $wp_block );
+		$this->assertNotEmpty( $block_css_declarations_property->getValue() );
+	}
+
 	/**
 	 * Data provider.
 	 *