Add Unit testing for duotone enhanced pagination. (#55542)

* Add unit testing to duotone CSS declarations

* Rename test to include gutenberg prefix

* Update tests to be like the ones in Core

phpunit/class-wp-duotone-test.php

@@ -45,6 +45,44 @@ public function test_gutenberg_render_duotone_support_custom() {
 		$this->assertMatchesRegularExpression( $expected, WP_Duotone_Gutenberg::render_duotone_support( $block_content, $block ) );
 	}
 
+
+	/**
+	 * 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.
+	 *
+	 * @covers ::render_duotone_support
+	 */
+	public function test_gutenberg_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( $block );
+
+		/*
+		 * Handling to access the static WP_Duotone::$block_css_declarations property.
+		 *
+		 * Why is an instance needed?
+		 * WP_Duotone is a static class by design, meaning it only contains static properties and methods.
+		 * In production, it should not be instantiated. However, as of PHP 8.3, ReflectionProperty::setValue()
+		 * needs an object.
+		 */
+		$wp_duotone                      = new WP_Duotone_Gutenberg();
+		$block_css_declarations_property = new ReflectionProperty( 'WP_Duotone_Gutenberg', 'block_css_declarations' );
+		$block_css_declarations_property->setAccessible( true );
+		$previous_value = $block_css_declarations_property->getValue();
+		$block_css_declarations_property->setValue( $wp_duotone, array() );
+		WP_Duotone_Gutenberg::render_duotone_support( '', $block, $wp_block );
+		$actual = $block_css_declarations_property->getValue();
+
+		// Reset the property.
+		$block_css_declarations_property->setValue( $wp_duotone, $previous_value );
+		$block_css_declarations_property->setAccessible( false );
+
+		$this->assertNotEmpty( $actual );
+	}
+
 	public function data_get_slug_from_attribute() {
 		return array(
 			'pipe-slug'                       => array( 'var:preset|duotone|blue-orange', 'blue-orange' ),