Add the body class the header's full-width rule keys on

Core removes #wpcontent's left padding on the two screens that use this
header, keyed on a body class. Without it the header sits 20px in from
the menu instead of spanning the screen — reported as "the margin on the
left not pulling the header across".

The class name comes from PageHeader::body_class() rather than being
written here, so the rule and the class cannot drift apart.

Author: arraypress

src/SettingFields.php

@@ -128,6 +128,7 @@ public function __construct( string $id, array $config ) {
 		add_action( 'admin_menu', [ $this, 'register_menu' ] );
 		add_action( 'admin_enqueue_scripts', [ $this, 'enqueue' ] );
 		add_filter( 'screen_settings', [ $this, 'render_screen_tools' ], 10, 2 );
+		add_filter( 'admin_body_class', [ $this, 'body_class' ] );
 		add_action( 'admin_post_' . $this->action_slug( 'export' ), [ $this, 'handle_export' ] );
 		add_action( 'admin_post_' . $this->action_slug( 'import' ), [ $this, 'handle_import' ] );
 		add_action( 'admin_post_' . $this->action_slug( 'reset' ), [ $this, 'handle_reset' ] );
@@ -243,6 +244,26 @@ public function register_menu(): void {
 		}
 	}
 
+	/**
+	 * Mark the screen as one using the kit's page header.
+	 *
+	 * The header only spans the screen if `#wpcontent`'s left padding is
+	 * removed, and core does that with a body class rather than on the header
+	 * itself. The class name comes from the kit so the rule and the class
+	 * cannot drift apart.
+	 *
+	 * @param string $classes Existing body classes.
+	 *
+	 * @return string
+	 */
+	public function body_class( string $classes ): string {
+		if ( '' === $this->hook_suffix || get_current_screen()?->id !== $this->hook_suffix ) {
+			return $classes;
+		}
+
+		return trim( $classes . ' ' . PageHeader::body_class() );
+	}
+
 	/**
 	 * Register the help tabs.
 	 *

tests/SettingFieldsTest.php

@@ -698,6 +698,29 @@ public function test_the_header_renders_outside_the_wrap(): void {
 		$this->assertLessThan( $wrap, strpos( $html, 'wp-header-end' ) );
 	}
 
+	/**
+	 * The screen carries the body class the header's rule keys on.
+	 *
+	 * Without it #wpcontent keeps its 20px left padding and the header sits
+	 * inset from the menu instead of spanning the screen.
+	 */
+	public function test_the_screen_carries_the_header_body_class(): void {
+		$page = $this->page();
+		$page->register_menu();
+
+		$GLOBALS['sf_screen'] = new \SF_Screen( $page->get_hook_suffix() );
+
+		$this->assertStringContainsString(
+			\ArrayPress\FieldKit\Support\PageHeader::body_class(),
+			$page->body_class( 'existing-class' )
+		);
+
+		// And another screen is left exactly as it was.
+		$GLOBALS['sf_screen'] = new \SF_Screen( 'edit-post' );
+
+		$this->assertSame( 'existing-class', $page->body_class( 'existing-class' ) );
+	}
+
 	/**
 	 * The tools are not in the header.
 	 *