Remove Page Attributes from Specific Page.

class-wp-offline-page.php

<?php

class WP_Offline_Page {
	
	/**
	 * Initializes the instance.
	 */
	public function init() {
		add_action( 'current_screen', array( $this, 'remove_page_attributes' ) );
		// other code removed.
	}
	
	/**
	 * Remove the page attributes for the Offline Page.
	 *
	 * @return bool Returns true on success.
	 */
	public function remove_page_attributes() {
		// Bail out if this is not the right page.
		$screen = get_current_screen();
		if ( ! $screen instanceof WP_Screen || 'page' !== $screen->post_type ) {
			return false;
		}

		// When on the edit page, enqueue the inline edit script.
		if ( 'edit-page' === $screen->id ) {
			add_action( 'admin_enqueue_scripts', array( $this, 'enqueue_remove_page_attributes_script' ) );
			return true;
		}

		// When on the Offline Page's page UI, remove the page attributes support.
		if ( 'page' === $screen->id && isset( $_GET['post'] ) && (int) $_GET['post'] === $this->get_offline_page_id() ) { // WPCS: CSRF ok.
			remove_post_type_support( 'page', 'page-attributes' );
			return true;
		}

		return false;
	}

	/**
	 * Enqueue the script to remove the page attributes fields from the Quick Edit UI.
	 */
	public function enqueue_remove_page_attributes_script() {
		if ( $this->get_offline_page_id() < 1 ) {
			return;
		}

		wp_enqueue_script(
			'pwa-offline-page-inline-edit',
			plugins_url( '/offline-page/js/inline-edit-offline-page.js', __FILE__ ),
			array( 'jquery' ),
			PWA_VERSION,
			true
		);

		wp_localize_script( 'pwa-offline-page-inline-edit', 'offlinePageID', array(
			'id' => $this->offline_page_id,
		) );
	}
}

inline-edit-offline-page.js

/* global offlinePageID */

/**
 * Offline Page's inline edit handler to remove the page attribute fields from the quick edit UI.
 *
 * Note: This script uses MutationObserver, which is not supported by less modern browsers, such as
 * less than IE11.
 */
( function( $, window, document ) {
    'use strict';

    /**
     * Offline Page's Inline Edit Handler.
     *
     * @class
     */
    class InlineEditOfflinePage {

        /**
         * Creates the handler.
         *
         * @constructor
         * @param {Number} pageID
         */
        constructor(pageID) {
            this.pageID = pageID;
            this.editID = 'edit-' + this.pageID;
            this.listEl = document.getElementById('the-list');
            this.observer = new MutationObserver(this.mutationHandler.bind(this));
        }

        /**
         * Initializes the observer.
         *
         * @returns {boolean}
         */
        init() {
            this.observer.observe(this.listEl, {
                childList: true,
                subtree: true
            });
        }

        /**
         * Callback for the observer's mutations.  Iterates through the list, searching
         * for an added Offline Page node.  When found, it calls to remove its page attributes
         * fields from the Quick Edit UI.
         *
         * @param mutationsList
         */
        mutationHandler(mutationsList) {
            for (let mutation of mutationsList) {
                if (this.isOfflinePageNode(mutation)) {
                    this.removePageAttributes();
                    break;
                }
            }
        }

        /**
         * Checks if this mutation is an offline page node.
         *
         * @param {Object} mutation Mutation Record.
         * @returns {boolean}
         */
        isOfflinePageNode(mutation) {
            if (mutation.addedNodes.length === 0) {
                return false;
            }

            // Search to find the right node.  If found, return true.
            for (let nodeItem of mutation.addedNodes) {
                return  this.editID === nodeItem.id;
            }

            return false;
        }

        /**
         * Removes the page attributes from the quick edits.
         */
        removePageAttributes() {
            $('#post_parent').parent('label').remove();
            $('.inline-edit-menu-order-input').closest('label').remove();
        }
    }

    $( document ).ready( function() {
        // Bail out if something goes wrong.
        if ( ! 'MutationObserver' in window || typeof offlinePageID === 'undefined' ) {
            return;
        }

        // If this list does not include the offline page, there's nothing to do. Bail out.
        if ( null === $( '#post-' + offlinePageID.id ) ) {
            return;
        }

        const inlineEdit = new InlineEditOfflinePage(offlinePageID.id);
        inlineEdit.init();
    });

})( jQuery, window, document );

test.php

<?php

/**
 * Tests for class WP_Offline_Page.
 */
class Test_WP_Offline_Page extends WP_UnitTestCase {

	/**
	 * Tested instance.
	 *
	 * @var WP_Offline_Page
	 */
	public $instance;

	/**
	 * Setup.
	 *
	 * @inheritdoc
	 */
	public function setUp() {
		parent::setUp();
		$this->instance = new WP_Offline_Page();
	}

	/**
	 * Test init.
	 *
	 * @covers WP_Offline_Page::init()
	 */
	public function test_init() {
		$this->instance->init();
		$this->assertEquals( 10, has_action( 'current_screen', array( $this->instance, 'remove_page_attributes' ) ) );
	}

	/**
	 * Test remove_page_attributes.
	 *
	 * @covers WP_Offline_Page::remove_page_attributes()
	 */
	public function test_remove_page_attributes() {
		$this->assertArrayNotHasKey( 'post', $_GET ); // WPCS: CSRF ok.
		$this->assertFalse( $this->instance->remove_page_attributes() );

		set_current_screen( 'page' );
		$page_id      = $this->factory()->post->create( array( 'post_type' => 'page' ) );
		$_GET['post'] = $page_id; // WPCS: CSRF ok.

		// Check that false returns when the page is not the Offline Page.
		$this->assertFalse( $this->instance->remove_page_attributes() );
		add_option( WP_Offline_Page::OPTION_NAME, $page_id + 99 );
		$this->instance->get_offline_page_id( true );
		$this->assertFalse( $this->instance->remove_page_attributes() );

		// Check that the page attributes were removed for the Offline Page.
		update_option( WP_Offline_Page::OPTION_NAME, $page_id );
		$this->instance->get_offline_page_id( true );
		$this->assertArrayHasKey( 'page-attributes', get_all_post_type_supports( 'page' ) );
		$this->assertTrue( $this->instance->remove_page_attributes() );
		$this->assertArrayNotHasKey( 'page-attributes', get_all_post_type_supports( 'page' ) );
	}
}