Admin Bar: Prevent cache poisoning from device-specific mobile class

src/wp-includes/class-wp-admin-bar.php

@@ -463,9 +463,12 @@ final protected function _render( $root ) {
 		/*
 		 * Add browser classes.
 		 * We have to do this here since admin bar shows on the front end.
+		 * The 'mobile' class is only added for logged-in users to avoid cache poisoning:
+		 * logged-out pages are commonly served from a full-page cache, so a device-specific
+		 * class set by the first visitor would be served to all subsequent visitors.
 		 */
-		$class = 'nojq nojs';
-		if ( wp_is_mobile() ) {
+		$class = 'nojs';
+		if ( is_user_logged_in() && wp_is_mobile() ) {
 			$class .= ' mobile';
 		}
 

tests/phpunit/tests/adminbar.php

@@ -822,6 +822,56 @@ public function test_proto_property_is_not_defined() {
 		$this->assertFalse( isset( $admin_bar->proto ), 'WP_Admin_Bar::$proto should not be defined.' );
 	}
 
+	/**
+	 * Tests that the 'mobile' class is not added to the admin bar for logged-out users on mobile,
+	 * preventing cache poisoning when full-page caching is active.
+	 *
+	 * @ticket 28117
+	 * @covers WP_Admin_Bar::render
+	 */
+	public function test_admin_bar_does_not_add_mobile_class_for_logged_out_users() {
+		add_filter( 'wp_is_mobile', '__return_true' );
+
+		$admin_bar      = new WP_Admin_Bar();
+		$admin_bar_html = get_echo( array( $admin_bar, 'render' ) );
+
+		remove_filter( 'wp_is_mobile', '__return_true' );
+
+		$this->assertStringNotContainsString( 'class="nojs mobile"', $admin_bar_html, 'The "mobile" class should not be present for logged-out users.' );
+	}
+
+	/**
+	 * Tests that the 'mobile' class is added to the admin bar for logged-in users on mobile.
+	 *
+	 * @ticket 28117
+	 * @covers WP_Admin_Bar::render
+	 */
+	public function test_admin_bar_adds_mobile_class_for_logged_in_users_on_mobile() {
+		wp_set_current_user( self::$admin_id );
+		add_filter( 'wp_is_mobile', '__return_true' );
+
+		$admin_bar      = new WP_Admin_Bar();
+		$admin_bar_html = get_echo( array( $admin_bar, 'render' ) );
+
+		remove_filter( 'wp_is_mobile', '__return_true' );
+		wp_set_current_user( 0 );
+
+		$this->assertStringContainsString( 'class="nojs mobile"', $admin_bar_html, 'The "mobile" class should be present for logged-in users on mobile.' );
+	}
+
+	/**
+	 * Tests that the 'nojq' class is never present in the admin bar output.
+	 *
+	 * @ticket 28117
+	 * @covers WP_Admin_Bar::render
+	 */
+	public function test_admin_bar_does_not_output_nojq_class() {
+		$admin_bar      = new WP_Admin_Bar();
+		$admin_bar_html = get_echo( array( $admin_bar, 'render' ) );
+
+		$this->assertStringNotContainsString( 'nojq', $admin_bar_html, 'The "nojq" class should not be present in the admin bar output.' );
+	}
+
 	/**
 	 * This test ensures that WP_Admin_Bar::$menu is declared as a "regular" class property.
 	 *