From 628417ec8bea19534b62d9752e54bcd1e2bc3137 Mon Sep 17 00:00:00 2001
From: Sainath Poojary <spoojary614@gmail.com>
Date: Tue, 29 Jul 2025 15:24:04 +0530
Subject: [PATCH 1/2] Menus: Fix `nav_menu_item_args` filter side effects
 between menu items

---
 src/wp-includes/class-walker-nav-menu.php | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/src/wp-includes/class-walker-nav-menu.php b/src/wp-includes/class-walker-nav-menu.php
index 3d9f442bb8e87..bcb553fd2038b 100644
--- a/src/wp-includes/class-walker-nav-menu.php
+++ b/src/wp-includes/class-walker-nav-menu.php
@@ -157,7 +157,8 @@ public function start_el( &$output, $data_object, $depth = 0, $args = null, $cur
 		// Restores the more descriptive, specific name for use within this method.
 		$menu_item = $data_object;
 
-		if ( isset( $args->item_spacing ) && 'discard' === $args->item_spacing ) {
+		$args = (array) $args;
+		if ( isset( $args['item_spacing'] ) && 'discard' === $args['item_spacing'] ) {
 			$t = '';
 			$n = '';
 		} else {
@@ -178,7 +179,7 @@ public function start_el( &$output, $data_object, $depth = 0, $args = null, $cur
 		 * @param WP_Post  $menu_item Menu item data object.
 		 * @param int      $depth     Depth of menu item. Used for padding.
 		 */
-		$args = apply_filters( 'nav_menu_item_args', $args, $menu_item, $depth );
+		$args = apply_filters( 'nav_menu_item_args', (object) $args, $menu_item, $depth );
 
 		/**
 		 * Filters the CSS classes applied to a menu item's list item element.

From cae145a15938f5eb6d733dbae5a8152c1913b0a3 Mon Sep 17 00:00:00 2001
From: Sainath Poojary <spoojary614@gmail.com>
Date: Wed, 30 Jul 2025 17:59:00 +0530
Subject: [PATCH 2/2] Menus: Add tests to prevent nav_menu_item_args filter
 leakage between items

---
 tests/phpunit/tests/menu/walker-nav-menu.php | 68 ++++++++++++++++++++
 1 file changed, 68 insertions(+)

diff --git a/tests/phpunit/tests/menu/walker-nav-menu.php b/tests/phpunit/tests/menu/walker-nav-menu.php
index 3ff725cbb0228..660ffa37b9e24 100644
--- a/tests/phpunit/tests/menu/walker-nav-menu.php
+++ b/tests/phpunit/tests/menu/walker-nav-menu.php
@@ -435,4 +435,72 @@ public function data_build_atts_should_build_attributes() {
 			),
 		);
 	}
+
+	/**
+	 * Tests that modifications to `$args` in the `nav_menu_item_args` filter do not leak between menu items.
+	 *
+	 * @ticket 37344
+	 *
+	 * @covers Walker_Nav_Menu::start_el
+	 */
+	public function test_walker_nav_menu_start_el_should_not_leak_nav_menu_item_args_filter_changes_between_items() {
+		add_filter( 'nav_menu_item_args', array( $this, 'filter_nav_menu_item_args_append_marker' ), 10, 3 );
+
+		$item1 = array(
+			'ID'      => 1,
+			'title'   => 'Item 1',
+			'classes' => array(),
+			'current' => false,
+			'target'  => '',
+			'xfn'     => '',
+			'url'     => 'http://example.com/item1',
+		);
+
+		$item2 = array(
+			'ID'      => 2,
+			'title'   => 'Item 2',
+			'classes' => array(),
+			'current' => false,
+			'target'  => '',
+			'xfn'     => '',
+			'url'     => 'http://example.com/item2',
+		);
+
+		$args = array(
+			'before'      => '',
+			'after'       => '',
+			'link_before' => '',
+			'link_after'  => '',
+		);
+
+		$output1 = '';
+		$this->walker->start_el( $output1, (object) $item1, 0, (object) $args );
+
+		$output2 = '';
+		$this->walker->start_el( $output2, (object) $item2, 0, (object) $args );
+
+		remove_filter( 'nav_menu_item_args', array( $this, 'filter_nav_menu_item_args_append_marker' ), 10 );
+
+		// Both menu items should have exactly one 'X' marker, not accumulating.
+		$this->assertStringContainsString( '>X<a href="http://example.com/item1">Item 1</a>', $output1 );
+		$this->assertStringContainsString( '>X<a href="http://example.com/item2">Item 2</a>', $output2 );
+
+		// Ensure no accumulation - Item 2 should not contain 'XX'.
+		$this->assertStringNotContainsString( '>XX<a href="http://example.com/item2">Item 2</a>', $output2 );
+	}
+
+	/**
+	 * Filter callback for testing nav_menu_item_args filter side effects.
+	 *
+	 * Appends a marker character to the 'before' argument to test for accumulation.
+	 *
+	 * @param stdClass $args      An object of wp_nav_menu() arguments.
+	 * @param WP_Post  $menu_item Menu item data object.
+	 * @param int      $depth     Depth of menu item. Used for padding.
+	 * @return stdClass Modified arguments object.
+	 */
+	public function filter_nav_menu_item_args_append_marker( $args, $menu_item, $depth ) {
+		$args->before .= 'X';
+		return $args;
+	}
 }