Move assign category ids to beforeToHtml instead inside load collection

app/code/Magento/Catalog/Block/Product/ListProduct.php

@@ -3,6 +3,7 @@
  * Copyright © Magento, Inc. All rights reserved.
  * See COPYING.txt for license details.
  */
+declare(strict_types=1);
 
 namespace Magento\Catalog\Block\Product;
 
@@ -141,14 +142,7 @@ public function getLayer()
      */
     public function getLoadedProductCollection()
     {
-        $collection = $this->_getProductCollection();
-
-        $categoryId = $this->getLayer()->getCurrentCategory()->getId();
-        foreach ($collection as $product) {
-            $product->setData('category_id', $categoryId);
-        }
-
-        return $collection;
+        return $this->_getProductCollection();
     }
 
     /**
@@ -205,6 +199,14 @@ protected function _beforeToHtml()
             $collection->load();
         }
 
+        $categoryId = $this->getLayer()->getCurrentCategory()->getId();
+
+        if ($categoryId) {
+            foreach ($collection as $product) {
+                $product->setData('category_id', $categoryId);
+            }
+        }
+
         return parent::_beforeToHtml();
     }
 

dev/tests/integration/testsuite/Magento/Catalog/Block/Product/ListTest.php

@@ -3,22 +3,32 @@
  * Copyright © Magento, Inc. All rights reserved.
  * See COPYING.txt for license details.
  */
+declare(strict_types=1);
+
 namespace Magento\Catalog\Block\Product;
 
+use PHPUnit\Framework\TestCase;
+use PHPUnit\Framework\MockObject\MockObject;
+
 /**
  * Test class for \Magento\Catalog\Block\Product\List.
  *
  * @magentoDataFixture Magento/Catalog/_files/product_simple.php
  * @magentoAppArea frontend
  * @magentoDbIsolation disabled
  */
-class ListTest extends \PHPUnit\Framework\TestCase
+class ListTest extends TestCase
 {
     /**
      * @var \Magento\Catalog\Block\Product\ListProduct
      */
     protected $_block;
 
+    /**
+     * @var \Magento\Catalog\Model\ResourceModel\Product\Collection|MockObject
+     */
+    private $collectionProductMock;
+
     protected function setUp(): void
     {
         \Magento\TestFramework\Helper\Bootstrap::getObjectManager()->get(\Magento\Framework\App\State::class)
@@ -28,6 +38,8 @@ protected function setUp(): void
         )->createBlock(
             \Magento\Catalog\Block\Product\ListProduct::class
         );
+
+        $this->collectionProductMock = $this->createMock(\Magento\Catalog\Model\ResourceModel\Product\Collection::class);
     }
 
     public function testGetLayer()
@@ -42,6 +54,7 @@ public function testGetLoadedProductCollection()
         $this->assertInstanceOf(\Magento\Catalog\Model\ResourceModel\Product\Collection::class, $collection);
         /* Check that root category was defined for Layer as current */
         $this->assertEquals(2, $this->_block->getLayer()->getCurrentCategory()->getId());
+        $this->collectionProductMock->expects($this->never())->method('load');
     }
 
     /**