Fix productlink collection provider missing items

app/code/Magento/Catalog/Model/ProductLink/CollectionProvider.php

@@ -47,22 +47,26 @@ public function getCollection(\Magento\Catalog\Model\Product $product, $type)
 
         $products = $this->providers[$type]->getLinkedProducts($product);
         $converter = $this->converterPool->getConverter($type);
-        $output = [];
         $sorterItems = [];
         foreach ($products as $item) {
-            $output[$item->getId()] = $converter->convert($item);
+            $sorterItems[$item->getId()] = $converter->convert($item);
         }
 
-        foreach ($output as $item) {
-            $itemPosition = $item['position'];
-            if (!isset($sorterItems[$itemPosition])) {
-                $sorterItems[$itemPosition] = $item;
-            } else {
-                $newPosition = $itemPosition + 1;
-                $sorterItems[$newPosition] = $item;
-            }
-        }
-        ksort($sorterItems);
+        usort($sorterItems, [$this, 'comparePosition']);
+
         return $sorterItems;
     }
+
+    /**
+     * Compare item by key 'position'.
+     *
+     * @param array $itemA
+     * @param array $itemB
+     * @return int Return -1, 0 or 1 when $itemA['position'] is
+     * respectively less than, equal to, or greater than $itemB['position']
+     */
+    public function comparePosition(array $itemA, array $itemB): int
+    {
+        return (int)$itemA['position'] <=> (int)$itemB['position'];
+    }
 }

app/code/Magento/Catalog/Test/Unit/Model/CollectionProviderTest.php

@@ -57,10 +57,12 @@ public function testGetCollection()
         $linkedProductOneMock = $this->createMock(Product::class);
         $linkedProductTwoMock = $this->createMock(Product::class);
         $linkedProductThreeMock = $this->createMock(Product::class);
+        $linkedProductFourMock = $this->createMock(Product::class);
 
         $linkedProductOneMock->expects($this->once())->method('getId')->willReturn(1);
         $linkedProductTwoMock->expects($this->once())->method('getId')->willReturn(2);
         $linkedProductThreeMock->expects($this->once())->method('getId')->willReturn(3);
+        $linkedProductFourMock->expects($this->once())->method('getId')->willReturn(4);
 
         $this->converterPoolMock->expects($this->once())
             ->method('getConverter')
@@ -71,9 +73,10 @@ public function testGetCollection()
             [$linkedProductOneMock, ['name' => 'Product One', 'position' => 10]],
             [$linkedProductTwoMock, ['name' => 'Product Two', 'position' => 2]],
             [$linkedProductThreeMock, ['name' => 'Product Three', 'position' => 2]],
+            [$linkedProductFourMock, ['name' => 'Product Four', 'position' => 5]],
         ];
 
-        $this->converterMock->expects($this->exactly(3))->method('convert')->willReturnMap($map);
+        $this->converterMock->expects($this->exactly(4))->method('convert')->willReturnMap($map);
 
         $this->providerMock->expects($this->once())
             ->method('getLinkedProducts')
@@ -82,14 +85,16 @@ public function testGetCollection()
                 [
                     $linkedProductOneMock,
                     $linkedProductTwoMock,
-                    $linkedProductThreeMock
+                    $linkedProductThreeMock,
+                    $linkedProductFourMock,
                 ]
             );
 
         $expectedResult = [
-            2 => ['name' => 'Product Two', 'position' => 2],
-            3 => ['name' => 'Product Three', 'position' => 2],
-            10 => ['name' => 'Product One', 'position' => 10],
+            0 => ['name' => 'Product Three', 'position' => 2],
+            1 => ['name' => 'Product Two', 'position' => 2],
+            2 => ['name' => 'Product Four', 'position' => 5],
+            3 => ['name' => 'Product One', 'position' => 10],
         ];
 
         $actualResult = $this->model->getCollection($this->productMock, 'crosssell');