diff --git a/.github/workflows/mutation.yml b/.github/workflows/mutation.yml
new file mode 100644
index 0000000..c95057f
--- /dev/null
+++ b/.github/workflows/mutation.yml
@@ -0,0 +1,27 @@
+on:
+  pull_request:
+    paths-ignore:
+      - 'docs/**'
+      - 'README.md'
+      - 'CHANGELOG.md'
+      - '.gitignore'
+      - '.gitattributes'
+
+  push:
+    paths-ignore:
+      - 'docs/**'
+      - 'README.md'
+      - 'CHANGELOG.md'
+      - '.gitignore'
+      - '.gitattributes'
+
+name: mutation test
+
+jobs:
+  mutation:
+    uses: php-forge/actions/.github/workflows/infection.yml@main
+    with:
+      development: true
+      phpstan: true
+    secrets:
+      STRYKER_DASHBOARD_API_KEY: ${{ secrets.STRYKER_DASHBOARD_API_KEY }}
diff --git a/composer.json b/composer.json
index 826163d..03dfc90 100644
--- a/composer.json
+++ b/composer.json
@@ -41,6 +41,7 @@
     "config": {
         "sort-packages": true,
         "allow-plugins": {
+            "infection/extension-installer": true,
             "phpstan/extension-installer": true,
             "yiisoft/yii2-composer": true
         }
@@ -48,6 +49,7 @@
     "scripts": {
         "check-dependencies": "./vendor/bin/composer-require-checker check",
         "ecs": "./vendor/bin/ecs --fix",
+        "mutation": "./vendor/bin/infection --threads=4 --ignore-msi-with-no-mutations --only-covered --min-msi=100 --min-covered-msi=100",
         "rector": "./vendor/bin/rector process src",
         "static": "./vendor/bin/phpstan --memory-limit=512M",
         "tests": "./vendor/bin/phpunit"
diff --git a/infection.json.dist b/infection.json5
similarity index 62%
rename from infection.json.dist
rename to infection.json5
index d06b334..15361d9 100644
--- a/infection.json.dist
+++ b/infection.json5
@@ -1,16 +1,17 @@
 {
-    "source": {
-        "directories": [
-            "src"
-        ]
-    },
+    "$schema": "vendor/infection/infection/resources/schema.json",
     "logs": {
         "text": "php:\/\/stderr",
         "stryker": {
             "report": "main"
         }
     },
-    "mutators": {
-        "@default": true
-    }
+    "source": {
+        "directories": [
+            "src"
+        ]
+    },
+    "phpStan": {
+        "configDir": "."
+    },
 }
diff --git a/phpunit.xml.dist b/phpunit.xml.dist
index 3ddadb0..2215bca 100644
--- a/phpunit.xml.dist
+++ b/phpunit.xml.dist
@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <phpunit
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
-    xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/10.2/phpunit.xsd"
+    xsi:noNamespaceSchemaLocation="vendor/phpunit/phpunit/phpunit.xsd"
     bootstrap="tests/bootstrap.php"
     cacheDirectory="runtime/.phpunit.cache"
     colors="true"
diff --git a/src/NestedSetsBehavior.php b/src/NestedSetsBehavior.php
index 038bcf6..67756e7 100644
--- a/src/NestedSetsBehavior.php
+++ b/src/NestedSetsBehavior.php
@@ -1054,14 +1054,19 @@ protected function beforeInsertNode(int|null $value, int $depth): void
             $this->getOwner()->setAttribute($this->rightAttribute, $value + 1);
         }
 
-        $nodeDepthValue = $this->node?->getAttribute($this->depthAttribute) ?? 0;
-        $this->getOwner()->setAttribute($this->depthAttribute, $nodeDepthValue + $depth);
+        $nodeDepthValue = $this->node?->getAttribute($this->depthAttribute);
 
-        if ($this->treeAttribute !== false) {
-            $this->getOwner()->setAttribute($this->treeAttribute, $this->node?->getAttribute($this->treeAttribute));
+        if ($nodeDepthValue !== null) {
+            $this->getOwner()->setAttribute($this->depthAttribute, $nodeDepthValue + $depth);
+        }
+
+        if ($this->treeAttribute !== false && $this->node !== null) {
+            $this->getOwner()->setAttribute($this->treeAttribute, $this->node->getAttribute($this->treeAttribute));
         }
 
-        $this->shiftLeftRightAttribute($value ?? 0, 2);
+        if ($value !== null) {
+            $this->shiftLeftRightAttribute($value, 2);
+        }
     }
 
     /**
@@ -1225,7 +1230,7 @@ protected function moveNode(ActiveRecord $node, int $value, int $depth): void
                 );
             }
 
-            $this->shiftLeftRightAttribute($rightValue + 1, -$delta);
+            $this->shiftLeftRightAttribute($rightValue, -$delta);
         } else {
             $leftAttribute = $db->quoteColumnName($this->leftAttribute);
             $rightAttribute = $db->quoteColumnName($this->rightAttribute);
diff --git a/tests/NestedSetsBehaviorTest.php b/tests/NestedSetsBehaviorTest.php
index d311816..8c469bf 100644
--- a/tests/NestedSetsBehaviorTest.php
+++ b/tests/NestedSetsBehaviorTest.php
@@ -678,7 +678,7 @@ public function testThrowExceptionWhenAppendToTargetIsSame(): void
         $this->expectException(Exception::class);
         $this->expectExceptionMessage('Can not move a node when the target node is same.');
 
-        $node->appendTo($childOfNode);
+        $node->appendTo($childOfNode, false);
     }
 
     public function testThrowExceptionWhenAppendToTargetIsChild(): void
@@ -1073,7 +1073,7 @@ public function testThrowExceptionWhenInsertAfterTargetIsNewRecord(): void
         $this->expectException(Exception::class);
         $this->expectExceptionMessage('Can not move a node when the target node is new record.');
 
-        $node->insertAfter(new Tree());
+        $node->insertAfter(new Tree(), false);
     }
 
     public function testThrowExceptionWhenInsertAfterTargetIsSame(): void
@@ -1620,4 +1620,152 @@ public function makeRoot(): bool
 
         $node->makeRoot();
     }
+
+    public function testAppendNodeToExistingChildNode(): void
+    {
+        $this->generateFixtureTree();
+
+        $node = new Tree(['name' => 'New node']);
+
+        $childOfNode = Tree::findOne(2);
+
+        self::assertNotNull(
+            $childOfNode,
+            'Child node with ID \'2\' should exist before appending.',
+        );
+
+        self::assertTrue(
+            $node->appendTo($childOfNode),
+            'Appending a new node to the existing child node should return \'true\'.',
+        );
+    }
+
+    public function testAppendToSetsCorrectDepthForNewNode(): void
+    {
+        $this->createDatabase();
+
+        $root = new Tree(['name' => 'Root']);
+
+        $root->makeRoot();
+
+        $parent = new Tree(['name' => 'Parent']);
+
+        $parent->appendTo($root);
+
+        $parent->refresh();
+
+        self::assertEquals(1, $parent->depth);
+
+        $child = new Tree(['name' => 'Child']);
+
+        $child->appendTo($parent);
+
+        $child->refresh();
+
+        self::assertEquals(2, $child->depth);
+
+        $grandchild = new Tree(['name' => 'Grandchild']);
+
+        $grandchild->appendTo($child);
+        $grandchild->refresh();
+
+        self::assertEquals(3, $grandchild->depth);
+    }
+
+    public function testAppendToAndPrependToHaveSameDepthBehavior(): void
+    {
+        $this->createDatabase();
+
+        $root = new Tree(['name' => 'Root']);
+
+        $root->makeRoot();
+
+        $parent = new Tree(['name' => 'Parent']);
+
+        $parent->appendTo($root);
+
+        $child1 = new Tree(['name' => 'Child1']);
+
+        $child1->prependTo($parent);
+
+        $child2 = new Tree(['name' => 'Child2']);
+
+        $child2->appendTo($parent);
+
+        $child1->refresh();
+
+        self::assertEquals($child1->depth, $child2->depth);
+
+        $child2->refresh();
+
+        self::assertEquals(2, $child1->depth);
+    }
+
+    public function testAppendToSetsCorrectDepthForMultipleTree(): void
+    {
+        $this->createDatabase();
+
+        $root = new MultipleTree(['name' => 'Root']);
+
+        $root->makeRoot();
+
+        $level1 = new MultipleTree(['name' => 'Level1']);
+
+        $level1->appendTo($root);
+
+        $level2 = new MultipleTree(['name' => 'Level2']);
+
+        $level2->appendTo($level1);
+
+        $level3 = new MultipleTree(['name' => 'Level3']);
+
+        $level3->appendTo($level2);
+
+        $level1->refresh();
+
+        self::assertEquals(1, $level1->depth);
+
+        $level2->refresh();
+
+        self::assertEquals(2, $level2->depth);
+
+        $level3->refresh();
+
+        self::assertEquals(3, $level3->depth);
+    }
+
+    public function testDepthDiffersBetweenSiblingsAndChildren(): void
+    {
+        $this->createDatabase();
+
+        $root = new Tree(['name' => 'Root']);
+
+        $root->makeRoot();
+
+        $parent = new Tree(['name' => 'Parent']);
+
+        $parent->appendTo($root);
+
+        $sibling = new Tree(['name' => 'Sibling']);
+
+        $sibling->insertAfter($parent);
+
+        $child = new Tree(['name' => 'Child']);
+
+        $child->appendTo($parent);
+
+        $parent->refresh();
+
+        self::assertEquals($parent->depth, $sibling->depth);
+        self::assertEquals($parent->depth + 1, $child->depth);
+        self::assertEquals(1, $parent->depth);
+
+        $sibling->refresh();
+
+        self::assertEquals(1, $sibling->depth);
+
+        $child->refresh();
+
+        self::assertEquals(2, $child->depth);
+    }
 }