Add unit test

Author: Epica3055

src/System.Windows.Forms.Design/tests/UnitTests/System/Windows/Forms/Design/Behavior/SnapLineTests.cs

@@ -236,4 +236,59 @@ public void SnapLine_ToString(string filter, string expected)
 
         Assert.Equal(expected, snapLine.ToString());
     }
+
+    // Unit test for https://github.com/dotnet/winforms/issues/13305
+    [Fact]
+    public void SnapLine_ReturnOverrided()
+    {
+        using ControlDesigner baseDesigner = new();
+        using Control control = new();
+        baseDesigner.Initialize(control);
+        baseDesigner.SnapLines.Count.Should().Be(8);
+
+        ControlDesigner derivedDesigner = new ButtonBaseDesigner();
+        using Button button = new();
+        derivedDesigner.Initialize(button);
+        derivedDesigner.SnapLines.Count.Should().Be(9);
+
+        derivedDesigner = new ComboBoxDesigner();
+        using ComboBox comboBox = new();
+        derivedDesigner.Initialize(comboBox);
+        derivedDesigner.SnapLines.Count.Should().Be(9);
+
+        derivedDesigner = new DateTimePickerDesigner();
+        using DateTimePicker dateTimePicker = new();
+        derivedDesigner.Initialize(dateTimePicker);
+        derivedDesigner.SnapLines.Count.Should().Be(9);
+
+        derivedDesigner = new LabelDesigner();
+        using Label label = new();
+        derivedDesigner.Initialize(label);
+        derivedDesigner.SnapLines.Count.Should().Be(9);
+
+        derivedDesigner = new ParentControlDesigner();
+        derivedDesigner.Initialize(control);
+        derivedDesigner.SnapLines.Count.Should().Be(12);
+
+        derivedDesigner = new SplitterPanelDesigner();
+        using SplitContainer splitContainer = new();
+        using SplitterPanel splitterPanel = new(splitContainer);
+        derivedDesigner.Initialize(splitterPanel);
+        derivedDesigner.SnapLines.Count.Should().Be(4);
+
+        derivedDesigner = new TextBoxBaseDesigner();
+        using TextBox textBox = new();
+        derivedDesigner.Initialize(textBox);
+        derivedDesigner.SnapLines.Count.Should().Be(9);
+
+        derivedDesigner = new ToolStripContentPanelDesigner();
+        using ToolStripContentPanel toolStripContentPanel = new();
+        derivedDesigner.Initialize(toolStripContentPanel);
+        derivedDesigner.SnapLines.Count.Should().Be(4);
+
+        derivedDesigner = new UpDownBaseDesigner();
+        using DomainUpDown domainUpDown = new();
+        derivedDesigner.Initialize(domainUpDown);
+        derivedDesigner.SnapLines.Count.Should().Be(9);
+    }
 }