Add unit test for TextBoxBaseDesigner (#12433)

* Add unit test for TextBoxBaseDesigner

* Update Initialize function and remove blank line

* Remove white space, add line before assertion and check default textbox text

* update testcase

* Removed whitespace

src/System.Windows.Forms.Design/tests/UnitTests/System/Windows/Forms/Design/TextBoxBaseDesignerTests.cs

@@ -0,0 +1,93 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+#nullable enable
+
+using Moq;
+using System.ComponentModel.Design;
+using System.ComponentModel;
+using System.Windows.Forms.Design.Behavior;
+
+namespace System.Windows.Forms.Design.Tests;
+
+public class TextBoxBaseDesignerTests : IDisposable
+{
+    private readonly TextBoxBaseDesigner _designer;
+    private readonly TextBox _textbox;
+
+    public TextBoxBaseDesignerTests()
+    {
+        _designer = new();
+        _textbox = new();
+        Mock<ISite> site = new();
+        site.Setup(s => s.GetService(typeof(IDesignerHost))).Returns(new Mock<IDesignerHost>().Object);
+        _textbox.Site = site.Object;
+    }
+
+    public void Dispose()
+    {
+        _designer.Dispose();
+        _textbox.Dispose();
+    }
+
+    [Fact]
+    public void Constructor_SetsAutoResizeHandlesToTrue_AND_SelectionRules_ReturnsCorrectRules()
+    {
+        _designer.AutoResizeHandles.Should().BeTrue();
+        _designer.Initialize(_textbox);
+
+        SelectionRules rules = _designer.SelectionRules;
+
+        rules.Should().NotHaveFlag(SelectionRules.TopSizeable | SelectionRules.BottomSizeable);
+    }
+
+    [WinFormsTheory]
+    [InlineData(BorderStyle.None, 0)]
+    [InlineData(BorderStyle.FixedSingle, 2)]
+    [InlineData(BorderStyle.Fixed3D, 3)]
+    public void SnapLines_ReturnsCorrectSnapLines(BorderStyle borderStyle, int expectedBaselineOffset)
+    {
+        _textbox.BorderStyle = borderStyle;
+        _designer.Initialize(_textbox);
+
+        List<SnapLine> snapLines = (List<SnapLine>)_designer.SnapLines;
+
+        snapLines.Should().NotBeNull();
+
+        SnapLine? baselineSnapLine = snapLines.Cast<SnapLine>().FirstOrDefault(sl => sl.SnapLineType == SnapLineType.Baseline);
+
+        baselineSnapLine.Should().NotBeNull();
+        baselineSnapLine!.Priority.Should().Be(SnapLinePriority.Medium);
+        int expectedBaseline = DesignerUtils.GetTextBaseline(_textbox, Drawing.ContentAlignment.TopLeft) + expectedBaselineOffset;
+
+        baselineSnapLine.Offset.Should().Be(expectedBaseline);
+    }
+
+    [WinFormsTheory]
+    [InlineData(false, true, SelectionRules.LeftSizeable | SelectionRules.RightSizeable)]
+    [InlineData(true, true, SelectionRules.AllSizeable)]
+    [InlineData(false, false, SelectionRules.AllSizeable)]
+    public void SelectionRules_ReturnsCorrectRules(bool multiline, bool autoSize, SelectionRules expectedRules)
+    {
+        _textbox.Multiline = multiline;
+        _textbox.AutoSize = autoSize;
+        _designer.Initialize(_textbox);
+
+        _designer.SelectionRules.Should().HaveFlag(expectedRules);
+    }
+
+    [Fact]
+    public void InitializeNewComponent_ClearsTextProperty()
+    {
+        _textbox.Text.Should().BeEmpty();
+
+        _textbox.Text = "Test Text";
+        _designer.Initialize(_textbox);
+
+        _textbox.Text.Should().Be("Test Text");
+
+        _designer.InitializeNewComponent(null);
+
+        _textbox.Text.Should().BeEmpty();
+    }
+}