Add code coverage for TabRenderer (#13482)

* Add code coverage for TabRenderer

Author: Zheng-Li01

src/test/unit/System.Windows.Forms/System/Windows/Forms/TabRendererTests.cs

@@ -0,0 +1,140 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Drawing;
+using System.Windows.Forms.VisualStyles;
+
+namespace System.Windows.Forms.Tests;
+
+public class TabRendererTests : IDisposable
+{
+    private readonly Bitmap _bitmap;
+    private readonly Graphics _graphics;
+    private readonly Rectangle _bounds;
+
+    public TabRendererTests()
+    {
+        _bitmap = new(100, 30);
+        _graphics = Graphics.FromImage(_bitmap);
+        _bounds = new(0, 0, 100, 30);
+    }
+
+    public void Dispose()
+    {
+        _graphics.Dispose();
+        _bitmap.Dispose();
+    }
+
+    [Fact]
+    public void IsSupported_Matches_VisualStyleRenderer_IsSupported() =>
+        TabRenderer.IsSupported.Should().Be(VisualStyleRenderer.IsSupported);
+
+    [WinFormsFact]
+    public void DrawTabItem_Basic_ChangesTabStateSuccessfully()
+    {
+        Color colorBeforeDraw = _bitmap.GetPixel(10, 10);
+
+        foreach (TabItemState state in Enum.GetValues<TabItemState>())
+        {
+            TabRenderer.DrawTabItem(_graphics, _bounds, state);
+            TabRenderer.DrawTabItem(_graphics, _bounds, focused: true, state);
+        }
+
+        Color colorAfterDraw = _bitmap.GetPixel(10, 10);
+        colorAfterDraw.Should().NotBe(colorBeforeDraw);
+    }
+
+    [WinFormsFact]
+    public void DrawTabItem_WithText_ChangesTabStateSuccessfully()
+    {
+        using Font font = new("Arial", 8);
+        Color colorBeforeDraw = _bitmap.GetPixel(10, 10);
+
+        foreach (TabItemState state in Enum.GetValues<TabItemState>())
+        {
+            TabRenderer.DrawTabItem(_graphics, _bounds, "TabText", font, state);
+        }
+
+        Color colorAfterDraw = _bitmap.GetPixel(10, 10);
+        colorAfterDraw.Should().NotBe(colorBeforeDraw);
+    }
+
+    [WinFormsFact]
+    public void DrawTabItem_WithText_Focused_ChangesTabStateSuccessfully()
+    {
+        using Font font = new("Arial", 8);
+        Color colorBeforeDraw = _bitmap.GetPixel(10, 10);
+
+        foreach (TabItemState state in Enum.GetValues<TabItemState>())
+        {
+            TabRenderer.DrawTabItem(_graphics, _bounds, "TabText", font, focused: true, state);
+        }
+
+        Color colorAfterDraw = _bitmap.GetPixel(10, 10);
+        colorAfterDraw.Should().NotBe(colorBeforeDraw);
+    }
+
+    [WinFormsFact]
+    public void DrawTabItem_WithText_TextFormatFlags_Focused_ChangesTabStateSuccessfully()
+    {
+        using Font font = new("Arial", 8);
+        Color colorBeforeDraw = _bitmap.GetPixel(10, 10);
+
+        foreach (TabItemState state in Enum.GetValues<TabItemState>())
+        {
+            TabRenderer.DrawTabItem(_graphics, _bounds, "TabText", font, TextFormatFlags.Default, focused: true, state);
+        }
+
+        Color colorAfterDraw = _bitmap.GetPixel(10, 10);
+        colorAfterDraw.Should().NotBe(colorBeforeDraw);
+    }
+
+    [WinFormsFact]
+    public void DrawTabItem_WithImage_ChangesTabStateSuccessfully()
+    {
+        using Bitmap image = new(16, 16);
+        Rectangle imageRect = new(5, 5, 16, 16);
+        Color colorBeforeDraw = _bitmap.GetPixel(10, 10);
+
+        foreach (TabItemState state in Enum.GetValues<TabItemState>())
+        {
+            TabRenderer.DrawTabItem(_graphics, _bounds, image, imageRect, focused: false, state);
+            TabRenderer.DrawTabItem(_graphics, _bounds, image, imageRect, focused: true, state);
+        }
+
+        Color colorAfterDraw = _bitmap.GetPixel(10, 10);
+        colorAfterDraw.Should().NotBe(colorBeforeDraw);
+    }
+
+    [WinFormsFact]
+    public void DrawTabItem_WithTextAndImage_ChangesTabStateSuccessfully()
+    {
+        using Font font = new("Arial", 8);
+        using Bitmap image = new(16, 16);
+        Rectangle imageRect = new(5, 5, 16, 16);
+        Color colorBeforeDraw = _bitmap.GetPixel(10, 10);
+
+        foreach (TabItemState state in Enum.GetValues<TabItemState>())
+        {
+            TabRenderer.DrawTabItem(_graphics, _bounds, "TabText", font, image, imageRect, focused: false, state);
+            TabRenderer.DrawTabItem(_graphics, _bounds, "TabText", font, TextFormatFlags.Default, image, imageRect, focused: true, state);
+        }
+
+        Color colorAfterDraw = _bitmap.GetPixel(10, 10);
+        colorAfterDraw.Should().NotBe(colorBeforeDraw);
+    }
+
+    [WinFormsFact]
+    public void DrawTabPage_ChangesTabPageStateSuccessfully()
+    {
+        using Bitmap bmp = new(100, 100);
+        using Graphics g = Graphics.FromImage(bmp);
+        Rectangle bounds = new(0, 0, 100, 100);
+        Color colorBeforeDraw = bmp.GetPixel(10, 10);
+
+        TabRenderer.DrawTabPage(g, bounds);
+
+        Color colorAfterDraw = bmp.GetPixel(10, 10);
+        colorAfterDraw.Should().NotBe(colorBeforeDraw);
+    }
+}