Fixing ShowInTaskbar=false doesn't work in some cases (#6421) (#6989)

Fixed #6421 - do not exit the form when changing ShowInTaskbar property and re-creating handle
* Add unit test
* Add manual test to Form.ShowInTaskbar

src/System.Windows.Forms/src/System/Windows/Forms/Form.cs

@@ -6361,7 +6361,10 @@ private void WmNCDestroy(ref Message m)
 
             if (Modal && _dialogResult == DialogResult.None)
             {
-                DialogResult = DialogResult.Cancel;
+                if (GetState(States.Recreate) == false)
+                {
+                    DialogResult = DialogResult.Cancel;
+                }
             }
         }
 

src/System.Windows.Forms/tests/IntegrationTests/System.Windows.Forms.IntegrationTests.Common/MainFormControlsTabOrder.cs

@@ -28,6 +28,7 @@ public enum MainFormControlsTabOrder
         RichTextBoxesButton,
         PictureBoxesButton,
         FormBorderStylesButton,
+        FormShowInTaskbarButton,
         ToggleIconButton,
         ErrorProviderButton,
         TaskDialogButton,

src/System.Windows.Forms/tests/IntegrationTests/WinformsControlsTest/FormShowInTaskbar.cs

@@ -0,0 +1,63 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.Drawing;
+using System.Windows.Forms;
+
+namespace WinformsControlsTest
+{
+    public class FormShowInTaskbar : Form
+    {
+        public FormShowInTaskbar()
+        {
+            Width = 600;
+            Height = 460;
+            StartPosition = FormStartPosition.CenterScreen;
+
+            var btnTest = new Button()
+            {
+                Text = "Click here to open new Form",
+                Location = new Point(10, 10),
+                Height = 60,
+                AutoSize = true,
+            };
+
+            btnTest.Click += BtnTest_Click;
+            Controls.Add(btnTest);
+        }
+
+        private void BtnTest_Click(object sender, EventArgs e)
+        {
+            using var form = new Form()
+            {
+                Width = 680,
+                Height = 400,
+                StartPosition = FormStartPosition.CenterScreen,
+            };
+
+            var btnTest = new Button()
+            {
+                Text = $"Click here to test ShowInTaskbar.{Environment.NewLine}If the test result is failed, this dialog will automatically close, or it will throw an exception.",
+                Location = new Point(10, 10),
+                Height = 60,
+                AutoSize = true,
+            };
+
+            btnTest.Click += (object sender, EventArgs e) =>
+            {
+                IntPtr formHandle = form.Handle;
+                form.ShowInTaskbar = !form.ShowInTaskbar;
+
+                if (form.IsHandleCreated == false)
+                    throw new Exception();
+
+                if (formHandle == form.Handle)
+                    throw new Exception();
+            };
+
+            form.Controls.Add(btnTest);
+            form.ShowDialog(this);
+        }
+    }
+}

src/System.Windows.Forms/tests/IntegrationTests/WinformsControlsTest/MainForm.cs

@@ -143,6 +143,10 @@ public MainForm()
                 MainFormControlsTabOrder.FormBorderStylesButton,
                 new InitInfo("FormBorderStyles", (obj, e) => new FormBorderStyles().Show(this))
             },
+            {
+                MainFormControlsTabOrder.FormShowInTaskbarButton,
+                new InitInfo("FormShowInTaskbar", (obj, e) => new FormShowInTaskbar().Show(this))
+            },
             {
                 MainFormControlsTabOrder.ToggleIconButton,
                 new InitInfo("ToggleFormIcon", (obj, e) => ShowIcon = !ShowIcon)

src/System.Windows.Forms/tests/UnitTests/System/Windows/Forms/FormTests.cs

@@ -1820,6 +1820,36 @@ public void Form_TransparencyKey_SetNotTopLevelWithHandle_GetReturnsExpected(boo
             Assert.Equal(0, createdCallCount);
         }
 
+        [WinFormsFact]
+        public void Form_ShowInTaskbar_SetFalse_GetReturnsExpected()
+        {
+            // Regression test for https://github.com/dotnet/winforms/issues/6421
+
+            using var form = new Form
+            {
+                ShowInTaskbar = true,
+            };
+
+            DialogResult expectedDialogResult = DialogResult.OK;
+
+            form.Load += (object sender, EventArgs e) =>
+            {
+                IntPtr formHandle = form.Handle;
+                form.ShowInTaskbar = false;
+
+                Assert.True(form.IsHandleCreated);
+                Assert.NotEqual(formHandle, form.Handle);
+            };
+
+            form.Shown += (object sender, EventArgs e) =>
+            {
+                form.DialogResult = expectedDialogResult;
+            };
+
+            Assert.Equal(expectedDialogResult, form.ShowDialog());
+            Assert.Equal(expectedDialogResult, form.DialogResult);
+        }
+
         public static IEnumerable<object[]> Visible_Set_TestData()
         {
             foreach (DialogResult dialogResult in Enum.GetValues(typeof(DialogResult)))