Pass SendKeys state by ref (dotnet#6711)

Regression introduced in 90f7c1d.

Fixes dotnet#6666

Co-authored-by: Jeremy Kuhne <jkuhne@microsoft.com>

src/System.Windows.Forms/src/System/Windows/Forms/SendKeys.cs

@@ -170,7 +170,7 @@ private static bool AddSimpleKey(
                 }
 
                 AddMsgsForVK(vk & 0xff, repeat, haveKeys.HaveAlt > 0 && haveKeys.HaveCtrl == 0, hwnd);
-                CancelMods(haveKeys, UnknownGrouping, hwnd);
+                CancelMods(ref haveKeys, UnknownGrouping, hwnd);
             }
             else
             {
@@ -205,7 +205,7 @@ private static void AddMsgsForVK(int vk, int repeat, bool altnoctrldown, IntPtr
         ///  Called whenever there is a closing parenthesis, or the end of a character. This generates events for the
         ///  end of a modifier.
         /// </summary>
-        private static void CancelMods((int HaveShift, int HaveCtrl, int HaveAlt) haveKeys, int level, IntPtr hwnd)
+        private static void CancelMods(ref (int HaveShift, int HaveCtrl, int HaveAlt) haveKeys, int level, IntPtr hwnd)
         {
             if (haveKeys.HaveShift == level)
             {
@@ -519,7 +519,7 @@ private static void ParseKeys(string keys, IntPtr hwnd)
                             }
 
                             AddMsgsForVK(vk, repeat, haveKeys.HaveAlt > 0 && haveKeys.HaveCtrl == 0, hwnd);
-                            CancelMods(haveKeys, UnknownGrouping, hwnd);
+                            CancelMods(ref haveKeys, UnknownGrouping, hwnd);
                         }
                         else if (keyName.Length == 1)
                         {
@@ -604,7 +604,7 @@ private static void ParseKeys(string keys, IntPtr hwnd)
                             throw new ArgumentException(string.Format(SR.InvalidSendKeysString, keys));
                         }
 
-                        CancelMods(haveKeys, cGrp, hwnd);
+                        CancelMods(ref haveKeys, cGrp, hwnd);
                         cGrp--;
                         if (cGrp == 0)
                         {
@@ -632,7 +632,7 @@ private static void ParseKeys(string keys, IntPtr hwnd)
                 throw new ArgumentException(SR.SendKeysGroupDelimError);
             }
 
-            CancelMods(haveKeys, UnknownGrouping, hwnd);
+            CancelMods(ref haveKeys, UnknownGrouping, hwnd);
         }
 
         /// <summary>

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

@@ -0,0 +1,41 @@
+// 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 Xunit;
+
+namespace System.Windows.Forms.Tests
+{
+    public class SendKeysTests
+    {
+        [WinFormsFact(Skip = "This test depends on focus and should be run manually.")]
+        public void SendKeysGrouping()
+        {
+            // Regression https://github.com/dotnet/winforms/issues/6666
+
+            using var form = new CaptureForm();
+            form.Show();
+            form.Focus();
+            SendKeys.SendWait("^(a)^(c)");
+
+            Assert.Equal(4, form.KeyEvents.Count);
+            Assert.Equal(Keys.ControlKey, form.KeyEvents[0].KeyCode);
+            Assert.Equal(Keys.A, form.KeyEvents[1].KeyCode);
+            Assert.Equal(Keys.Control, form.KeyEvents[1].Modifiers);
+            Assert.Equal(Keys.ControlKey, form.KeyEvents[2].KeyCode);
+            Assert.Equal(Keys.C, form.KeyEvents[3].KeyCode);
+            Assert.Equal(Keys.Control, form.KeyEvents[3].Modifiers);
+        }
+
+        private class CaptureForm : Form
+        {
+            public List<KeyEventArgs> KeyEvents { get; } = new();
+
+            protected override void OnKeyDown(KeyEventArgs e)
+            {
+                KeyEvents.Add(e);
+                base.OnKeyDown(e);
+            }
+        }
+    }
+}