Fixing the incorrect ScreenReader blue rectangle for a new character when typing (Text pattern area)

src/System.Windows.Forms/src/System/Windows/Forms/RichTextBox.cs

@@ -578,6 +578,11 @@ private bool ProtectedError
             set { richTextBoxFlags[protectedErrorSection] = value ? 1 : 0; }
         }
 
+        private protected override void RaiseAccessibilityTextChangedEvent()
+        {
+            // Do not do anything because Win32 provides unmanaged Text pattern for RichTextBox
+        }
+
         /// <summary>
         ///  Returns the name of the action that will be performed if the user
         ///  Redo's their last Undone operation. If no operation can be redone,

src/System.Windows.Forms/src/System/Windows/Forms/TextBoxBase.cs

@@ -1,4 +1,4 @@
-// Licensed to the .NET Foundation under one or more agreements.
+// 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.
 
@@ -1608,8 +1608,16 @@ protected override void OnTextChanged(EventArgs e)
             // the text changes.
             CommonProperties.xClearPreferredSizeCache(this);
             base.OnTextChanged(e);
+
+            if (UiaCore.UiaClientsAreListening().IsTrue())
+            {
+                RaiseAccessibilityTextChangedEvent();
+            }
         }
 
+        private protected virtual void RaiseAccessibilityTextChangedEvent()
+            => AccessibilityObject.RaiseAutomationEvent(UiaCore.UIA.Text_TextChangedEventId);
+
         /// <summary>
         ///  Returns the character nearest to the given point.
         /// </summary>