[WinUI3]Fix Windows 10 title bar borders (#36429)

* Fix borders for windows in the Settings UI

* Fix HOSTS window

* Fix Advanced Paste

* Fix Environment Variables

* Fix File Locksmith

* Fix Peek, with a caveat

* Fix Registry Preview

* Remove unused imports

* Clean up imports in OobeShellPage

* Move OSVersionHelper from Common.UI up into ManagedCommon

src/common/ManagedCommon/NativeMethods.cs

@@ -42,6 +42,9 @@ namespace ManagedCommon
         [DllImport("user32.dll")]
         internal static extern uint SendInput(uint nInputs, INPUT[] pInputs, int cbSize);
 
+        [DllImport("dwmapi")]
+        internal static extern IntPtr DwmExtendFrameIntoClientArea(IntPtr hWnd, ref MARGINS pMarInset);
+
         [StructLayout(LayoutKind.Sequential)]
         public struct INPUT
         {
@@ -100,5 +103,14 @@ namespace ManagedCommon
             INPUT_KEYBOARD = 1,
             INPUT_HARDWARE = 2,
         }
+
+        [StructLayout(LayoutKind.Sequential)]
+        internal struct MARGINS
+        {
+            public int cxLeftWidth;
+            public int cxRightWidth;
+            public int cyTopHeight;
+            public int cyBottomHeight;
+        }
     }
 }

src/common/ManagedCommon/OSVersionHelper.cs

@@ -0,0 +1,26 @@
+// Copyright (c) Microsoft Corporation
+// The Microsoft Corporation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+
+namespace ManagedCommon
+{
+    public static class OSVersionHelper
+    {
+        public static bool IsWindows10()
+        {
+            return Environment.OSVersion.Version.Major >= 10 && Environment.OSVersion.Version.Minor < 22000;
+        }
+
+        public static bool IsWindows11()
+        {
+            return Environment.OSVersion.Version.Major >= 10 && Environment.OSVersion.Version.Build >= 22000;
+        }
+
+        public static bool IsGreaterThanWindows11_21H2()
+        {
+            return Environment.OSVersion.Version.Major >= 10 && Environment.OSVersion.Version.Build > 22000;
+        }
+    }
+}

src/common/ManagedCommon/WindowHelpers.cs

@@ -35,5 +35,20 @@ namespace ManagedCommon
                 }
             }
         }
+
+        /// <summary>
+        /// Workaround for a WinUI bug on Windows 10 in which a window's top border is always
+        /// black. Calls <c>DwmExtendFrameIntoClientArea()</c> with a <c>cyTopHeight</c> of 2 to force
+        /// the window's top border to be visible.<br/><br/>
+        /// Is a no-op on versions other than Windows 10.
+        /// </summary>
+        public static void ForceTopBorder1PixelInsetOnWindows10(IntPtr handle)
+        {
+            if (OSVersionHelper.IsWindows10())
+            {
+                var margins = new NativeMethods.MARGINS { cxLeftWidth = 0, cxRightWidth = 0, cyBottomHeight = 0, cyTopHeight = 2 };
+                NativeMethods.DwmExtendFrameIntoClientArea(handle, ref margins);
+            }
+        }
     }
 }