Update Anchor Layout to support high DPI machines.

src/System.Windows.Forms.Primitives/src/System/LocalAppContextSwitches/LocalAppContextSwitches.cs

@@ -10,15 +10,35 @@ namespace System.Windows.Forms.Primitives
     // Borrowed from https://github.com/dotnet/runtime/blob/main/src/libraries/Common/src/System/LocalAppContextSwitches.Common.cs
     internal static partial class LocalAppContextSwitches
     {
+        // Switch names declared internal below are used in unit/integration tests. Refer to
+        // https://github.com/microsoft/winforms/blob/tree/main/docs/design/anchor_layout_changes_in_net80.md
+        // for more details on how to enable these switches in the application.
         private const string ScaleTopLevelFormMinMaxSizeForDpiSwitchName = "System.Windows.Forms.ScaleTopLevelFormMinMaxSizeForDpi";
+        internal const string AnchorLayoutV2SwitchName = "System.Windows.Forms.AnchorLayoutV2";
 
         private static int s_scaleTopLevelFormMinMaxSizeForDpi;
+        private static int s_AnchorLayoutV2;
+
         public static bool ScaleTopLevelFormMinMaxSizeForDpi
         {
             [MethodImpl(MethodImplOptions.AggressiveInlining)]
             get => GetCachedSwitchValue(ScaleTopLevelFormMinMaxSizeForDpiSwitchName, ref s_scaleTopLevelFormMinMaxSizeForDpi);
         }
 
+        /// <summary>
+        ///  Indicates whether AnchorLayoutV2 feature is enabled.
+        /// </summary>
+        /// <devdoc>
+        ///  Returns AnchorLayoutV2 switch value from runtimeconfig.json. Defaults to true if application is targeting .NET 8.0 and beyond.
+        ///  Refer to
+        ///  https://github.com/microsoft/winforms/blob/tree/main/docs/design/anchor_layout_changes_in_net80.md for more details.
+        /// </devdoc>
+        public static bool AnchorLayoutV2
+        {
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+            get => GetCachedSwitchValue(AnchorLayoutV2SwitchName, ref s_AnchorLayoutV2);
+        }
+
         private static readonly FrameworkName? s_targetFrameworkName = GetTargetFrameworkName();
 
         private static readonly bool s_isNetCoreApp = (s_targetFrameworkName?.Identifier) == ".NETCoreApp";
@@ -33,9 +53,14 @@ private static bool GetCachedSwitchValue(string switchName, ref int cachedSwitch
         {
             // The cached switch value has 3 states: 0 - unknown, 1 - true, -1 - false
             if (cachedSwitchValue < 0)
+            {
                 return false;
+            }
+
             if (cachedSwitchValue > 0)
+            {
                 return true;
+            }
 
             return GetSwitchValue(switchName, ref cachedSwitchValue);
         }
@@ -64,6 +89,8 @@ static bool GetSwitchDefaultValue(string switchName)
                     return false;
                 }
 
+                // We are introducing switch defaults in .NET 8.0+ and support matrix for this product is
+                // limited to Windows 10 and above versions.
                 if (OsVersion.IsWindows10_1703OrGreater())
                 {
                     if (s_targetFrameworkName!.Version.CompareTo(new Version("8.0")) >= 0)
@@ -72,6 +99,11 @@ static bool GetSwitchDefaultValue(string switchName)
                         {
                             return true;
                         }
+
+                        if (switchName == AnchorLayoutV2SwitchName)
+                        {
+                            return true;
+                        }
                     }
                 }
 

src/System.Windows.Forms/src/System/Windows/Forms/Control.cs

@@ -15,6 +15,7 @@
 using System.Text;
 using System.Windows.Forms.Automation;
 using System.Windows.Forms.Layout;
+using System.Windows.Forms.Primitives;
 using Microsoft.Win32;
 using static Interop;
 using Encoding = System.Text.Encoding;
@@ -4686,6 +4687,7 @@ internal virtual void AssignParent(Control? value)
                 OnParentChanged(EventArgs.Empty);
             }
 
+            UpdateAnchorsIfRequired();
             SetState(States.CheckedHost, false);
             ParentInternal?.LayoutEngine.InitLayout(this, BoundsSpecified.All);
         }
@@ -7714,8 +7716,7 @@ internal virtual void OnParentBecameInvisible()
                 {
                     for (int i = 0; i < controlsCollection.Count; i++)
                     {
-                        Control ctl = controlsCollection[i];
-                        ctl.OnParentBecameInvisible();
+                        controlsCollection[i].OnParentBecameInvisible();
                     }
                 }
             }
@@ -10210,8 +10211,9 @@ but we break things at every step.
                 {
                     for (int i = 0; i < controlsCollection.Count; i++)
                     {
-                        LayoutEngine.InitLayout(controlsCollection[i], BoundsSpecified.All);
-                        CommonProperties.xClearPreferredSizeCache(controlsCollection[i]);
+                        Control control = controlsCollection[i];
+                        LayoutEngine.InitLayout(control, BoundsSpecified.All);
+                        CommonProperties.xClearPreferredSizeCache(control);
                     }
                 }
             }
@@ -10583,10 +10585,15 @@ protected virtual void ScaleControl(SizeF factor, BoundsSpecified specified)
             Size scaledSize = LayoutUtils.IntersectSizes(rawScaledBounds.Size, maximumSize);
             scaledSize = LayoutUtils.UnionSizes(scaledSize, minSize);
 
-            if (DpiHelper.IsScalingRequirementMet && (ParentInternal is not null) && (ParentInternal.LayoutEngine == DefaultLayout.Instance))
+            if (DpiHelper.IsScalingRequirementMet
+                // In the v2 layout, anchors are updated/computed after the controls bounds changed
+                // and, thus, don't need scaling.
+                && !DefaultLayout.UseAnchorLayoutV2(this)
+                && ParentInternal is { } parent
+                && (parent.LayoutEngine == DefaultLayout.Instance))
             {
                 // We need to scale AnchorInfo to update distances to container edges
-                DefaultLayout.ScaleAnchorInfo((IArrangedElement)this, factor);
+                DefaultLayout.ScaleAnchorInfo(this, factor);
             }
 
             // Set in the scaled bounds as constrained by the newly scaled min/max size.
@@ -10808,6 +10815,16 @@ public void SetBounds(int x, int y, int width, int height)
             }
         }
 
+        private void UpdateAnchorsIfRequired()
+        {
+            if (!LocalAppContextSwitches.AnchorLayoutV2)
+            {
+                return;
+            }
+
+            DefaultLayout.UpdateAnchorInfoV2(this);
+        }
+
         /// <summary>
         ///  Sets the bounds of the control.
         /// </summary>
@@ -10837,6 +10854,7 @@ public void SetBounds(int x, int y, int width, int height, BoundsSpecified speci
                 _height != height)
             {
                 SetBoundsCore(x, y, width, height, specified);
+                UpdateAnchorsIfRequired();
 
                 // WM_WINDOWPOSCHANGED will trickle down to an OnResize() which will
                 // have refreshed the interior layout or the resized control.  We only need to layout
@@ -12195,6 +12213,7 @@ private void WmCreate(ref Message m)
             _parent?.UpdateChildZOrder(this);
 
             UpdateBounds();
+            UpdateAnchorsIfRequired();
 
             // Let any interested sites know that we've now created a handle
             OnHandleCreated(EventArgs.Empty);