Fix bug where other buttons would fail due to wrong adapter request.

Author: KlausLoeffelmann

src/System.Windows.Forms/System/Windows/Forms/Controls/Buttons/ButtonBase.cs

@@ -984,7 +984,6 @@ internal ButtonBaseAdapter Adapter
                             break;
                         case FlatStyle.Flat:
                             _adapter = CreateFlatAdapter();
-                            ;
                             break;
                         default:
                             Debug.Fail($"Unsupported FlatStyle: \"{FlatStyle}\"");
@@ -1020,8 +1019,10 @@ internal virtual ButtonBaseAdapter CreateStandardAdapter()
 
     internal virtual ButtonBaseAdapter CreateDarkModeAdapter()
     {
-        Debug.Fail("Derived classes need to provide a meaningful implementation.");
-        return null;
+        // When a button-derived class does not have a dedicated DarkMode adapter implementation,
+        // we're falling back to the standard adapter, to not _force_ the derived class to implement
+        // a dark mode adapter.
+        return CreateStandardAdapter();
     }
 
     internal virtual StringFormat CreateStringFormat()

src/System.Windows.Forms/System/Windows/Forms/Controls/Buttons/ButtonInternal/ButtonDarkModeAdapter.cs

@@ -8,12 +8,6 @@ namespace System.Windows.Forms.ButtonInternal;
 
 internal class ButtonDarkModeAdapter : ButtonBaseAdapter
 {
-    // Magic numbers for PushButtonState mapping
-    private const PushButtonState DisabledPushButtonState = PushButtonState.Disabled;
-    private const PushButtonState NormalPushButtonState = PushButtonState.Normal;
-    private const PushButtonState PressedPushButtonState = PushButtonState.Pressed;
-    private const PushButtonState HotPushButtonState = PushButtonState.Hot;
-
     internal ButtonDarkModeAdapter(ButtonBase control) : base(control) { }
 
     internal override void PaintUp(PaintEventArgs e, CheckState state)
@@ -55,7 +49,7 @@ internal override void PaintDown(PaintEventArgs e, CheckState state)
             e.Graphics,
             Control.ClientRectangle,
             Control.FlatStyle,
-            PressedPushButtonState,
+            PushButtonState.Pressed,
             Control.IsDefault,
             Control.Focused,
             Control.ShowFocusCues,
@@ -84,7 +78,7 @@ internal override void PaintOver(PaintEventArgs e, CheckState state)
             e.Graphics,
             Control.ClientRectangle,
             Control.FlatStyle,
-            HotPushButtonState,
+            PushButtonState.Hot,
             Control.IsDefault,
             Control.Focused,
             Control.ShowFocusCues,
@@ -122,13 +116,13 @@ private ColorOptions PaintDarkModeRender(IDeviceContext deviceContext) =>
     private static PushButtonState ToPushButtonState(CheckState state, bool enabled)
     {
         return !enabled
-            ? DisabledPushButtonState
+            ? PushButtonState.Disabled
             : state switch
             {
-                CheckState.Unchecked => NormalPushButtonState,
-                CheckState.Checked => PressedPushButtonState,
-                CheckState.Indeterminate => HotPushButtonState,
-                _ => NormalPushButtonState
+                CheckState.Unchecked => PushButtonState.Normal,
+                CheckState.Checked => PushButtonState.Pressed,
+                CheckState.Indeterminate => PushButtonState.Hot,
+                _ => PushButtonState.Normal
             };
     }
 }

src/System.Windows.Forms/System/Windows/Forms/Controls/Buttons/DarkModeRenderer/ButtonDarkModeRenderer.cs

@@ -41,7 +41,11 @@ internal static partial class ButtonDarkModeRenderer
     /// </summary>
     public static void DrawButtonBorder(Graphics graphics, GraphicsPath path, Color borderColor, int borderWidth)
     {
-        using var borderPen = borderColor.GetCachedPenScope(borderWidth);
+        using var borderPen = new Pen(borderColor, borderWidth)
+        {
+            Alignment = PenAlignment.Inset // Align pen inward
+        };
+
         graphics.DrawPath(borderPen, path);
     }