Put DrawButtonBorder after PaintImage and removing the rounded border in Flat mode

Author: Leaf Shi

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

@@ -549,15 +549,13 @@ internal void PaintImage(PaintEventArgs e, LayoutData layout)
     {
         if (Application.IsDarkModeEnabled && Control.DarkModeRequestState is true && Control.BackgroundImage is not null)
         {
-            Rectangle bounds = Control.ClientRectangle;
-            bounds.Inflate(-ButtonBorderSize, -ButtonBorderSize);
             ControlPaint.DrawBackgroundImage(
                 e.GraphicsInternal,
                 Control.BackgroundImage,
                 Color.Transparent,
                 Control.BackgroundImageLayout,
                 Control.ClientRectangle,
-                bounds,
+                Control.ClientRectangle,
                 Control.DisplayRectangle.Location,
                 Control.RightToLeft);
         }

src/System.Windows.Forms/System/Windows/Forms/Controls/Buttons/ButtonInternal/DarkMode/ButtonDarkModeRendererBase.cs

@@ -66,6 +66,8 @@ public void RenderButton(
             // Paint image and field using the provided delegates
             paintImage(contentBounds);
 
+            DrawButtonBorder(graphics, bounds, state, isDefault);
+
             paintField();
 
             if (focused && showFocusCues)
@@ -76,6 +78,8 @@ public void RenderButton(
         }
     }
 
+    public abstract void DrawButtonBorder(Graphics graphics, Rectangle bounds, PushButtonState state, bool isDefault);
+
     public abstract Rectangle DrawButtonBackground(Graphics graphics, Rectangle bounds, PushButtonState state, bool isDefault, Color backColor);
 
     public abstract void DrawFocusIndicator(Graphics graphics, Rectangle contentBounds, bool isDefault);

src/System.Windows.Forms/System/Windows/Forms/Controls/Buttons/ButtonInternal/DarkMode/FlatButtonDarkModeRenderer.cs

@@ -17,8 +17,6 @@ namespace System.Windows.Forms;
 internal sealed class FlatButtonDarkModeRenderer : ButtonDarkModeRendererBase
 {
     private const int FocusIndicatorInflate = -3;
-    private const int CornerRadius = 6;
-    private static readonly Size s_corner = new(CornerRadius, CornerRadius);
 
     private protected override Padding PaddingCore { get; } = new(0);
 
@@ -28,10 +26,6 @@ public override Rectangle DrawButtonBackground(
         // fill background
         using var back = backColor.GetCachedSolidBrushScope();
         graphics.FillRectangle(back, bounds);
-
-        // draw border identical to Win32
-        DrawButtonBorder(graphics, bounds, state, isDefault);
-
         // return inner content area (border + 1 px system padding)
         return Rectangle.Inflate(bounds, -3, -3);
     }
@@ -77,7 +71,7 @@ public override Color GetBackgroundColor(PushButtonState state, bool isDefault)
                 _ => DefaultColors.StandardBackColor
             };
 
-    private static void DrawButtonBorder(Graphics g, Rectangle bounds, PushButtonState state, bool isDefault)
+    public override void DrawButtonBorder(Graphics g, Rectangle bounds, PushButtonState state, bool isDefault)
     {
         g.SmoothingMode = SmoothingMode.AntiAlias;
 
@@ -98,12 +92,9 @@ private static void DrawSingleBorder(Graphics g, Rectangle rect, Color color)
     {
         g.SmoothingMode = SmoothingMode.AntiAlias;
 
-        using var path = new GraphicsPath();
-        path.AddRoundedRectangle(rect, s_corner);
-
         // a 1‑px stroke, aligned *inside*, is exactly what Win32 draws
         using var pen = new Pen(color) { Alignment = PenAlignment.Inset };
-        g.DrawPath(pen, path);
+        g.DrawRectangle(pen, rect);
     }
 
     private static Color GetBorderColor(PushButtonState state) =>

src/System.Windows.Forms/System/Windows/Forms/Controls/Buttons/ButtonInternal/DarkMode/PopupButtonDarkModeRenderer.cs

@@ -120,7 +120,7 @@ public override Color GetBackgroundColor(PushButtonState state, bool isDefault)
     /// <summary>
     ///  Draws the 3D effect border for the button.
     /// </summary>
-    private static void DrawButtonBorder(Graphics graphics, Rectangle bounds, PushButtonState state, bool isDefault)
+    public override void DrawButtonBorder(Graphics graphics, Rectangle bounds, PushButtonState state, bool isDefault)
     {
         // Save original smoothing mode to restore later
         SmoothingMode originalMode = graphics.SmoothingMode;

src/System.Windows.Forms/System/Windows/Forms/Controls/Buttons/ButtonInternal/DarkMode/SystemButtonDarkModeRenderer.cs

@@ -108,6 +108,9 @@ public override Color GetBackgroundColor(PushButtonState state, bool isDefault)
                 _ => DefaultColors.StandardBackColor
             };
 
+    public override void DrawButtonBorder(Graphics graphics, Rectangle bounds, PushButtonState state, bool isDefault) =>
+        DrawButtonBorder(graphics, bounds, state, isDefault, false);
+
     /// <summary>
     ///  Draws the button border based on the current state, using anti-aliasing and an additional inner border.
     /// </summary>