Fix the Button border and color with FlatStyle = Flat: FlatAppearance properties

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

@@ -53,6 +53,7 @@ private Color GetButtonBackColor(PushButtonState state)
     {
         Color backColor;
 
+        // Set the initial back color.
         if (Control.BackColor != Forms.Control.DefaultBackColor)
         {
             backColor = Control.BackColor;
@@ -67,6 +68,21 @@ private Color GetButtonBackColor(PushButtonState state)
             backColor = ButtonDarkModeRenderer.GetBackgroundColor(state, Control.IsDefault);
         }
 
+        // Override the back color for FlatStyle buttons if specific colors are set.
+        if (Control.FlatStyle == FlatStyle.Flat)
+        {
+            if (state == PushButtonState.Pressed && !Control.FlatAppearance.MouseDownBackColor.IsEmpty)
+            {
+                backColor = Control.FlatAppearance.MouseDownBackColor;
+            }
+            else if (state == PushButtonState.Hot
+                && !Control.FlatAppearance.MouseOverBackColor.IsEmpty
+                && !IsHighContrastHighlighted())
+            {
+                backColor = Control.FlatAppearance.MouseOverBackColor;
+            }
+        }
+
         return backColor;
     }
 
@@ -86,6 +102,8 @@ internal override void PaintUp(PaintEventArgs e, CheckState state)
                 g,
                 Control.ClientRectangle,
                 Control.FlatStyle,
+                Control.FlatAppearance.BorderSize,
+                Control.FlatAppearance.BorderColor,
                 pushButtonState,
                 Control.IsDefault,
                 Control.Focused,
@@ -124,6 +142,8 @@ internal override void PaintDown(PaintEventArgs e, CheckState state)
                 g,
                 Control.ClientRectangle,
                 Control.FlatStyle,
+                Control.FlatAppearance.BorderSize,
+                Control.FlatAppearance.BorderColor,
                 PushButtonState.Pressed,
                 Control.IsDefault,
                 Control.Focused,
@@ -162,6 +182,8 @@ internal override void PaintOver(PaintEventArgs e, CheckState state)
                 g,
                 Control.ClientRectangle,
                 Control.FlatStyle,
+                Control.FlatAppearance.BorderSize,
+                Control.FlatAppearance.BorderColor,
                 PushButtonState.Hot,
                 Control.IsDefault,
                 Control.Focused,

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

@@ -32,6 +32,8 @@ public void RenderButton(
         Graphics graphics,
         Rectangle bounds,
         FlatStyle flatStyle,
+        int borderSize,
+        Color borderColor,
         PushButtonState state,
         bool isDefault,
         bool focused,
@@ -61,11 +63,16 @@ public void RenderButton(
                 height: bounds.Height - padding.Vertical);
 
             // Draw button background and get content bounds
-            Rectangle contentBounds = DrawButtonBackground(graphics, paddedBounds, state, isDefault, backColor);
+            Rectangle contentBounds =
+                DrawButtonBackground(graphics, paddedBounds, state, isDefault, backColor, borderSize, borderColor);
 
-            // Paint image and field using the provided delegates
+            // Paint image using the provided delegates
             paintImage(contentBounds);
 
+            // Draw button border
+            DrawButtonBorder(graphics, bounds, borderSize, borderColor, state, isDefault);
+
+            // Paint field using the provided delegates
             paintField();
 
             if (focused && showFocusCues)
@@ -76,7 +83,13 @@ public void RenderButton(
         }
     }
 
-    public abstract Rectangle DrawButtonBackground(Graphics graphics, Rectangle bounds, PushButtonState state, bool isDefault, Color backColor);
+    public virtual void DrawButtonBorder(Graphics graphics, Rectangle bounds, int borderSize,
+        Color borderColor, PushButtonState state, bool isDefault)
+    {
+    }
+
+    public abstract Rectangle DrawButtonBackground(Graphics graphics, Rectangle bounds,
+        PushButtonState state, bool isDefault, Color backColor, int borderSize, Color borderColor);
 
     public abstract void DrawFocusIndicator(Graphics graphics, Rectangle contentBounds, bool isDefault);
 

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

@@ -17,20 +17,20 @@ 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);
 
     public override Rectangle DrawButtonBackground(
-        Graphics graphics, Rectangle bounds, PushButtonState state, bool isDefault, Color backColor)
+        Graphics graphics, Rectangle bounds, PushButtonState state, bool isDefault,
+        Color backColor, int borderSize, Color borderColor)
     {
         // fill background
         using var back = backColor.GetCachedSolidBrushScope();
-        graphics.FillRectangle(back, bounds);
+        Rectangle rectangle = bounds;
+        // Shrink the drawing area to avoid overlapping the border, consistent with DrawButtonBorder's inset logic
+        rectangle.Inflate(-borderSize - 1, -borderSize - 1);
 
-        // draw border identical to Win32
-        DrawButtonBorder(graphics, bounds, state, isDefault);
+        graphics.FillRectangle(back, rectangle);
 
         // return inner content area (border + 1 px system padding)
         return Rectangle.Inflate(bounds, -3, -3);
@@ -77,33 +77,29 @@ 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, int borderSize, Color borderColor, PushButtonState state, bool isDefault)
     {
         g.SmoothingMode = SmoothingMode.AntiAlias;
 
         // Win32 draws its stroke fully *inside* the control → inset by 1 px
         Rectangle outer = Rectangle.Inflate(bounds, -1, -1);
 
-        DrawSingleBorder(g, outer, GetBorderColor(state));
+        DrawSingleBorder(g, outer, borderSize, borderColor, GetBorderColor(state));
 
         // Default button gets a second 1‑px border one pixel further inside
         if (isDefault)
         {
             Rectangle inner = Rectangle.Inflate(outer, -1, -1);
-            DrawSingleBorder(g, inner, DefaultColors.AcceptFocusIndicatorBackColor);
+            DrawSingleBorder(g, inner, borderSize, borderColor, DefaultColors.AcceptFocusIndicatorBackColor);
         }
     }
 
-    private static void DrawSingleBorder(Graphics g, Rectangle rect, Color color)
+    private static void DrawSingleBorder(Graphics g, Rectangle rect, int borderSize, Color borderColor, 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);
+        using var pen = new Pen(!borderColor.IsEmpty ? borderColor : color, borderSize) { Alignment = PenAlignment.Inset };
+        g.DrawRectangle(pen, rect);
     }
 
     private static Color GetBorderColor(PushButtonState state) =>

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

@@ -32,6 +32,8 @@ static void DrawButtonBorder(Graphics graphics, GraphicsPath path, Color borderC
     /// <param name="graphics">The graphics context to draw on.</param>
     /// <param name="bounds">The bounds of the button.</param>
     /// <param name="flatStyle">The flat style of the button.</param>
+    /// <param name="borderSize">Size of the button border</param>
+    /// <param name="borderColor">Color of the button border</param>
     /// <param name="state">The visual state of the button (normal, hot, pressed, disabled, default).</param>
     /// <param name="isDefault">True if the button is the default button; otherwise, false.</param>
     /// <param name="focused">True if the button is focused; otherwise, false.</param>
@@ -43,6 +45,8 @@ void RenderButton(
         Graphics graphics,
         Rectangle bounds,
         FlatStyle flatStyle,
+        int borderSize,
+        Color borderColor,
         PushButtonState state,
         bool isDefault,
         bool focused,
@@ -59,8 +63,17 @@ void RenderButton(
     /// <param name="bounds">Bounds of the button</param>
     /// <param name="state">State of the button (normal, hot, pressed, disabled)</param>
     /// <param name="isDefault">True if button is the default button</param>
+    /// <param name="borderSize">Size of the button border</param>
+    /// <param name="borderColor">Color of the button border</param>
     /// <returns>The content bounds (area inside the button for text/image)</returns>
-    Rectangle DrawButtonBackground(Graphics graphics, Rectangle bounds, PushButtonState state, bool isDefault, Color backColor);
+    Rectangle DrawButtonBackground(
+        Graphics graphics,
+        Rectangle bounds,
+        PushButtonState state,
+        bool isDefault,
+        Color backColor,
+        int borderSize,
+        Color borderColor);
 
     /// <summary>
     ///  Draws focus indicator appropriate for this style.

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

@@ -30,7 +30,8 @@ internal class PopupButtonDarkModeRenderer : ButtonDarkModeRendererBase
     /// <summary>
     ///  Draws button background with popup styling, including subtle 3D effect.
     /// </summary>
-    public override Rectangle DrawButtonBackground(Graphics graphics, Rectangle bounds, PushButtonState state, bool isDefault, Color backColor)
+    public override Rectangle DrawButtonBackground(Graphics graphics, Rectangle bounds, PushButtonState state,
+        bool isDefault, Color backColor, int borderSize, Color borderColor)
     {
         // Use padding from ButtonDarkModeRenderer
         Padding padding = PaddingCore;

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

@@ -30,7 +30,8 @@ internal class SystemButtonDarkModeRenderer : ButtonDarkModeRendererBase
     /// <summary>
     ///  Draws button background with system styling (larger rounded corners).
     /// </summary>
-    public override Rectangle DrawButtonBackground(Graphics graphics, Rectangle bounds, PushButtonState state, bool isDefault, Color backColor)
+    public override Rectangle DrawButtonBackground(Graphics graphics, Rectangle bounds, PushButtonState state,
+        bool isDefault, Color backColor, int borderSize, Color borderColor)
     {
         // Shrink for DarkBorderGap and FocusBorderThickness
         Rectangle fillBounds = Rectangle.Inflate(bounds, -SystemStylePadding, -SystemStylePadding);