Fix disabled ComboBox rendering to use system disabled palette in modern visual styles

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

@@ -32,9 +32,6 @@ internal sealed class ModernButtonDarkModeRenderer : ButtonDarkModeRendererBase
     private static readonly Color s_darkNormal = Color.FromArgb(0x2D, 0x2D, 0x2D);
     private static readonly Color s_darkNormalHover = Color.FromArgb(0x32, 0x32, 0x32);
     private static readonly Color s_darkNormalPressed = Color.FromArgb(0x2A, 0x2A, 0x2A);
-    private static readonly Color s_darkDisabled = Color.FromArgb(0x25, 0x25, 0x25);
-
-    private static readonly Color s_darkDisabledText = Color.FromArgb(0x88, 0x88, 0x88);
 
     private static readonly Color s_darkGap = Color.FromArgb(0x0A, 0x0A, 0x0A);
     private static readonly Color s_darkFocusRing = Color.White;
@@ -43,11 +40,8 @@ internal sealed class ModernButtonDarkModeRenderer : ButtonDarkModeRendererBase
     private static readonly Color s_lightNormal = Color.FromArgb(0xFB, 0xFB, 0xFB);
     private static readonly Color s_lightNormalHover = Color.FromArgb(0xF9, 0xF9, 0xF9);
     private static readonly Color s_lightNormalPressed = Color.FromArgb(0xF5, 0xF5, 0xF5);
-    private static readonly Color s_lightDisabled = Color.FromArgb(0xFA, 0xFA, 0xFA);
     private static readonly Color s_lightBorder = Color.FromArgb(0xD0, 0xD0, 0xD0);
 
-    private static readonly Color s_lightDisabledText = Color.FromArgb(0xA0, 0xA0, 0xA0);
-
     private static bool IsDark => Application.IsDarkModeEnabled;
 
     private int FocusRingThickness
@@ -182,25 +176,16 @@ public override void DrawFocusIndicator(Graphics graphics, Rectangle bounds, boo
 
     public override Color GetTextColor(PushButtonState state, bool isDefault, Color backColor)
     {
-        if (state == PushButtonState.Disabled)
-        {
-            Color preferredForeColor = IsDark
-                ? s_darkDisabledText
-                : s_lightDisabledText;
-
-            return ModernControlColorMath.GetDisabledTextColor(
-                preferredForeColor,
-                backColor);
-        }
-
-        return ModernButtonColorMath.GetReadableForeColor(backColor);
+        return state == PushButtonState.Disabled
+            ? ModernControlColorMath.GetDisabledForeColor(backColor)
+            : ModernButtonColorMath.GetReadableForeColor(backColor);
     }
 
     public override Color GetBackgroundColor(PushButtonState state, bool isDefault)
     {
         if (state == PushButtonState.Disabled)
         {
-            return IsDark ? s_darkDisabled : s_lightDisabled;
+            return ModernControlColorMath.GetDisabledSurfaceColor();
         }
 
         if (isDefault)

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

@@ -19,9 +19,7 @@ internal sealed class ModernFlatButtonRenderer : ButtonDarkModeRendererBase
     private static readonly Color s_lightBorder = Color.FromArgb(0xD0, 0xD0, 0xD0);
     private static readonly Color s_darkBorder = Color.FromArgb(0x55, 0x55, 0x55);
     private static readonly Color s_lightNormal = Color.FromArgb(0xFB, 0xFB, 0xFB);
-    private static readonly Color s_lightDisabled = Color.FromArgb(0xFA, 0xFA, 0xFA);
     private static readonly Color s_darkNormal = Color.FromArgb(0x2D, 0x2D, 0x2D);
-    private static readonly Color s_darkDisabled = Color.FromArgb(0x25, 0x25, 0x25);
 
     private static bool IsDark => Application.IsDarkModeEnabled;
 
@@ -96,25 +94,16 @@ public override void DrawFocusIndicator(Graphics graphics, Rectangle contentBoun
 
     public override Color GetTextColor(PushButtonState state, bool isDefault, Color backColor)
     {
-        if (state == PushButtonState.Disabled)
-        {
-            Color preferredForeColor = IsDark
-                ? Color.FromArgb(0x88, 0x88, 0x88)
-                : Color.FromArgb(0xA0, 0xA0, 0xA0);
-
-            return ModernControlColorMath.GetDisabledTextColor(
-                preferredForeColor,
-                backColor);
-        }
-
-        return ModernButtonColorMath.GetReadableForeColor(backColor);
+        return state == PushButtonState.Disabled
+            ? ModernControlColorMath.GetDisabledForeColor(backColor)
+            : ModernButtonColorMath.GetReadableForeColor(backColor);
     }
 
     public override Color GetBackgroundColor(PushButtonState state, bool isDefault)
     {
         if (state == PushButtonState.Disabled)
         {
-            return IsDark ? s_darkDisabled : s_lightDisabled;
+            return ModernControlColorMath.GetDisabledSurfaceColor();
         }
 
         if (isDefault)

src/System.Windows.Forms/System/Windows/Forms/Controls/ComboBox/ComboBox.ModernComboAdapter.cs

@@ -139,10 +139,8 @@ private static void DrawStandardFrame(
             Rectangle clientBounds,
             Rectangle borderBounds)
         {
-            Color background = GetEffectiveBackColor(comboBox);
             Color borderColor = GetBorderColor(
                 comboBox,
-                background,
                 useAccent: false);
 
             CutOutRoundedCorners(
@@ -166,10 +164,8 @@ private static void DrawFlatFrame(
                 return;
             }
 
-            Color background = GetEffectiveBackColor(comboBox);
             Color borderColor = GetBorderColor(
                 comboBox,
-                background,
                 useAccent: false);
             using var pen = borderColor.GetCachedPenScope(
                 GetBorderThickness(comboBox));
@@ -182,10 +178,8 @@ private static void DrawPopupFrame(
             Rectangle clientBounds,
             Rectangle borderBounds)
         {
-            Color background = GetEffectiveBackColor(comboBox);
             Color borderColor = GetBorderColor(
                 comboBox,
-                background,
                 useAccent: true);
             CutOutRoundedCorners(
                 comboBox,
@@ -223,9 +217,7 @@ private void DrawDropDownButton(
                         0.035f);
             if (!comboBox.Enabled)
             {
-                buttonColor = PopupButtonColorMath.Mute(
-                    buttonColor,
-                    0.55f);
+                buttonColor = ModernControlColorMath.GetDisabledSurfaceColor();
             }
 
             using (var brush = buttonColor.GetCachedSolidBrushScope())
@@ -235,9 +227,7 @@ private void DrawDropDownButton(
 
             Color chevronColor = comboBox.Enabled
                 ? PopupButtonColorMath.GetReadableForeColor(buttonColor)
-                : ModernControlColorMath.GetDisabledTextColor(
-                    comboBox.ForeColor,
-                    buttonColor);
+                : ModernControlColorMath.GetDisabledForeColor(buttonColor);
             int halfWidth = Math.Max(
                 2,
                 ScaleHelper.ScaleToDpi(3, _deviceDpi));
@@ -316,9 +306,7 @@ private void DrawDropDownListText(
                         ? comboBox.ForeColor
                         : PopupButtonColorMath.GetReadableForeColor(
                             background)
-                    : ModernControlColorMath.GetDisabledTextColor(
-                        comboBox.ForeColor,
-                        background);
+                    : ModernControlColorMath.GetDisabledForeColor(background);
             TextFormatFlags flags = TextFormatFlags.SingleLine
                 | TextFormatFlags.VerticalCenter
                 | TextFormatFlags.EndEllipsis
@@ -421,20 +409,16 @@ private static void DrawRoundedBorder(
                 parentColor);
         }
 
-        private static Color GetBorderColor(
-            ComboBox comboBox,
-            Color background,
-            bool useAccent)
+        private static Color GetBorderColor(ComboBox comboBox, bool useAccent)
         {
-            Color borderColor = useAccent
+            if (!comboBox.Enabled)
+            {
+                return ModernControlColorMath.GetDisabledBorderColor();
+            }
+
+            return useAccent
                 ? Application.SystemVisualSettings.AccentColor
                 : comboBox.ForeColor;
-
-            return comboBox.Enabled
-                ? borderColor
-                : ModernControlColorMath.GetDisabledTextColor(
-                    borderColor,
-                    background);
         }
 
         private static int GetBorderThickness(ComboBox comboBox)
@@ -480,16 +464,13 @@ private static Color GetEffectiveBackColor(ComboBox comboBox)
                     ?? SystemColors.Window;
 
         /// <summary>
-        ///  Returns the field surface color, muted when the ComboBox is disabled so a disabled
-        ///  control no longer shows its full custom <see cref="Control.BackColor"/> (issue #14797).
+        ///  Returns the field surface color. A disabled ComboBox does not honor user-set
+        ///  <see cref="Control.BackColor"/> values and uses the shared modern disabled surface
+        ///  instead, which adapts to the current color mode (issue #14797).
         /// </summary>
         private static Color GetEffectiveFieldColor(ComboBox comboBox)
-        {
-            Color background = GetEffectiveBackColor(comboBox);
-
-            return comboBox.Enabled
-                ? background
-                : PopupButtonColorMath.Mute(background, 0.55f);
-        }
+            => comboBox.Enabled
+                ? GetEffectiveBackColor(comboBox)
+                : ModernControlColorMath.GetDisabledSurfaceColor();
     }
 }

src/System.Windows.Forms/System/Windows/Forms/Controls/ComboBox/ComboBox.cs

@@ -3826,6 +3826,27 @@ protected override unsafe void WndProc(ref Message m)
                     return;
                 }
 
+                // Modern NET11 mode, disabled DropDown: the native edit child sends
+                // WM_CTLCOLORSTATIC and the OS default would produce a white background,
+                // inconsistent with the disabled surface ModernComboAdapter paints for the rest
+                // of the field. In light mode, explicitly set disabled colors via system values.
+                // In dark mode, ModernComboAdapter already renders the disabled surface correctly,
+                // and the edit child inherits appropriate rendering from the parent control.
+                if (hwndChild == _childEdit?.HWND
+                    && UsesModernComboAdapter
+                    && !Enabled
+                    && !Application.IsDarkModeEnabled)
+                {
+                    PInvokeCore.SetBkColor(
+                        (HDC)m.WParamInternal,
+                        ColorTranslator.ToWin32(SystemColors.ButtonFace));
+                    PInvokeCore.SetTextColor(
+                        (HDC)m.WParamInternal,
+                        ColorTranslator.ToWin32(SystemColors.GrayText));
+                    m.ResultInternal = (LRESULT)(nint)PInvokeCore.GetSysColorBrush(SystemColors.ButtonFace);
+                    return;
+                }
+
                 // Additional handling for Simple style listbox when disabled
                 if (DropDownStyle == ComboBoxStyle.Simple
                     && Application.IsDarkModeEnabled

src/System.Windows.Forms/System/Windows/Forms/Rendering/ModernControlColorMath.cs

@@ -1,4 +1,4 @@
-// Licensed to the .NET Foundation under one or more agreements.
+// Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Drawing;
@@ -16,6 +16,50 @@ internal static class ModernControlColorMath
     private const float DisabledMuteAmount = 0.45f;
     private const int ContrastSearchIterations = 10;
 
+    // Shared disabled-state palette for modern renderers. Modern controls do not honor user-set
+    // BackColor/ForeColor while disabled, so these fixed surfaces replace them. This is the single
+    // source of truth: the modern Button renderers and the modern ComboBox adapter all read from
+    // here, so a disabled Button and a disabled ComboBox cannot drift apart.
+    private static readonly Color s_darkModeDisabledSurface = Color.FromArgb(0x25, 0x25, 0x25);
+    private static readonly Color s_lightModeDisabledSurface = Color.FromArgb(0xFA, 0xFA, 0xFA);
+    private static readonly Color s_darkModeDisabledBorder = Color.FromArgb(0x55, 0x55, 0x55);
+    private static readonly Color s_lightModeDisabledBorder = Color.FromArgb(0xD0, 0xD0, 0xD0);
+    private static readonly Color s_darkModeDisabledForeground = Color.FromArgb(0x88, 0x88, 0x88);
+    private static readonly Color s_lightModeDisabledForeground = Color.FromArgb(0xA0, 0xA0, 0xA0);
+
+    /// <summary>
+    ///  Gets the surface color for a disabled modern control, honoring the current color mode
+    ///  and high contrast settings.
+    /// </summary>
+    internal static Color GetDisabledSurfaceColor()
+        => SystemInformation.HighContrast
+            ? SystemColors.Control
+            : Application.IsDarkModeEnabled
+                ? s_darkModeDisabledSurface
+                : s_lightModeDisabledSurface;
+
+    /// <summary>
+    ///  Gets the border color for a disabled modern control, honoring the current color mode
+    ///  and high contrast settings.
+    /// </summary>
+    internal static Color GetDisabledBorderColor()
+        => SystemInformation.HighContrast
+            ? SystemColors.GrayText
+            : Application.IsDarkModeEnabled
+                ? s_darkModeDisabledBorder
+                : s_lightModeDisabledBorder;
+
+    /// <summary>
+    ///  Gets the contrast-adjusted foreground color for content drawn on
+    ///  <see cref="GetDisabledSurfaceColor"/>.
+    /// </summary>
+    internal static Color GetDisabledForeColor(Color backColor)
+        => GetDisabledTextColor(
+            Application.IsDarkModeEnabled
+                ? s_darkModeDisabledForeground
+                : s_lightModeDisabledForeground,
+            backColor);
+
     internal static Color GetDisabledTextColor(
         Color preferredForeColor,
         Color backColor)