Fix ComboBox dark mode BackColor bug.

Implement dark mode fix for disabled RichTextBox.

Author: KlausLoeffelmann

src/System.Windows.Forms/PublicAPI.Unshipped.txt

@@ -0,0 +1,2 @@
+override System.Windows.Forms.ComboBox.OnEnabledChanged(System.EventArgs! e) -> void
+override System.Windows.Forms.RichTextBox.OnEnabledChanged(System.EventArgs! e) -> void

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

@@ -298,11 +298,6 @@ public unsafe partial class Control :
 
     internal byte LayoutSuspendCount { get; private set; }
 
-    // Flag for the youngest control in the descendant-inheritance hierarchy as the
-    // ultimate truth if the control - by having set the ControlStyles. -
-    // indicates to participate in automatic dark mode theming or not.
-    internal bool? _darkModeRequestState;
-
     /// <summary>
     ///  Initializes a new instance of the <see cref="Control"/> class.
     /// </summary>
@@ -491,6 +486,32 @@ internal int OriginalDeviceDpiInternal
         }
     }
 
+    /// <summary>
+    ///  Caches whether the youngest control in the inheritance hierarchy has requested
+    ///  participation in automatic dark mode theming based on its <see cref="ControlStyles"/> settings.
+    /// </summary>
+    /// <remarks>
+    ///  <para>
+    ///   This property acts as a cache for the dark mode theming request state due to the architectural pattern
+    ///   of Control inheritance in WinForms. The constructor of <see cref="Control"/> calls <see cref="CreateParams"/>,
+    ///   which means that <see cref="CreateParams"/> in derived controls is invoked before their respective constructors run.
+    ///  </para>
+    ///  <para>
+    ///   As a result, if a control indicates its intent to participate in dark mode theming by setting
+    ///   <see cref="ControlStyles.ApplyThemingImplicitly"/>, we need to cache this state in this internal property
+    ///   rather than relying on the setting it does in its constructor. This is necessary because theming decisions
+    ///   and related logic may be triggered during the base <see cref="Control"/> construction process, before the
+    ///   derived control's constructor has a chance to execute.
+    ///  </para>
+    ///  <para>
+    ///   Furthermore, since base classes (such as <c>TextBoxBase</c> or <c>ButtonBase</c>) may request dark mode
+    ///   theming, but a more derived control might opt out, this <see cref="DarkModeRequestState"/> property serves
+    ///   as the definitive source of truth for a particular control's dark mode participation. It ensures that the
+    ///   most derived control's preference is respected, regardless of what its ancestors may have set.
+    ///  </para>
+    /// </remarks>
+    internal bool? DarkModeRequestState { get; set; }
+
     /// <summary>
     ///  The Accessibility Object for this Control
     /// </summary>
@@ -7384,7 +7405,7 @@ protected virtual void OnHandleCreated(EventArgs e)
 
 #pragma warning disable WFO5001 // Type is for evaluation purposes only and is subject to change or removal in future updates.
             if (Application.IsDarkModeEnabled
-                && _darkModeRequestState is true
+                && DarkModeRequestState is true
                 && !RecreatingHandle)
             {
                 _ = PInvoke.SetWindowTheme(
@@ -9349,7 +9370,7 @@ internal virtual void RecreateHandleCore()
             // ensure that the theming is applied to all child controls as well.
 #pragma warning disable WFO5001
             if (Application.IsDarkModeEnabled
-                && _darkModeRequestState is true)
+                && DarkModeRequestState is true)
             {
                 _ = PInvoke.SetWindowTheme(
                     hwnd: HWND,
@@ -10345,9 +10366,9 @@ protected void SetStyle(ControlStyles flag, bool value)
         // reset those settings - the source of truth ultimately is the value which gets set
         // for the first time a control sets or clears this style explicitly in CreateParams.
         if ((flag & ControlStyles.ApplyThemingImplicitly) == ControlStyles.ApplyThemingImplicitly
-            && !_darkModeRequestState.HasValue)
+            && !DarkModeRequestState.HasValue)
         {
-            _darkModeRequestState = value;
+            DarkModeRequestState = value;
         }
     }
 #pragma warning restore WFO5001 // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
@@ -10412,7 +10433,7 @@ protected virtual void SetVisibleCore(bool value)
                     // We shouldn't mess with the color mode if users haven't specifically set it.
                     // https://github.com/dotnet/winforms/issues/12014
                     // And we shouldn't prepare dark mode, if the form opted out.
-                    if (value && Application.ColorModeSet && _darkModeRequestState is true)
+                    if (value && Application.ColorModeSet && DarkModeRequestState is true)
                     {
                         PrepareDarkMode(HWND, Application.IsDarkModeEnabled);
                     }

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

@@ -168,7 +168,7 @@ private protected override bool OwnerDraw
                 && Image is null
 
                 // ...the user wants to opt out of implicit DarkMode rendering.
-                && _darkModeRequestState is true
+                && DarkModeRequestState is true
 
                 // And all of this only counts for FlatStyle.Standard. For the
                 // rest, we're using specific renderers anyway, which check

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

@@ -249,15 +249,31 @@ public AutoCompleteStringCollection AutoCompleteCustomSource
         }
     }
 
+#pragma warning disable WFO5001 // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
     /// <summary>
     ///  The background color of this control. This is an ambient property and
     ///  will always return a non-null value.
     /// </summary>
     public override Color BackColor
     {
-        get => ShouldSerializeBackColor() ? base.BackColor : SystemColors.Window;
+        get
+        {
+            if (ShouldSerializeBackColor())
+            {
+                return base.BackColor;
+            }
+            else
+            {
+                return Application.IsDarkModeEnabled
+                    && DarkModeRequestState is true
+                        ? SystemColors.ControlDarkDark
+                        : SystemColors.Window;
+            }
+        }
+
         set => base.BackColor = value;
     }
+#pragma warning restore WFO5001 // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
 
     [Browsable(false)]
     [EditorBrowsable(EditorBrowsableState.Never)]
@@ -3620,6 +3636,9 @@ private unsafe void WmReflectMeasureItem(ref Message m)
         m.ResultInternal = (LRESULT)1;
     }
 
+    private static readonly IntPtr s_darkEditBrush
+        = PInvokeCore.CreateSolidBrush(ColorTranslator.ToWin32(Color.FromArgb(64, 64, 64)));
+
     /// <summary>
     ///  The ComboBox's window procedure. Inheriting classes can override this
     ///  to add extra functionality, but should not forget to call
@@ -3672,7 +3691,32 @@ protected override unsafe void WndProc(ref Message m)
                 }
 
                 break;
+
             case PInvokeCore.WM_CTLCOLOREDIT:
+                // Only handle if the ComboBox is disabled
+#pragma warning disable WFO5001 // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
+                if (IsHandleCreated
+                    && !Enabled
+                    && Application.IsDarkModeEnabled
+                    && DarkModeRequestState is true)
+                {
+                    PInvokeCore.SetBkColor(
+                        (HDC)m.WParamInternal,
+                        ColorTranslator.ToWin32(Color.FromArgb(64, 64, 64)));
+
+                    PInvokeCore.SetTextColor(
+                        (HDC)m.WParamInternal,
+                        ColorTranslator.ToWin32(Color.FromArgb(180, 180, 180)));
+
+                    m.ResultInternal = (LRESULT)s_darkEditBrush;
+                }
+                else
+                {
+                    m.ResultInternal = (LRESULT)(nint)InitializeDCForWmCtlColor((HDC)(nint)m.WParamInternal, m.MsgInternal);
+                }
+#pragma warning restore WFO5001 // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
+
+                break;
             case PInvokeCore.WM_CTLCOLORLISTBOX:
                 m.ResultInternal = (LRESULT)(nint)InitializeDCForWmCtlColor((HDC)(nint)m.WParamInternal, m.MsgInternal);
                 break;

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

@@ -4673,7 +4673,7 @@ protected override void OnHandleCreated(EventArgs e)
     private void ApplyDarkModeOnDemand()
     {
         if (Application.IsDarkModeEnabled
-            && _darkModeRequestState is true)
+            && DarkModeRequestState is true)
         {
             // Enable double buffering when in dark mode to reduce flicker.
             uint exMask = PInvoke.LVS_EX_ONECLICKACTIVATE | PInvoke.LVS_EX_TWOCLICKACTIVATE |

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

@@ -2412,6 +2412,24 @@ protected override void OnGotFocus(EventArgs e)
         }
     }
 
+    protected override void OnEnabledChanged(EventArgs e)
+    {
+        base.OnEnabledChanged(e);
+        HandleDarkModeDisabledBackground();
+    }
+
+    private void HandleDarkModeDisabledBackground()
+    {
+#pragma warning disable WFO5001 // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
+        if (Application.IsDarkModeEnabled
+            && !Enabled
+            && DarkModeRequestState is true)
+        {
+            Invalidate();
+        }
+#pragma warning restore WFO5001 // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
+    }
+
     protected override void OnHandleCreated(EventArgs e)
     {
         // base.OnHandleCreated is called somewhere in the middle of this
@@ -2458,6 +2476,8 @@ protected override void OnHandleCreated(EventArgs e)
         // base sets the Text property. It's important to do this *after* setting EM_AUTOUrlDETECT.
         base.OnHandleCreated(e);
 
+        HandleDarkModeDisabledBackground();
+
         // For some reason, we need to set the OleCallback before setting the RTF property.
         UpdateOleCallback();
 
@@ -3445,6 +3465,24 @@ protected override void WndProc(ref Message m)
     {
         switch (m.MsgInternal)
         {
+#pragma warning disable WFO5001 // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
+            case PInvokeCore.WM_PAINT:
+                if (IsHandleCreated && !Enabled)
+                {
+                    if (Application.IsDarkModeEnabled && DarkModeRequestState is true)
+                    {
+                        // If the control is in dark mode, we need to paint the background
+                        // with the dark mode color.
+                        using Graphics g = Graphics.FromHwnd(Handle);
+                        g.Clear(SystemColors.Control);
+                    }
+                }
+
+                base.WndProc(ref m);
+                break;
+
+#pragma warning restore WFO5001 // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
+
             case MessageId.WM_REFLECT_NOTIFY:
                 WmReflectNotify(ref m);
                 break;

src/System.Windows.Forms/System/Windows/Forms/Controls/TextBox/TextBoxBase.cs

@@ -287,10 +287,9 @@ public override Color BackColor
             else
             {
                 return ReadOnly
-
                     // If we're ReadOnly and in DarkMode, we are using a different background color.
                     ? Application.IsDarkModeEnabled
-                        && _darkModeRequestState is true
+                        && DarkModeRequestState is true
                             ? SystemColors.ControlDarkDark
                             : SystemColors.Control
                     : SystemColors.Window;
@@ -955,7 +954,7 @@ private void EnsureReadonlyBackgroundColor(bool value)
     {
         // If we have no specifically defined back color, we set the back color in case we're in dark mode.
         if (Application.IsDarkModeEnabled
-            && _darkModeRequestState is true)
+            && DarkModeRequestState is true)
         {
             base.BackColor = value ? SystemColors.ControlLight : SystemColors.Window;
             Invalidate();