Fix ComboBox dark mode theme on first TabPage

[Copilot]

Proposed changes

• Modified TabControl.StyleChildren() to only apply dark mode theme to direct children (TabPages), not descendant controls
• Added parent check using PInvoke.GetParent() to identify direct children
• Preserved return value of SetWindowTheme() for proper error handling

Customer Impact

• ComboBox controls on the first TabPage now correctly display in dark mode
• Affects applications using Application.SetColorMode(SystemColorMode.Dark) with TabControl containing ComboBox controls

Regression?

• Yes (regression from .NET 9.0)

Risk

• Low. Surgical fix changes only theme application scope in TabControl
• Only affects dark mode rendering path
• Controls with specific dark mode themes (ComboBox, etc.) now preserve their themes

Screenshots

Before

After

Test methodology

• Code review completed
• CodeQL security scan passed
• Manual testing required on Windows to validate visual rendering

Test environment(s)

• .NET 10.0.100

---

Technical Details:

The issue occurred because TabControl.ApplyDarkModeOnDemand() calls EnumChildWindows() which recursively enumerates all descendant windows. The StyleChildren callback was applying the generic DarkMode_Explorer theme to every window, overwriting ComboBox's specific DarkMode_ComboBoxButton theme for its button component.

The fix restricts theme application to only TabPage windows (direct children of TabControl), allowing controls to maintain their specific themes:

private BOOL StyleChildren(HWND handle)
{
    // Only apply theme to direct children (TabPages), not to controls within TabPages.
    HWND parent = PInvoke.GetParent(handle);
    if (parent == HWND)
    {
        return PInvoke.SetWindowTheme(handle, $"{DarkModeIdentifier}_{ExplorerThemeIdentifier}", null)
            .Succeeded;
    }

    return true;
}

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>[Dark Mode] the combobox refuses dark theme when on first tabpage of a tabcontrol</issue_title> <issue_description>### .NET version

latest .NET 10.0 build

Did it work in .NET Framework?

No

Did it work in any of the earlier releases of .NET Core or .NET 5+?

This is a regression issue, not reproduce .NET 9.0

Issue description

The combobox refuses dark theme when on first tabpage of a tabcontrol Expected Results:

Actual Results:

Steps to reproduce

1. Create a .NET winforms project with a TabCpntrol control
2. Add a Combobox contorl in the tabPage1
3. Set app under Dark mode in program.cs file Application.SetColorMode(SystemColorMode.Dark);
4. Run the application

More info:

1. The issue can reproduce when setting the DropDownStyle as DropDown/DropDownList, not reproduce when setting the DropDownStyle as Sample</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes dotnet/winforms#14107

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[codecov[bot]]

Codecov Report

:x: Patch coverage is 0% with 8 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 77.15885%. Comparing base (ceb6911) to head (c0b6d94).

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #14128         +/-   ##
===================================================
+ Coverage   77.15337%   77.15885%   +0.00548%     
===================================================
  Files           3279        3279                 
  Lines         645325      645331          +6     
  Branches       47720       47721          +1     
===================================================
+ Hits          497890      497930         +40     
+ Misses        143748      143713         -35     
- Partials        3687        3688          +1     

Flag	Coverage Δ
Debug	77.15885% <0.00000%> (+0.00548%)	:arrow_up:
integration	18.99499% <0.00000%> (+0.00100%)	:arrow_up:
production	52.02862% <0.00000%> (+0.01280%)	:arrow_up:
test	97.40749% <ø> (ø)
unit	49.47336% <0.00000%> (+0.01738%)	:arrow_up:

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