Fix ComboBox dark mode theme on first TabPage

[SimonZhao888]

Fixes #14107

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

• Manual test

Test environment(s)

• 11.0.100-preview.5.26227.104

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

Fixes a dark-mode regression where ComboBox controls placed on the first TabPage of a TabControl lost their specialized dark theming. TabControl.ApplyDarkModeOnDemand calls EnumChildWindows, which enumerates all descendant windows (not just direct children). The previous StyleChildren callback unconditionally applied DarkMode_Explorer to every descendant, overwriting the DarkMode_CFD theme that ComboBox.OnHandleCreated had set on itself. The fix scopes the theme application to direct children (i.e. TabPages) by comparing PInvoke.GetParent(handle) against the TabControl's HWND.

Changes:

• Guard StyleChildren so SetWindowTheme(DarkMode_Explorer) is only applied when the enumerated handle's native parent is the TabControl itself.
• Return true for non-direct descendants so enumeration continues, leaving control-specific dark themes (e.g. ComboBox) intact.

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[SimonZhao888]

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