Fix TabControl spinner button not applying Dark Mode when TabPages added dynamically

[LeafShi1]

Fixes #14159

Root Cause:

• The TabControl's spinner button is created on-demand by Windows when TabPages exceed available display space and Multiline = false
• ApplyDarkModeOnDemand() only executes during handle creation (OnHandleCreated) or recreation (RecreateHandleCore)
• When TabPages are added dynamically (e.g., through PropertyGrid), the spinner button child window is created without Dark Mode theme applied

Proposed changes

• Added WM_PARENTNOTIFY message handling in TabControl's WndProc method to apply Dark Mode theme to dynamically created child windows (specifically the spinner button).

Customer Impact

• Spinner buttons now correctly render in Dark Mode when TabPages are added dynamically through PropertyGrid

Regression?

• No

Risk

• Minimal

Screenshots

Before

The Spinner button of Tabcontrol not in DarkMode when adding the TabPage items by using TabPages property in the PropretyGrid control

TabControlIssue.mp4

After

Dynamically created TabControl child windows (spinner buttons) now automatically receive Dark Mode theme when created

AfterChange.mp4

Test methodology

• Manually

Test environment(s)

• .net 11.0.0-alpha.1.25617.103

Microsoft Reviewers: Open in CodeFlow

[codecov]

Codecov Report

❌ Patch coverage is 28.57143% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.15607%. Comparing base (01eefd4) to head (4abb279).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #14162         +/-   ##
===================================================
+ Coverage   77.15180%   77.15607%   +0.00427%     
===================================================
  Files           3279        3279                 
  Lines         645333      645340          +7     
  Branches       47720       47721          +1     
===================================================
+ Hits          497886      497919         +33     
+ Misses        143754      143731         -23     
+ Partials        3693        3690          -3     

Flag	Coverage Δ
Debug	77.15607% <28.57143%> (+0.00427%)	⬆️
integration	18.99353% <28.57143%> (+0.01346%)	⬆️
production	52.02427% <28.57143%> (+0.01020%)	⬆️
test	97.40559% <ø> (ø)
unit	49.46797% <28.57143%> (+0.00052%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.