Fix 13627 DemoConsole app: The open button cannot be seen in TreeView Tasks dialog in Dark Mode

[Epica3055]

Fix #13627

Proposed changes

• We need to set OwnerDraw true to be able to draw the button ourselves. So we set FlatStyle to Popup to break parent class's OwnerDraw logic.

Screenshots

Before

After

Test methodology

• manual test

Microsoft Reviewers: Open in CodeFlow

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 76.60452%. Comparing base (08485db) to head (e5b2814).
Report is 1 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #13631         +/-   ##
===================================================
- Coverage   76.60828%   76.60452%   -0.00376%     
===================================================
  Files           3253        3253                 
  Lines         640996      641000          +4     
  Branches       47439       47439                 
===================================================
- Hits          491056      491035         -21     
- Misses        146290      146316         +26     
+ Partials        3650        3649          -1     

Flag	Coverage Δ
Debug	76.60452% <0.00000%> (-0.00376%)	⬇️
integration	18.98137% <0.00000%> (-0.01248%)	⬇️
production	51.06099% <0.00000%> (-0.00802%)	⬇️
test	97.41175% <ø> (ø)
unit	48.44031% <0.00000%> (-0.00659%)	⬇️

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