ComboBox with DropDownStyle.Simple renders incorrectly and displays an unexpected vertical scrollbar

[Olina-Zhang]

.NET version

.NET 11.0.100-preview.7.26365.114 + Private dlls built from Net11/Integration-4 branch of https://github.com/KlausLoeffelmann/winforms repo

Did it work in .NET Framework?

Not tested/verified

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

Not support, new feature of #14792

Issue description

When VisualStylesMode is set to NET11, a ComboBox with DropDownStyle set to Simple renders incorrectly. An unexpected vertical scrollbar is displayed on the right side of the control.

Steps to reproduce

1. Create a WinForms application.
2. Set the Form's VisualStylesMode to NET11.
3. Add a ComboBox to the form.
4. Set:

• DropDownStyle = ComboBoxStyle.Simple
• Add several items (e.g. 1, 2, 3, 4).

5. Run the application.