.NET version

.NET 10.0.100-preview.2.25118.3

Did it work in .NET Framework?

No

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

No, also repro in .NET 9.0 which it started to be supported.

Issue description

The Form title bar is white color when the MDIParent1 MainMenuStrip property is set to none.

Steps to reproduce

1. Create a winforms .NET Core project.
2. Enable DarkMode by Application.SetColorMode(SystemColorMode.Dark) in Program.cs file.
3. Ctrl+Shift+A to add a MDI Parent Form.
4. Set MDIParent1 MainMenuStrip property to none.
5. Build and run MDIParent1.
6. Click File -> New, select Window0's Maximize button.

More Info:

1. This issue also repro when click MDI Parent button in WinformsControlTest app.