When I create a Windows Forms project for .net 8 with VisualStudio 2022, I cannot work with the form designer. Instead, a second view opens with the source code.
F7 / Shift-F7 only switches between the two source code views.
My approach:
Create new project dialog
Filter “RemObjects Oxygene,” “Windows,” ‘Desktop’
In the list, I select “Windows Forms Application (.NET Core)”
This is also the case for existing projects that I have converted from .NET Framework 4.8 to .NET 8.
Yes, unfortunately Microsoft explictly disabed/prevented the form designers (WinForms and WPF) to not work with third party languages, for .NET Core. We don’t know why, except that its on purpose.
The designers continue to work for Classic .NET 4.8 projects.
That’s shocking news. 
But fortunately, it’s not a big problem for my existing projects. I had these as .net 4.8 projects, and for .net 8, I created an additional project that uses the same source files.
So if I need to make changes to the forms, I load the .net 4.8 project.
For new projects, I then have to work with two parallel projects, as before. 
Yeah it’s a bit annoying. i really don’t understand why they did this.