We have a Winforms project upgraded to .NET 8. The conversion had only a few problems that where easily fixed, so we continued on this path.
However, now I found a problem that I suspect is a bug in Winforms designer on .NET 8.

I have a custom control, and a panel on that control marked as a “dropzone”, so you can drop controls on this custom control in the designer. This worked well until the move to .NET 8.

Here is a snippet of the custom control’s code:

[Designer(typeof(UserControlDesigner))]
[Serializable]
public partial class gttDXGridDashboard : UserControl
{ 
  // define a property called "DropZone"
  [Category("Appearance")]
  [DesignerSerializationVisibility(DesignerSerializationVisibility.Content)]
  public PanelControl DropZone
  {
      get { return panelControlCustom; }
  }

  // ...
}

public class UserControlDesigner : ParentControlDesigner
{
    public override void Initialize(IComponent component)
    {
        base.Initialize(component);

        if (Control is gttDXGridDashboard dashboard)
        {
            bool result = EnableDesignMode(dashboard.DropZone, "DropZone");
            // this writeline never shows, maybe because it's in a control from the toolbox ?
            Console.WriteLine($"EnableDesignMode(dashboard.DropZone, "DropZone") = {result}");
        }
    }
}

Now for the problem: in a form that has a gttDXGridDashboard on it, and has a button in the dropzone, the designer.cs looks like this:

  // gttDXGridDashboardList.DropZone
  // 
  this.gttDXGridDashboardList.DropZone.Controls.Add(this.gttDXSimpleButtonAddMultipleChassis);

This worked well while the project was running on the .NET framework, but since the move to .NET 8, this line in the designer.cs is removed from the moment I drop another control on the dropzone, or even when I for example move the existing button (gttDXSimpleButtonAddMultipleChassis) a bit to the right or left

At that moment, the button gttDXSimpleButtonAddMultipleChassis still exists on the form, but has no parent anymore and thus is invisible and unusable.

I tried updating all packages, rebuilding all sources, restarted VS, but no results.

Then I wanted to make a workaround, and write some code that will put all the controls without a parent back onto the dropzone, but I don’t know how to search for controls that have no parent, I cannot loop over this.Controls because these controls have no parent anymore.

So I need a little help here.

Did I do something wrong with defining the dropzone?

Is this a know bug/problem and if so is there a fix/workaround?

If all fails, how can I find controls that have no parent?

I know I can always write code in the forms that have this problem to put the controls back, but that would mean writing almost the same code again in several forms, I want a more generic workaround.