New Form is not shown while drag & drop a Custom Control from toolbox to Designer in WinForms in NetCore

Question

In our Control, we have opened a new form to update the content of our control in the Designer. The new Form is not shown in the NetCore but works properly in the NetFrameWork. I have replicated the same issue in a simple control.

    [ToolboxItem(true)] // This attribute makes the control visible in the Toolbox
    public class CustomButton : Button
    {
        // Constructor
        public CustomButton()
        {
            // Set default properties
            this.BackColor = Color.Blue;
            this.ForeColor = Color.White;
            this.Text = "Custom Button";

            Form FileLoadForm = new Form();
            FileLoadForm.Controls.Add(new Button());
            FileLoadForm.ShowDialog();
        }
    }

Answer 1

Perhaps the window is hidden behind it.

[ToolboxItem(true)] // This attribute makes the control visible in the Toolbox
public class CustomButton : Button
{
    [DllImport("user32.dll", CharSet = CharSet.Auto)]
    [return: MarshalAs(UnmanagedType.Bool)]
    private static extern bool SetForegroundWindow(IntPtr hWnd);

    // Constructor
    public CustomButton() {
        // Set default properties
        this.BackColor = Color.Blue;
        this.ForeColor = Color.White;
        this.Text = "Custom Button";

        Form FileLoadForm = new Form();
        FileLoadForm.Controls.Add(new Button());
        FileLoadForm.HandleCreated += (s, e) => {
            SetForegroundWindow(((Control?)s)!.Handle);
        };
        FileLoadForm.ShowDialog();
    }
}