UIElements in the GameView

using System;
using System.Reflection;
using UnityEditor;
using UnityEngine;
using UnityEngine.UIElements;

public class ScreenDragTest : MonoBehaviour
{
    public Vector2 ScreenPos = new Vector2(100, 100);
    public bool ConstantRefresh;
}

class SquareDragger : MouseManipulator
{
    private Vector2 m_Start;
    protected bool m_Active;

    public SquareDragger()
    {
        activators.Add(new ManipulatorActivationFilter { button = MouseButton.LeftMouse });
        m_Active = false;
    }

    protected override void RegisterCallbacksOnTarget()
    {
        target.RegisterCallback<MouseDownEvent>(OnMouseDown);
        target.RegisterCallback<MouseMoveEvent>(OnMouseMove);
        target.RegisterCallback<MouseUpEvent>(OnMouseUp);
    }

    protected override void UnregisterCallbacksFromTarget()
    {
        target.UnregisterCallback<MouseDownEvent>(OnMouseDown);
        target.UnregisterCallback<MouseMoveEvent>(OnMouseMove);
        target.UnregisterCallback<MouseUpEvent>(OnMouseUp);
    }

    protected void OnMouseDown(MouseDownEvent e)
    {
        if (m_Active)
        {
            e.StopImmediatePropagation();
            return;
        }

        if (CanStartManipulation(e))
        {
            m_Start = e.localMousePosition;

            m_Active = true;
            target.CaptureMouse();
            e.StopPropagation();
        }
    }

    protected void OnMouseMove(MouseMoveEvent e)
    {
        if (!m_Active || !target.HasMouseCapture())
            return;

        Vector2 diff = e.localMousePosition - m_Start;

        target.style.top = target.layout.y + diff.y;
        target.style.left = target.layout.x + diff.x;

        e.StopPropagation();
    }

    protected void OnMouseUp(MouseUpEvent e)
    {
        if (!m_Active || !target.HasMouseCapture() || !CanStopManipulation(e))
            return;

        m_Active = false;
        target.ReleaseMouse();
        e.StopPropagation();
    }
}

[CustomEditor(typeof(ScreenDragTest))]
class ScreenDragTestEditor : Editor
{
    protected virtual void OnEnable()
    {
        System.Reflection.Assembly assembly = typeof( UnityEditor.EditorWindow ).Assembly;
        System.Type type = assembly.GetType( "UnityEditor.GameView" );
        var gameView = EditorWindow.GetWindow(type);

        var myImage = new Image(){ image = Texture2D.whiteTexture, tintColor = Color.red };
        gameView.rootVisualElement.Add(myImage);

        var screenDrag = target as ScreenDragTest;
        myImage.BringToFront();
        myImage.style.width = 20;
        myImage.style.height = 20;
        myImage.style.top = screenDrag.ScreenPos.y;
        myImage.style.left = screenDrag.ScreenPos.x;

        var manipulator = new SquareDragger();
        myImage.AddManipulator(manipulator);
    }
}

Just an example of how we can embed custom editor elements into the GameView