Example of drawing an overlay over another UI Element.

UIElement_OverlayExample.cs

using UnityEngine;
using UnityEngine.UIElements;
 
public class Example : MonoBehaviour
{
    public UIDocument document;
 
    void Start()
    {
        var document = GetComponent<UIDocument>();
        var progressBar = document.rootVisualElement.Q<ProgressBar>();
 
        var progressBarOverlay = new VisualElement { style = { flexDirection = FlexDirection.Row } };
        document.rootVisualElement.Add(progressBarOverlay);
 
        progressBar.RegisterCallback<GeometryChangedEvent>(evt =>
        {
            progressBarOverlay.style.position = Position.Absolute;
       
            progressBarOverlay.style.top = evt.newRect.y;
            progressBarOverlay.style.left = evt.newRect.x;
            progressBarOverlay.style.width = evt.newRect.width;
            progressBarOverlay.style.height = evt.newRect.height;
            progressBarOverlay.BringToFront();
        });
 
        // Add some markers
        for (int i = 0; i < 10; i++)
        {
            var marker = new VisualElement
            {
                style =
                {
                    backgroundColor = new StyleColor(Color.green),
                    width = 15,
                    height = Length.Percent(100),
                    left = Length.Percent(10 * i),
                }
            };
            progressBarOverlay.Add(marker);
        }
    }
}

More info here https://forum.unity.com/threads/creating-a-markable-progress-bar-timeline-in-unity-inspector-and-editor.1596972/#post-9865482