Simple Unity script to create a scrolling marquee.

Marquee.cs

using UnityEngine;

public class Marquee : MonoBehaviour
{
    public string message = "Where we're going, we don't need roads.";
    public float scrollSpeed = 50;
    
    Rect messageRect;
    
    void OnGUI ()
    {
        // Set up message's rect if we haven't already.
        if (messageRect.width == 0) {
            var dimensions = GUI.skin.label.CalcSize(new GUIContent(message));
            
            // Start message past the left side of the screen.
            messageRect.x = -dimensions.x;
            messageRect.width = dimensions.x;
            messageRect.height = dimensions.y;
        }
        
        messageRect.x += Time.deltaTime * scrollSpeed;
        
        // If message has moved past the right side, move it back to the left.
        if (messageRect.x > Screen.width) {
            messageRect.x = -messageRect.width;
        }
        
        GUI.Label(messageRect, message);
    }
}

I've recently run into this and with the new UIDocuments the above method didn't work well for me. This was my fix. Hope it helps someone.

    private IMGUIContainer container;
    private const float scroll_speed = 0.001f;

    private void OnEnable()
    {
        container = GetComponent<UIDocument>().rootVisualElement.Q<IMGUIContainer>("footer");
        StartCoroutine(AutoScroll());
    }
    private IEnumerator AutoScroll()
    {
        float position = 100f;
        while (position >= -100f)
        {
            container.style.left = Length.Percent(position);
            position = (position <= -9f) ? 100f : position -= 0.01f;
            yield return new WaitForSeconds(scroll_speed);
        }
    }

from there you can do stuff like container.Add(message_label); assuming Label message_label;