A behavior script for Unity: for moving an object in a rect shape by turning it at a right angle using a quaternion when outed a certain range.

RotateMoveBehaviour.cs

using UnityEngine;

public class CubeBehavior : MonoBehaviour
{
    const float threshold = 1f;
    private readonly Quaternion quaternion = Quaternion.AngleAxis(90, Vector3.down);

    public Vector3 direction = Vector3.forward * 0.1f;

    // Update is called once per frame
    void Update()
    {
        var previous = transform.position;
        var next = previous + direction;
        if (Mathf.Max(Mathf.Abs(next.x), Mathf.Abs(next.z)) >= threshold)
        {
            direction = quaternion * direction;
        }
        else {
            transform.position = next;
        }
    }
}