VoxelEditor

using UnityEngine;
using UnityEditor;

[CustomEditor(typeof(Transform))]
public class VoxelEditor : Editor
{
    // Called when you click any Transform in the scene
    void OnEnable ()
    {
        // remove a possibly existing listener
        SceneView.duringSceneGui -= OnScene;
        // add a listener
        SceneView.duringSceneGui += OnScene;
    }

    // In examples it is recommended to uninstall the delegate listener
    // But we need [CustomEditor(typeof(Transform))] and this means Unity will call our Script whenever a Transform
    // is getting selected. If we want to detect clicks anywhere (so that this listens to ALL OnScene calls) we simply
    // keep the listener installed. This won't overflow because we uninstall listeners before installing new ones. 
    // But if your Editor ever crashes or throws errors this miiight be related. But I think it's pretty secure.
    // And if you are fine with clicking on Transforms only then uncomment the code in OnDisable below. 
    // If you do, clicks in empty scene-areas are not detected.
    private void OnDisable()
    {
        // SceneView.duringSceneGui -= OnScene;
    }
    private static void OnScene(SceneView sceneview)
    {
        // uncomment if you want to make sure it runs:
        // Debug.Log("OnScene !");
        
        if (Event.current.type == EventType.MouseDown)
        {
            // This kind of works, but results in wrong coordinates, maybe because of the Window offset or something.
            //Ray ray = sceneview.camera.ScreenPointToRay(Event.current.mousePosition);

            // This works fine as it gets you the correct coordinates
            Vector2 mousePosition = Event.current.mousePosition;
            Ray ray = HandleUtility.GUIPointToWorldRay(mousePosition);
            
            RaycastHit hit;
            if (Physics.Raycast(ray, out hit))
            {
                Debug.Log(hit.point.x.ToString());
                // Draw a pointing upwards where we clicked. Just to make sure we SEE that the coordinates are correct.
                Debug.DrawLine(hit.point, hit.point + Vector3.up * 0.3f, Color.red, 0.1f);
            }
        }
    }
}