带有鼠标中键奇怪行为的EventType.MouseDown

Question

这种行为我不清楚。 在MonoBehaviour中，我有以下代码：

void OnGUI() {
   Event e = Event.current;
   Vector3 mousePosition = e.mousePosition;
   mousePosition.y = Screen.height - mousePosition.y;
   Ray ray = cam.ScreenPointToRay(mousePosition);
   RaycastHit hitInfo;
   if (Physics.Raycast(ray, out hitInfo)) {
      if (e.rawType == EventType.MouseDown && e.button == 0) {
          Debug.Log("DOWN 0");
       } else if (e.rawType == EventType.MouseDown && e.button == 1) {
          Debug.Log("DOWN 1");
       } else if (e.rawType == EventType.MouseDown && e.button == 2) {
          Debug.Log("DOWN 2");
       } else if (e.rawType == EventType.MouseUp && e.button == 0) {
          Debug.Log("UP 0");
       } else if (e.rawType == EventType.MouseUp && e.button == 1) {
          Debug.Log("UP 1");
       } else if (e.rawType == EventType.MouseUp && e.button == 2) {
          Debug.Log("UP 2");
       }
   }
}

所有情况都有效

if (e.rawType == EventType.MouseDown && e.button == 2) {
      Debug.Log("DOWN 2");
}

是否存在无法拦截鼠标中键向下的原因？