如何在fabricJS中通过鼠标选择被覆盖的对象？

var canvas = new fabric.Canvas("c");

fabric.util.addListener(canvas.upperCanvasEl, "dblclick", function (e) {
  var _canvas = canvas;
  var _mouse = _canvas.getPointer(e);
  var _active = _canvas.getActiveObject();
    
  if (e.target) {
    var _targets = _canvas.getObjects().filter(function (_obj) {
      return _obj.containsPoint(_mouse);
    });
      
    //console.warn(_targets);
      
    for (var _i=0, _max=_targets.length; _i<_max; _i+=1) {
      //check if target is currently active
      if (_targets[_i] == _active) {
       	//then select the one on the layer below
       	_targets[_i-1] && _canvas.setActiveObject(_targets[_i-1]);
         break;
        }
      }
    }
});

canvas
  .add(new fabric.Rect({
    top: 25,
    left: 25,
    width: 100,
    height: 100,
    fill: "red"
  }))
  .add(new fabric.Rect({
    top: 50,
    left: 50,
    width: 100,
    height: 100,
    fill: "green"
  }))
  .add(new fabric.Rect({
    top: 75,
    left: 75,
    width: 100,
    height: 100,
    fill: "blue"
  }))
  .renderAll();

canvas {
 border: 1px solid;
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/fabric.js/1.6.3/fabric.min.js"></script>
<canvas id="c" width="300" height="200"></canvas>

var canvas = new fabric.Canvas("c");

canvas.on("object:selected", function (e) {
  if (e.target) {
    e.target.bringToFront();
    this.renderAll();
  }
});

var _prevActive = 0;
var _layer = 0;

//
fabric.util.addListener(canvas.upperCanvasEl, "dblclick", function (e) {
    var _canvas = canvas;
    //current mouse position
    var _mouse = _canvas.getPointer(e);
    //active object (that has been selected on click)
    var _active = _canvas.getActiveObject();
    //possible dblclick targets (objects that share mousepointer)
    var _targets = _canvas.getObjects().filter(function (_obj) {
        return _obj.containsPoint(_mouse) && !_canvas.isTargetTransparent(_obj, _mouse.x, _mouse.y);
    });
    
    _canvas.deactivateAll();
      
    //new top layer target
    if (_prevActive !== _active) {
        //try to go one layer below current target
        _layer = Math.max(_targets.length-2, 0);
    }
    //top layer target is same as before
    else {
        //try to go one more layer down
        _layer = --_layer < 0 ? Math.max(_targets.length-2, 0) : _layer;
    }

    //get obj on current layer
    var _obj = _targets[_layer];

    if (_obj) {
    	_prevActive = _obj;
    	_obj.bringToFront();
    	_canvas.setActiveObject(_obj).renderAll();
    }
});

//create something to play with
canvas
  //fully covered rect is selectable with dblclicks
  .add(new fabric.Rect({
    top: 75,
    left: 75,
    width: 50,
    height: 50,
    fill: "black",
    stroke: "black",
    globalCompositeOperation: "xor",
    perPixelTargetFind: true
  }))
  .add(new fabric.Circle({
    top: 25,
    left: 25,
    radius: 50,
    fill: "rgba(255,0,0,.5)",
    stroke: "black",
    perPixelTargetFind: true
  }))
  .add(new fabric.Circle({
    top: 50,
    left: 50,
    radius: 50,
    fill: "rgba(0,255,0,.5)",
    stroke: "black",
    perPixelTargetFind: true
  }))
  .add(new fabric.Circle({
    top: 75,
    left: 75,
    radius: 50,
    fill: "rgba(0,0,255,.5)",
    stroke: "black",
    perPixelTargetFind: true
  }))
  .renderAll();

canvas {
 border: 1px solid;
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/fabric.js/1.6.4/fabric.min.js"></script>
<canvas id="c" width="300" height="200"></canvas>