example/dist/bundle.js
function (topLevelType, targetInst, nativeEvent, nativeEventTarget) {
    var isOverEvent = topLevelType === TOP_MOUSE_OVER || topLevelType === TOP_POINTER_OVER;
    var isOutEvent = topLevelType === TOP_MOUSE_OUT || topLevelType === TOP_POINTER_OUT;

    if (isOverEvent && (nativeEvent.relatedTarget || nativeEvent.fromElement)) {
      return null;
    }

    if (!isOutEvent && !isOverEvent) {
      // Must not be a mouse or pointer in or out - ignoring.
      return null;
    }

    var win = void 0;
    if (nativeEventTarget.window === nativeEventTarget) {
      // `nativeEventTarget` is probably a window object.
      win = nativeEventTarget;
    } else {
      // TODO: Figure out why `ownerDocument` is sometimes undefined in IE8.
      var doc = nativeEventTarget.ownerDocument;
      if (doc) {
        win = doc.defaultView || doc.parentWindow;
      } else {
        win = window;
      }
    }

    var from = void 0;
    var to = void 0;
    if (isOutEvent) {
      from = targetInst;
      var related = nativeEvent.relatedTarget || nativeEvent.toElement;
      to = related ? getClosestInstanceFromNode(related) : null;
    } else {
      // Moving to a node from outside the window.
      from = null;
      to = targetInst;
    }

    if (from === to) {
      // Nothing pertains to our managed components.
      return null;
    }

    var eventInterface = void 0,
        leaveEventType = void 0,
        enterEventType = void 0,
        eventTypePrefix = void 0;

    if (topLevelType === TOP_MOUSE_OUT || topLevelType === TOP_MOUSE_OVER) {
      eventInterface = SyntheticMouseEvent;
      leaveEventType = eventTypes$2.mouseLeave;
      enterEventType = eventTypes$2.mouseEnter;
      eventTypePrefix = 'mouse';
    } else if (topLevelType === TOP_POINTER_OUT || topLevelType === TOP_POINTER_OVER) {
      eventInterface = SyntheticPointerEvent;
      leaveEventType = eventTypes$2.pointerLeave;
      enterEventType = eventTypes$2.pointerEnter;
      eventTypePrefix = 'pointer';
    }

    var fromNode = from == null ? win : getNodeFromInstance$1(from);
    var toNode = to == null ? win : getNodeFromInstance$1(to);

    var leave = eventInterface.getPooled(leaveEventType, from, nativeEvent, nativeEventTarget);
    leave.type = eventTypePrefix + 'leave';
    leave.target = fromNode;
    leave.relatedTarget = toNode;

    var enter = eventInterface.getPooled(enterEventType, to, nativeEvent, nativeEventTarget);
    enter.type = eventTypePrefix + 'enter';
    enter.target = toNode;
    enter.relatedTarget = fromNode;

    accumulateEnterLeaveDispatches(leave, enter, from, to);

    return [leave, enter];
  }
function (topLevelType, targetInst, nativeEvent, nativeEventTarget) {
    var isOverEvent = topLevelType === TOP_MOUSE_OVER || topLevelType === TOP_POINTER_OVER;
    var isOutEvent = topLevelType === TOP_MOUSE_OUT || topLevelType === TOP_POINTER_OUT;

    if (isOverEvent && (nativeEvent.relatedTarget || nativeEvent.fromElement)) {
      return null;
    }

    if (!isOutEvent && !isOverEvent) {
      // Must not be a mouse or pointer in or out - ignoring.
      return null;
    }

    var win = void 0;
    if (nativeEventTarget.window === nativeEventTarget) {
      // `nativeEventTarget` is probably a window object.
      win = nativeEventTarget;
    } else {
      // TODO: Figure out why `ownerDocument` is sometimes undefined in IE8.
      var doc = nativeEventTarget.ownerDocument;
      if (doc) {
        win = doc.defaultView || doc.parentWindow;
      } else {
        win = window;
      }
    }

    var from = void 0;
    var to = void 0;
    if (isOutEvent) {
      from = targetInst;
      var related = nativeEvent.relatedTarget || nativeEvent.toElement;
      to = related ? getClosestInstanceFromNode(related) : null;
    } else {
      // Moving to a node from outside the window.
      from = null;
      to = targetInst;
    }

    if (from === to) {
      // Nothing pertains to our managed components.
      return null;
    }

    var eventInterface = void 0,
        leaveEventType = void 0,
        enterEventType = void 0,
        eventTypePrefix = void 0;

    if (topLevelType === TOP_MOUSE_OUT || topLevelType === TOP_MOUSE_OVER) {
      eventInterface = SyntheticMouseEvent;
      leaveEventType = eventTypes$2.mouseLeave;
      enterEventType = eventTypes$2.mouseEnter;
      eventTypePrefix = 'mouse';
    } else if (topLevelType === TOP_POINTER_OUT || topLevelType === TOP_POINTER_OVER) {
      eventInterface = SyntheticPointerEvent;
      leaveEventType = eventTypes$2.pointerLeave;
      enterEventType = eventTypes$2.pointerEnter;
      eventTypePrefix = 'pointer';
    }

    var fromNode = from == null ? win : getNodeFromInstance$1(from);
    var toNode = to == null ? win : getNodeFromInstance$1(to);

    var leave = eventInterface.getPooled(leaveEventType, from, nativeEvent, nativeEventTarget);
    leave.type = eventTypePrefix + 'leave';
    leave.target = fromNode;
    leave.relatedTarget = toNode;

    var enter = eventInterface.getPooled(enterEventType, to, nativeEvent, nativeEventTarget);
    enter.type = eventTypePrefix + 'enter';
    enter.target = toNode;
    enter.relatedTarget = fromNode;

    accumulateEnterLeaveDispatches(leave, enter, from, to);

    return [leave, enter];
  }
For almost every interaction we care about, there will be both a top-level
`mouseover` and `mouseout` event that occurs. Only use `mouseout` so that
we do not extract duplicate events. However, moving the mouse into the
browser from outside will not fire a `mouseout` event. In this case, we use
the `mouseover` top-level event.