title: Fullcalendar: draggable object rejects fullcalendar as droppable even though fullcalendar accepts drop
body: <p>I have set up FullCalendar to accept drops, which it does.  But the draggable object, which I have constructed with revert:'invalid' does not seem to recognize the dates on FullCalendar as droppable, and reverts back.  Here is my code:</p>
  
  <pre><code>&lt;!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"&gt;
  &lt;html&gt;&lt;head&gt;
  
    &lt;meta content="text/html; charset=ISO-8859-1" http-equiv="content-type"&gt;
    &lt;title&gt;mydrag&lt;/title&gt;
    &lt;script src="fullcalendar-bundle.js" type="text/javascript"&gt;&lt;/script&gt;
  &lt;/head&gt;&lt;body&gt;
  &lt;div id="mydrag" style="width: 200px; height: 100px; background-color: red;"&gt;My Drag&lt;/div&gt;
  &lt;div id="calendar"&gt;&lt;/div&gt;
  
  
  &lt;script type="text/javascript"&gt;
  function onExternalEventDrop(date, allDay) {
      alert("Dropped on " + date + " with allDay=" + allDay);
  }
  
  $('#mydrag').each(function() {
  
      // create an Event Object (http://arshaw.com/fullcalendar/docs/event_data/Event_Object/)
      // it doesn't need to have a start or end
      var eventObject = {
          title: 'MyDrag Title'
      };
  
      // store the Event Object in the DOM element so we can get to it later
      $(this).data('eventObject', eventObject);
  
      // make the event draggable using jQuery UI
      $(this).draggable({
          helper: 'clone',
          //revert: 'invalid',
          revert: function(droppableObj) {
              //if false then no socket object drop occurred.
              if(droppableObj === false) {
                  //revert the .myselector object by returning true
                  alert('Not a droppable object');
                  return true;
              }
              else {
                  //droppableObj was returned,
                  //we can perform additional checks here if we like
                  //alert(droppableObj.attr('id')); would work fine
                  //return false so that the .myselector object does not revert
                  return false;
              }
           },
          revertDuration: 500,  //  original position after the drag
          start: function(e, ui) {
              $(ui.helper).css('width', $(this).css('width'));
          }
      });
  
  });
  
  $('#calendar').fullCalendar({
      aspectRatio: 2.25,
      header: {
          left: '',
          center: 'title',
          right: 'prev,next'
      },
      columnFormat: {
          month: 'dddd'
      },
      droppable: true,
      drop: onExternalEventDrop
  });
  
  &lt;/script&gt;
  &lt;/body&gt;&lt;/html&gt;
  </code></pre>
  
  <p>When I drag the draggable element onto the calendar, the element reverts back (suggesting that the calendar date was not recognized as a valid droppable)....but the drop callback <em>is</em> triggered with the expected alert (suggesting that FullCalendar recognized the draggable as valid).  I would expect that the draggable should not revert back.  Am I doing or expecting something wrong?  I have searched all over, but not found anything to explain this.  Any help would be greatly appreciated.</p>
  
  <p>Update: Forgot to mention, what I have called "fullcalendar-bundle.js" is a file containing the following:</p>
  
  <ul>
  <li>jquery 1.5.2</li>
  <li>jquery ui 1.8.11</li>
  <li>fullcalendar 1.5.2 plugin</li>
  </ul>
  
  <p>Another update: I have just tried the FullCalendar 1.5.3 release, but see the same behavior.</p>