title: Extjs - Dragging window in Viewport
body: <p>I got a viewport and within the center region a window.
  When I try to move the window to the top of the center region, the window doesn't stay on the top.
  I note that the window is positioned at the top + the height of the north region.</p>
  
  <p>This is the code:</p>
  
  <pre><code>Ext.onReady(function(){
  
      Ext.create('Ext.container.Viewport', {
  
          layout: 'border',
          forceFit: true,
          hideMode: 'offsets',
          items: [
          {
              region:'north',
              id:'btns',
              height: 150,
              layout:
              {
                  type: 'hbox',
                  padding:'5',
                  align:'stretch'
              },
              items:[]
          }, {
              region: 'east',
              autoScroll: true,
              border: false,
              items:[{
                  xtype: 'container',
                  id: 'page_gauche1',
                  width: 500,
                  height: 920,
                  border: false
              }]
          }, {
              region: 'center',
              id: 'center',
              autoScroll: true,
              border: true,
              items: {
                  xtype: 'container',
                  id: 'page_gauche4',
                  width: 1400,
                  height: 920,
                  border: false,
                  hidden: true
              }
          }]
      });
  
      Ext.create('Ext.Window', {
  
          layout: 'fit',
          border: false,
          header: true,
          draggable: true,
          resizable: true,
          autoShow: true,
          constrain: true,
          renderTo: 'center',
          width: 300,
          height: 200
      });
  
  });
  </code></pre>