title: Modal window background doesn`t resize properly
body: <p>I have a problem with modal window in my application. All is good, except when resizing modal window mask: when browser size decreases, they increase. Modal window mask stays at a smaller window size.</p>
  
  <p><img src="https://i.stack.imgur.com/tYIM3.png" alt=""></p>
  
  <p>Here is my viewport</p>
  
  <pre><code>Ext.define('Bis.view.Viewport', {
      extend: 'Ext.container.Viewport',
      layout: 'border',
      items: [{
              region: 'north',
              xtype: 'container',
              height: 100,
              layout: 'border',
              items:[{
                  xtype: 'container',
                  layout: 'border',
                  region: 'north',
                  height: 70,
                  items: [{
                      xtype: 'container',
                      region: 'west',
                      styleHtmlCls: 'account-header-logo',
                      styleHtmlContent: true
                  }, {
                      xtype: 'component',
                      region: 'center'
                  }, {
                      xtype: 'container',
                      itemId: 'userControlsContainer',
                      region: 'east'
                  }]
              }, {
                  region: 'center'
              }]
  
          }, {
              region: 'center',
              bodyStyle: 'background:#79b5d8;'
          }, {
              region: 'south',
              xtype: 'container',
              items:[{
                  xtype: 'component',
                  width: 150,
                  styleHtmlCls: 'footer-logo',
                  styleHtmlContent: true
              }]
           }
      ]
  });
  </code></pre>
  
  <p>Modal window adding not to the viewport, but to the region: 'center'. When adding to the viewport, there are no such problems, but I need to add window to this region </p>