title: p:dialog appears on bottom when p:blockUI targets it
body: <p>I have a PrimeFaces dialog, that has two command buttons that executes some code in the backing bean. I want to block the dialog within the action. </p>
  
  <p>I managed to do it using blockUI, but when the blockUI is present, and I open the dialog, it appears at the bottom of the page.</p>
  
  <p>If I remove the blockUI component, the dialog opens at the center of the page, as I want. But I want it to be centered and with the blockUI.</p>
  
  <pre><code>        &lt;p:dialog header="Attention" id="dialog" position="center"
              widgetVar="dialog" modal="true" closable="false"
              dynamic="true" closeOnEscape="false"&gt;
  
          &lt;div class="internal-margin-top"&gt;
  
                  &lt;h:outputText value="Location" styleClass="ui-outputtext" /&gt;
                  &lt;p:inputText value="#{activityBean.location}"
                      id="inputLocation" maxlength="15"&gt;
                  &lt;/p:inputText&gt;
  
              &lt;/div&gt;
              &lt;div class="internal-margin-bottom"&gt;
                  &lt;p:commandButton id="closureYes" value="Yes"
                      styleClass="btn-green"
                      onstart="PF('block').show();"
                      oncomplete="PF('dialog').hide(); PF('block').hide();"
                      action="#{activityBean.processItem()}" process="@all"&gt;
                  &lt;/p:commandButton&gt;
                  &lt;p:commandButton id="closureNo" value="No"
                      styleClass="btn-red"
                      onstart="PF('block').show();"
                      oncomplete="PF('dialog').hide(); PF('block').hide();"
                      action="#{activityBean.processActivity()}" process="@all" /&gt;
              &lt;/div&gt;
          &lt;/p:dialog&gt;
  
              &lt;p:blockUI block="scrapDialog" widgetVar="block"&gt;
                  &lt;p:graphicImage library="images" name="loading_bar.gif" /&gt;
              &lt;/p:blockUI&gt;
  </code></pre>
  
  <p>Thanks in advance.</p>