Process primefaces menu before show - jsf

Question

I have a primefaces menu which items are disabled on a condition from backing bean.
The problem is, that the conditions are processed after the update.
If I select a node which has every item enabled and switch to a node where everything is disabled,
the menu is reloaded but shows everything enabled for a second before the items are disabled.
Is there a way to process the disabled-conditions before the menu is shown?
<h:form id="form">
<p:tree id="tree" ...>
<p:ajax event="select" update=":form:tree:menu" listener...>
<p:treeNode id="treenode">
<p:commandButton id="btn" type="button" />
<p:overlayPanel for="btn">
<p:menu id=menu">
<p:menuitem .... disabled="#{bean.condition1}" />
<p:menuitem .... disabled="#{bean.condition2}" />
...
</p:menu>
</p:overlayPanel>
</p:treenode>
</p:tree>

Answer 1

EDITED
Hide the menu items at the beggining of the ajax call and show them when it is completed. The source code in the page would be:
<p:tree id="tree" value="#{bean.root}" var="node" selectionMode="checkbox">
<p:treeNode id="treenode">
<p:commandButton id="btn" value="#{node}"
onstart="$('.ui-overlaypanel-content').css('display','none');"
oncomplete="$('.ui-overlaypanel-content').css('display','block');"
actionListener="#{bean.processAndUpdateConditions}" update="mymenu"/>
<p:overlayPanel for="btn">
<p:menu id="mymenu" >
<p:menuitem value="Condition 1" disabled="#{bean.condition1}" />
<p:menuitem value="Condition 2" disabled="#{bean.condition2}" />
</p:menu>
</p:overlayPanel>
</p:treeNode>
</p:tree>