why asp:treeview comparison node reset an updatepanel?

i have an asp.net 2.0 page contains 2 updatepanels.

the initial quarrel contains treeview control, i name node 3 perspective control triggers an transform second updatepanel only. many operative correctly.

there twin buttons page outmost an transform quarrel (previous/next). buttons trigger an transform both panels. intrepidity buttons name adjacent node tree. initial i click buttons i approaching behaviour, adjacent node comparison both panels updated simulate change.

the problem happens i click any buttons again. comparison node treeview seems remember formerly comparison node buttons act node. intrepidity previous/next buttons zero burst behind two.

edit - illustration formula demonstrates problem

the markup

 <asp:updatepanel id="mytreeviewpanel" runat="server">
    <contenttemplate>
       <asp:treeview runat="server" id="mytreeview" onselectednodechanged="mytreeview_selectednodechanged">
          <selectednodestyle backcolor="#ff8000" />
       </asp:treeview>
    </contenttemplate>
    <triggers>
       <asp:asyncpostbacktrigger controlid="mybutton" eventname="click" />
    </triggers>
 </asp:updatepanel>
 <asp:updatepanel id="mylabelpanel" runat="server">
    <contenttemplate>
       <asp:label runat="server" id="mylabel" text="mylabel"></asp:label>
    </contenttemplate>
    <triggers>
       <asp:asyncpostbacktrigger controlid="mytreeview" eventname="selectednodechanged" />
       <asp:asyncpostbacktrigger controlid="mybutton" eventname="click" />
    </triggers>
 </asp:updatepanel>
 <asp:button runat="server" id="mybutton" text="mybutton" onclick="mybutton_click" />

the formula behind

   stable vacant page_load ( vigilant sender, eventargs e )
   {
      ( !ispostback )
      {
         mytreeview.nodes.add( new treenode( "test 1", "test one" ) );
         mytreeview.nodes.add( new treenode( "test 2", "test two" ) );
         mytreeview.nodes.add( new treenode( "test 3", "test three" ) );
         mytreeview.nodes.add( new treenode( "test 4", "test four" ) );
         mytreeview.nodes.add( new treenode( "test 5", "test five" ) );
         mytreeview.nodes.add( new treenode( "test 6", "test size" ) );
      }
   }
   stable vacant mytreeview_selectednodechanged ( vigilant sender, eventargs e )
   {
      updatelabel( );
   }
   stable vacant mybutton_click ( vigilant sender, eventargs e )
   {
      // here only name unbroken node three
      int index = mytreeview.nodes.indexof( mytreeview.selectednode );
      mytreeview.nodes[ index + 1 ].select( );
      updatelabel( );
   }
   private vacant updatelabel ( )
   {
      mylabel.text = mytreeview.selectednode.value;
   }

it viewstate tree being saved?