Steps to reproduce

1. run index.zul with IE10 (http://zkfiddle.org/sample/10skd2k/16-Combobox-with-big-list#source-1)
   index.zul

   <zk>
     <vbox apply="org.zkoss.bind.BindComposer"  viewModel="@id('vm') @init('foo.ViewModel')">
       <label value="In IE the combobox has a wrong behavior when we opened the list of options if you are inside a window in modal mode and the window dimensions are set in% and if the component is positioned at the end of the window with scroll" />
       <button label="Open Window with dimensions setted in %" onClick="@command('openWindow1')" />
       <button label="Open Window with dimensions setted in px" onClick="@command('openWindow2')" />
     </vbox>
   </zk>
   

   package foo;
   
   import org.zkoss.zk.ui.*;
   import org.zkoss.zk.ui.event.*;
   import org.zkoss.zk.ui.util.*;
   import org.zkoss.zk.ui.ext.*;
   import org.zkoss.zk.au.*;
   import org.zkoss.zk.au.out.*;
   import org.zkoss.zul.*;
   import org.zkoss.bind.annotation.*;
   import java.util.Arrays;
   import java.util.TimeZone;
   
   public class ViewModel {
   	
     private ListModel<String> timeZones;
     
      
     public ListModel<String> getTimeZones(){
       return new ListModelList<String>(Arrays.asList(TimeZone.getAvailableIDs()));  
     }
     
     @Command
     public void openWindow1(){
       Executions.createComponents("window1.zul", null, null);  
     }
     
     @Command
     public void openWindow2(){
       Executions.createComponents("window2.zul", null, null);  
     }
     
   }
   

window1.zul

<zk>
 <window border="normal" mode="modal"  height="20%" title="Window with dimensions setted in %" apply="org.zkoss.bind.BindComposer"  viewModel="@id('vm') @init('foo.ViewModel')" contentStyle="overflow-y:scroll;"
  closable="true">
  <vbox>
    <label value="Window with dimensions setted in %"/>
    <space />
    <space />  	
    <space />
    <space />
    <combobox model="@load(vm.timeZones)" />
  </vbox>
 </window>

</zk>

window2.zul

<zk>
  <window border="normal" mode="modal" height="155px" title="Window with dimensions setted in px" apply="org.zkoss.bind.BindComposer"  viewModel="@id('vm') @init('pkg$.ViewModel')" contentStyle="overflow-y:scroll;"
    closable="true">
   <vbox>
      <label value="Window with dimensions setted in px"/>
      <space />
      <space />  
     <space />
     <space />
      <combobox model="@load(vm.timeZones)" />
  </vbox>
 </window> 
  
</zk>

1. open combobox

Actual result

dropdown list's position is wrong. Its CSS style top is wrong calculated.

Debug information

• If you set height with a fixed pixel e.g. height="155px" , the issue doesn't happen.
• When the combobox is near the Window's bottom border, its offsetHeight is incorrect. try with document.querySelector('.z-combobox-input').getBoundingClientRect()

Workaround

Increase the window's height

<window  height="25%" ...>