For example,
<?xml version="1.0" encoding="UTF-8"?>
<window title="Popup window" border="normal" width="200px">
<menupopup id="menupopup" onOpen='menupopup.getChildren().clear();menupopup.appendChild(new Menuitem("Long caption Long caption Long caption Long caption Long caption"));' >
<menuitem label="Item 1 - short"/>
</menupopup>
<label context="menupopup" value="Right-click me!"/>
</window>
Fixed since Jan. 21.