When focus is on a combobutton, "space" or "enter" key presses are not handled
(space or enter trigger "onclick' on normal buttons)
There is also no behavior to open the drop down menu.

steps to reproduce
select the combobutton (right click, then escape) then press enter / space / direction keys

<zk>
<window border="normal" title="hello" apply="org.zkoss.bind.BindComposer">
<hbox hflex="1">
<button onClick='alert("handle event")' label="Button" focus="true" />
<combobutton onClick='alert("handle event")' label="ComboButton" tabindex="1">
<menupopup>
<menuitem label="Item 1" />
</menupopup>
</combobutton>
</hbox>
</window>
</zk>