steps to reproduce

http://zkfiddle.org/sample/3jt28e/3-combobox-onChange-not-processed-onOpen
run fiddle
enter text in the combobox
without loosing focus from the input field, click the combobox button
combobox onOpen will trigger

actual result

popup open, combobox value = last known value (previously sent value, not current input field string)

expected result

popup open, combobox value = input field string value

debug info

Value is sent in the onOpen event data. Should be visible from server side.

root cause

Workaround

Use instant="true" to force sending the value