Listbox is not updated in webclient

[japink.m]

The user can add and delete items to a valuelist, by adding record to or deleting record from foundset. Valuelist is based on same table as foundset.
In smartclient the edits are directly updated in the listbox. In webclient the listbox is not update or after a while after some user action. But a combobox based on the same valuelist is directly updated. Why this difference between web/smart and listbox/combobox?
And what can I do about it. UpdateUI isn't working. Is there a way to force listbox update in webclient?

[jcompagner]

please make a case for this with a small sample solution. there should not be any difference