Sometimes when user clears one selected value on combobox, the binding doesn't update the value in viewmodel.
In atachmment I send one video taken from your combobox demo page demonstrating this problem:
http://www.zkoss.org/zkdemo/combobox/simple_combobox

Additional information (MDuchemin):
When selecting a item from the combobox, the combobox value is updated, but not the _defRawVal.
In InputWidget:updateChange_, the current value is compared to _defRawValue, and the onChange event is not sent if they are equal

                        if (!this._reVald && n == this._defRawVal) {
                            return false
                        }

Steps to reproduce in a combobox:
1-Enter a value (empty string or any string), and trigger an updateChange_ (by clicking outside of the combobox), _defRawVal will be updated with the current value and onchange event is sent to the server.
2-Select a combobox item. Combobox text / value is updated, onSelect event is sent to the server, but _defRawVal is not updated.
3-Enter the same value used in step 1. Because value is equal to _defRawVal, onChange event is not sent when clicking out of the combobox.

reproduced with : 8.0.1.FL.20151105-Eval