Combo box not behaving like it should

Question

I have a form in which I have 4 combo boxes that have more or less the same choices, however there is one that is common to all 4 (E078).
When "E078" is selected in any of the combo box, a new combo box will appear (and a white rectangle will be hidden so that the new combo box title can be seen). It works when I select that in one or more combo box, however to make it disappear I have to select some other choice in the same combo box twice!!! If I select anything only once, it will not make the new combo box hidden, but if re-select something (same or another choice) then the combo box will disappear!
I am at lost, and would be very thankful if someone could make it work seamlessly. I mean it works, nut it is just inconvenient and I cannot tell the user to make select something else twice if they want the field to disappear.
Here is my form:
https://workspaces.acrobat.com/?d=WjF6aV8IUuBX2w8iZN3k0g

Answer 1

When you use event.value in a combo box's Validate event it does not give you the export value, which is what you want. It is easier to use place the code in a custom Keystroke script where you can use the event.changeEx property to retrieve the export value of the selected item. See the documentation for more information: http://livedocs.adobe.com/acrobat_sdk/9.1/Acrobat9_1_HTMLHelp/JS_API_AcroJS.88.607.html