mx:TextInput disabled in Flex 4

we have big application written in flex 3 , being ported flex 4. our plan keep using mx: components existing code , gradually start using spark: components new code. 

on login screen have

     <mx:textinput id="username">

when running in flex 4 can see input, can't type in it. behaves
if disabled.

when in debugger username.enabled == true and
username.textfield.enabled == true. it's behaving if disabled.

 

on page:

http://help.adobe.com/en_us/flex/using/wsda78ed3a750d6b8f-26a13bbf123c441239d-8000.html

saw looked promising , tried setting

project > properties > flex compiler > use flash text engine in mx components

 

... still didn't work.  textinput still acts disabled.  has else run problem?  thanks.

is there specific reason why not using <s:textinput ?

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