<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2008/06/12/rounding-specific-corners-on-a-focus-rectangle-in-the-textinput-control-in-flex/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="middle"
        backgroundColor="white">

    <mx:TextInput id="textInput"
            focusThickness="10"
            cornerRadius="10"
            focusRoundedCorners="tl br" />

</mx:Application>