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<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute" xmlns:local="*">
<mx:Canvas width="100%" height="700"/>
</mx:Application>
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"Flex considers scroll bars in its sizing calculations only if you explicitly set the scroll policy to ScrollPolicy.ON. So, if you use an auto scroll policy (the default), the scroll bar overlaps the buttons. To prevent this behavior, you can set the height property for the HBox container or allow the HBox container to resize by setting a percentage-based width. Remember that changing the height of the HBox container causes other components in your application to move and resize according to their own sizing rules."
-- from Sizing Components in the Flex 3 help, under "Using Scroll bars"
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// In your Application £¬module or wherever you need this workaround.
override public function validateSize(recursive:Boolean = false):void {
super.validateSize(recursive);
if (!initialized) return;
if (height < measuredHeight) verticalScrollPolicy = ScrollPolicy.ON;
else verticalScrollPolicy = ScrollPolicy.OFF;
}