A single constraint column that occupies 20% of the Canvas width; 80% of the container is unallocated:

           

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<!-- constraints\BasicColumn_20Percent.mxml -->
<s:Application xmlns:fx="http://ns.adobe.com/mxml/2009"
    xmlns:mx="library://ns.adobe.com/flex/mx"
    xmlns:s="library://ns.adobe.com/flex/spark">
    <s:layout>
        <s:VerticalLayout />
    </s:layout>
    <mx:Canvas>
        <mx:constraintColumns>
            <mx:ConstraintColumn id="col1" width="20%" />
        </mx:constraintColumns>
    </mx:Canvas>
    <!-- Position child components based on 
        the constraint column. -->
</s:Application>

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A single constraint column that occupies 20% of the Canvas width; 80% of the container is unallocated: : Layout « Container « Flex

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