Change Canvas cornerRadius and borderStyle
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute">
<mx:Script>
[Bindable]
private var dp:Array =[
{label:'A', data:1},
{label:'B', data:2},
{label:'C', data:3},
{label:'D', data:4}
];
</mx:Script>
<mx:Canvas x="426" width="200" height="200" backgroundColor="#ffffff" cornerRadius="8" borderStyle="solid">
<mx:ComboBox x="10" y="10" dataProvider="{dp}" width="180"></mx:ComboBox>
<mx:DataGrid x="10" y="40" width="180" height="120" dataProvider="{dp}">
<mx:columns>
<mx:DataGridColumn headerText="Id" dataField="data" width="30" />
<mx:DataGridColumn headerText="Artist" dataField="label" />
</mx:columns>
</mx:DataGrid>
<mx:Button x="10" y="166" label="Button"/>
</mx:Canvas>
</mx:Application>
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