How to test if a Widget has been rendered if it's initially outside of the viewport

I've got this custom Widget component:

class CollectAssetDataButton extends StatelessWidget {
  const CollectAssetDataButton({ Key key }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return RaisedButton(
        child: Text(
      context
          .translate(I18NKeys.appointmentDetails.collectAssetsData)
          .toUpperCase(),
    ),
    onPressed: () {},
  );
  }
}

And I'm trying to test it using:

  final collectAssetsButtonFinder = find.byKey(WidgetKeysAppointmentDetails.collectAssetsButton);
  expect(collectAssetsButtonFinder, findsOneWidget);

as per a suggestion in https://github.com/flutter/flutter/issues/54651

But I'm still getting:

══╡ EXCEPTION CAUGHT BY FLUTTER TEST FRAMEWORK ╞════════════════════════════════════════════════════
The following TestFailure object was thrown running a test:
  Expected: exactly one matching node in the widget tree
  Actual: _KeyFinder:<zero widgets with key [<'___appointment_details_collect_assets_button___'>]
(ignoring offstage widgets)>
   Which: means none were found but one was expected

Thanks in advance!