Combined Geometry is not fully implemented.

XAML snippet bellow results in different outcome in MS Blend and in XAML Toy ( or in ue4)

<Grid
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" mc:Ignorable="d"

d:DesignWidth="1280" d:DesignHeight="720">

<Grid x:Name="LayoutRoot">
<Border Background="#00D1C1">
<Border.Clip>

<CombinedGeometry GeometryCombineMode="Exclude">
<CombinedGeometry.Geometry1>
<RectangleGeometry RadiusX="6" RadiusY="6" Rect="0,0,350,83"/>
</CombinedGeometry.Geometry1>
<CombinedGeometry.Geometry2>
<EllipseGeometry RadiusX="10" RadiusY="10" Center="3,3"/>
</CombinedGeometry.Geometry2>
</CombinedGeometry>

</Border.Clip>
</Border>
</Grid>
</Grid>