这在我的示例应用程序上有效。

<Grid RowDefinitions="Auto,Auto">
    <TextBlock Text="Test" Grid.Row="0" />
    <Grid
        Grid.Row="1"
        ColumnDefinitions="Auto,Auto"
        RowDefinitions="Auto,Auto">
        <ComboBox
            Grid.Row="1"
            Grid.Column="0"
            VerticalAlignment="Stretch"
            CornerRadius="0"
            SelectedIndex="0">
            <TextBlock Text="Auswahlen" />
        </ComboBox>
        <Button
            Grid.Row="1"
            Grid.Column="1"
            VerticalAlignment="Stretch"
            CornerRadius="0">
            <FontIcon
                FontFamily="Segoe MDL2 Assets"
                Glyph="&#xE819;" />
        </Button>
    </Grid>
</Grid>