WrapPanelStretchLastChildInNewRow.xaml

<Page x:Class="PrimitivesTests.WrapPanelStretchLastChildInNewRow"
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      xmlns:controls="using:CommunityToolkit.WinUI.Controls"
      xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
      xmlns:local="using:PrimitivesTests.WrapPanel"
      xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
      Background="{ThemeResource ApplicationPageBackgroundThemeBrush}"
      mc:Ignorable="d">

    <Grid>
        <controls:WrapPanel Width="132"
                            StretchChild="Last">
            <Rectangle Width="44"
                       Height="44"
                       Fill="Red" />
            <Rectangle Width="44"
                       Height="44"
                       Fill="Blue" />
            <Rectangle Width="44"
                       Height="44"
                       Fill="Green" />
            <Rectangle Height="44"
                       Fill="Orange" />
        </controls:WrapPanel>
    </Grid>

</Page>