MainWindow.xaml

<Window x:Class="ChatBubbles.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="ChatBubbles" Height="774.023" Width="500
        "
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"        
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
        xmlns:local="clr-namespace:ChatBubbles"
        mc:Ignorable="d" d:DesignWidth="480"
        d:DataContext="{d:DesignData Source=MessageCollectionDesignTime.xml}"
        FontFamily="Verdana"
        FontSize="12"
        Foreground="White" Background="Black" WindowStyle="ToolWindow" WindowStartupLocation="CenterScreen"
        >

    <Grid x:Name="ContentPanel" Grid.Row="1" Margin="12,0,12,0" VerticalAlignment="Stretch">
        <Grid local:GridUtils.RowDefinitions=",Auto">

            <ScrollViewer x:Name="ConversationScrollViewer"
                          HorizontalScrollBarVisibility="Hidden"
                          VerticalScrollBarVisibility="Hidden">
                <StackPanel Orientation="Vertical"
                      x:Name="ConversationContentContainer"
                      VerticalAlignment="Top">
                    <local:ConversationView x:Name="conversationView"/>
                </StackPanel>
            </ScrollViewer>

            <Grid Grid.Row="1"
              local:GridUtils.RowDefinitions=",,"
              Margin="0,10,0,0">
                <Rectangle Fill="White"
                    Grid.RowSpan="2"/>
                <TextBox Text=""
                    TextWrapping="Wrap"
                    AcceptsReturn="True"
                    Padding="0"
                    Height="40"
                    BorderThickness="0"
                    x:Name="TextInput" RenderTransformOrigin="0.5,1.588" TextChanged="TextInput_TextChanged"
                    GotFocus="TextInput_GotFocus"
                    LostFocus="TextInput_LostFocus" 
                    PreviewKeyDown="TextInput_PreviewKeyDown"/>
                <Path Data="m 0,0 l 16,0 l 0,16 l -16,-16"
                Fill="White"
                Margin="0,0,5,0"
                HorizontalAlignment="Right"
                Grid.Row="2"/>
            </Grid>
        </Grid>

    </Grid>
</Window>