关于wpf窗体里面的小窗口，可以拖动大小解决方法

关于wpf窗体里面的小窗口，可以拖动大小
像VS里面的解决方案资源管理器一样可以左右拖动，这个样wpf该怎样实现。。

楼主的实训作业就是想把上图的
TreeView做成可以拖动大小的
下面是布局代码，有点长

<Window x:Class="CustomerManage.MainWindow"

        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

        Title="MainWindow" Height="428" Width="525">

    <Grid>

        <Menu Height="26" VerticalAlignment="Top">

        </Menu>

        <TabControl Margin="0,30,0,0">

            <!--客户信息-->

            <TabItem Header="客户信息">

                <Grid>

                    <Grid.ColumnDefinitions>

                        <ColumnDefinition Width="100"/>

                        <ColumnDefinition/>

                    </Grid.ColumnDefinitions>

                    <Grid HorizontalAlignment="Center" VerticalAlignment="Top" Height="300" Margin="0,10,0,0">

                        <Grid.RowDefinitions>

                            <RowDefinition/>

                            <RowDefinition/>

                            <RowDefinition/>

                            <RowDefinition/>

                            <RowDefinition/>

                            <RowDefinition/>

                        </Grid.RowDefinitions>

                        <Button Content="添  加" Width="80" Margin="3,10,3,10" />

                        <Button Content="删  除" Grid.Row="1" Width="80" Margin="3,10,3,10" />

                        <Button Content="修  改" Grid.Row="2" Width="80" Margin="3,10,3,10" />

                        <Button Content="查  找" Grid.Row="3" Width="80" Margin="3,10,3,10" />

                        <Button Content="按部门显示" Grid.Row="4" Width="80" Margin="3,10,3,10" />

                        <Button Content="按公司显示" Grid.Row="5" Width="80" Margin="3,10,3,10" />

                        <Border BorderBrush="Silver" BorderThickness="2" Grid.RowSpan="6"/>

                    </Grid>

                    <DataGrid Grid.Column="1" Margin="2,10,2,2"/>

                </Grid>


            </TabItem>

            <!--员工信息-->

            <TabItem Header="员工信息">