Handle drag shape that be overlap - wpf

Question

i 'm doing a program that user could drag this shape.
The shape is:
It could be joined each other for making new block.
But the problem is, when i drag two [Blue] Shape into the [If] Shape, i cant not drag the [Blue] anymore, cause of the [If] Shape is collapsed and make the [Blue] Shape cant not be interact.
Here is my code:
The define of the shape
<UserControl x:Class="SampleCode.ActionShape"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:local="clr-namespace:SampleCode"
xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"
mc:Ignorable="d" >
<UserControl.Resources>
<local:ColorToBrushConverter x:Key="colorToBrushConverter"/>
<local:ThicknessConverter x:Key="thicknessConverter"/>
</UserControl.Resources>
<Grid>
<Path Data="{Binding SvgData}"
Fill="{Binding Color, Converter={StaticResource colorToBrushConverter}}"
Cursor="Hand">
</Path>
<TextBlock Foreground="{Binding TextColor, Converter={StaticResource colorToBrushConverter}}" FontSize="{Binding FontSize}" Margin="{Binding MarginText}" Text="{Binding Text}"></TextBlock>
</Grid>
</UserControl>
The mainwindow.xaml
<Window x:Class="SampleCode.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:SampleCode"
Title=""
Width="1400"
Height="800"
Loaded="Window_Loaded">
<Window.Resources>
<local:ColorToBrushConverter x:Key="ColorToBrushConverter"/>
<DataTemplate DataType="{x:Type local:BaseShapeMVVm}">
<local:ActionShape AllowDrop="True" MouseDown="Rectangle_MouseDown" MouseMove="Rectangle_MouseMove" MouseUp="Rectangle_MouseUp"></local:ActionShape>
</DataTemplate>
</Window.Resources>
<Window.DataContext>
<local:ViewModel />
</Window.DataContext>
<Grid>
<TreeView Name="ListBox" ItemsSource="{Binding Rectangles}">
<ItemsControl.ItemsPanel>
<ItemsPanelTemplate>
<Canvas />
</ItemsPanelTemplate>
</ItemsControl.ItemsPanel>
<ItemsControl.ItemContainerStyle>
<Style TargetType="TreeViewItem">
<Setter
Property="Canvas.Left" Value="{Binding X}"/>
<Setter
Property="Canvas.Top" Value="{Binding Y}"/>
</Style>
</ItemsControl.ItemContainerStyle>
</TreeView>
</Grid>
</Window>
could you give me some recommendation for this problem? i 'm very appoxiated