Mouse clicks on a canvas

I have the following XAML:

<ScrollViewer HorizontalScrollBarVisibility="Auto" VerticalScrollBarVisibility="Auto" Grid.Row="1" x:Name="DiagramScrollViewer">
    <Canvas x:Name="LayoutCanvas" MouseLeftButtonDown="LayoutCanvas_MouseLeftButtonDown" MouseLeftButtonUp="LayoutCanvas_MouseLeftButtonUp"
            MouseMove="LayoutCanvas_MouseMove">
    </Canvas>
</ScrollViewer>

 
I then add things to this canvas programmatically.  Now, here is something I have noticed: Let's say I have a canvas that is 400x400 in size, and there are 2 things on the canvas.  "Thing 1" is located at the top left of the canvas and is 100x100 in size.  "Thing 2" is located at the bottom right of the canvas and is also 100x100 in size.
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