The Placement property of Popup describes the placement of where a Popup control appears on the screen.
If you do not set any positioning or placement of a popup, the popup will appear in the left top corner of the window.
The PlacementMode enumeration represents the Placement property. The following code snippet sets the Placement property to AbsolutePoint.
<Popup Name="Popup1" HorizontalAlignment="Left"
VerticalAlignment="Top" Width="200" Height="50"
IsOpen="True" Placement="AbsolutePoint" >
<TextBlock Name="McTextBlock" Background="LightBlue" >
This is popup text
</TextBlock>
</Popup>