I have solved my problem.
I have modified the main window's XAML markup.
I have declared the notification user control and the grid containing main window's child controls in the same cell of a new grid.
I have set some properties in the notification user control's opening tag :
- Grid.ZIndex="1"
- HorizontalAlignment="Right"
- VerticalAlignment="Bottom"
- Margin="0 0 20 20"
When the notification user control is visible :
- The notification user control is over the grid containing main window's child controls
- The notification user control is in the bottom right corner of the main window