Did you intentionally add the blank title bar on top?
When the object is type of Border it can have CornerRadius propery
<Style x:Key="PathPanelGroupBorder" TargetType="{x:Type Border}">
...
<Setter Property="CornerRadius" Value="4" />
</Style>
or individual 4 values "4,0,0,4"
"topLeft,topRight,bottomRight,bottomLeft"