<StackPanel Background="Transparent" ... >
or
<Border Background="Transparent" ... >
should do the trick…
This makes the grid transparent, however visible for mouse-clicks.
Look here form more Information.
There are some other circumstances, where you have to use the PreviewXXX– event, this is when a child element “swallows” the event away, but in your case, the above should be what you’re looking for.