Library elements that display margin constraints (e.g. StackPane) let the user navigate left to right as follows: Top, Right, Bottom, Left. A more natural flow would be L R T B or even T L R B
Library elements that display AnchorPane constraints (e.g. Separator) let the user navigate from top to bottom as follows: L R T B

We should change the navigation flow of the margins to be consistent with the Anchor constraints navigation: L R T B