What is the purpose of the "second click" selection mode in the interface builder?

Question

When I click a UI element while designing a XIB, the UI element is selected like this:

If I click the same element again it is selected like this:

What is the purpose of this special selection mode?

Answer 1

Well, this is just a mechanism for traversing the view hierarchy (and lock focus on nested subviews). While it seems that there's no difference between two modes in your example (except of course the lack of the resize handles in the 'focused' one) consider a more complicated object such as a tableview on OSX (notice also how the light blue color indicates the view being edited while the focus ring indicates its superview's frame - also note that this drill-down is based on the location of the mouse pointer. In this example in order to travel all the way down to the cell we have to click inside the bounds of the innermost element we're aiming at):

1st Click - Selects the outer element / container (ScrollView)

2nd Click - Selects the TableView

3rd Click - Selects the TableColumn

4th Click - Selects the TableCell