The `NSStackView` is an OS X AppKit class introduced in OS X 10.9. It represents a horizontal or vertical stack of views.
From Apple's docs:
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NSStackView) is used to layout horizontal or vertical stacks of views using auto layout. Necessary constraints will automatically be created and modified when adding and removing views from a stack view to maintain a cohesive layout. This behavior is extended by customizing various properties of the stack view and the views it holds. Decreasing the clipping resistance priority allows the stack view to become smaller than the the minimum required to hold the provided views. Depending on the visibility priority associated with the internal views, they may either overflow and clip off the side, or drop from the view hierarchy. These dropped views are still retained by the stack view, and will be reattached if the stack view becomes the necessary size. Views can be forced to be dropped or reattached by setting the necessary visibility priority.
