NSSplitViewItem.CollapseBehavior.preferResizingSplitViewWithFixedSiblings
The item’s preference is to keep the other panes at their current size and position onscreen, potentially growing or shrinking the window in the direction to best preserve that.
Declaration
case preferResizingSplitViewWithFixedSiblingsDiscussion
The split view item breaks this preference in full-screen mode, and to keep the window fully onscreen during resizing.