Allows the delegate to determine whether a divider can be dragged or adjusted off the edge of the split view.
- macOS 10.5+
The split view that sent the message.
The zero-based index of the divider.
true if the divider should allow dragging off the edge of the split view, resulting in it not being visible.
If a split view has no delegate, or if its delegate does not respond to this message, the split view behaves as if it has a delegate that returns
false when sent this message.