MDPSplitView

The collapsible subview size the owner actually wants (0 = collapsed, negative = unknown, which disables restoring). Set it whenever a size is chosen on purpose: state restoration, a user action, or an explicit API call. Divider drags update it automatically on mouse-up. Assumes the collapsible subview is the one at index 1 (after the divider).

Repositions the divider to match intendedCollapsibleSize. Does nothing while the user drags, an animation runs, the size already matches, or the split view is (transiently) too small to honor it.

Whether the divider at the given index is currently being animated.

Set the position of a divider, possibly with an animation.

Helper method to toggle a (horizontally split) split view’s subview.

Helper method to toggle a (vertically split) split view’s subview.