Undo and redo

An UndoManager you own (the kit's `_kit/undo.ts`): undo and redo buttons, Ctrl/Cmd+Z and Shift+Ctrl/Cmd+Z, and the list of steps. A splitter drag is one step, and the content keeps its state across undo.

• UndoManager
• model.subscribe
• layout.load
• useModelState
• useSyncExternalStore