Gets or sets a value indicating whether the item is open: its children are shown. An item without a positive Count is closed.
public bool IsOpen { get; set; }
ISO 32000-2 §12.3.3, Table 151. Setting it rewrites the item's Count with the matching sign (its magnitude recomputed
from the children) and the Count of every ancestor and of the outline that shows the item's descendants. An item without
children has no Count to hold the state, so setting it then does nothing; an item opens when it gets its first child.