This signal is emitted when an item that has the delayedExpanding flag set is about to be expanded. The delayedExpanding flag is not reset; the slot that the signal is connected to should do so if necessary. The item being expanded is passed to the slot. The slot gets the opportunity to insert child items into that item. It should not change the item any other way. It can disallow the expansion by setting allow to false; in this case the item is not expanded.
The signal is always emitted, regardless whether the expansion was triggered by the user or by the program.
Referenced by expandSubTree().