collapseIcon property
The icon displayed when TreeNodeScope.node is collapsed and is not a Leaf. Defaults to NodeWidgetLeadingIcon.kCollapseIcon.
If you use a custom collapseIcon, make sure to give it a Key so that flutter animates the widget swap correctly. You can use the default key: NodeWidgetLeadingIcon.kCollapseIconKey.
Implementation
final Widget? collapseIcon;