subtreeHasCompositionCallbacks property Null safety
Whether the subtree rooted at this layer has any composition callback observers.
This only evaluates to true if the subtree rooted at this node has observers. For example, it may evaluate to true on a parent node but false on a child if the parent has observers but the child does not.
bool get subtreeHasCompositionCallbacks => _compositionCallbackCount > 0;