public final ObjectProperty<EventHandler<Event>> onInitializedProperty() {
    return onInitialized;
  }
Enables to define actions that should be performed when an overlay is initialized.

@return the {@link ObjectProperty} of the {@link EventHandler} which will be triggered upon
        initialization
@implNote an overlay is being considered as initialized, when the {@code overlay} node has been
          added to the scene graph and a layout pass has been performed, which means the values
          of {@link Region#widthProperty()} and {@link Region#heightProperty()} are defined.
          This enables to initially set the position of an overlay using its size dynamically,
          for example before an animation is being performed for the first time.<br>
          <b>This event will only be fired once!</b>
workbenchfx-core/src/main/java/com/dlsc/workbenchfx/model/WorkbenchOverlay.java