code: public static void updateActorPosition(final Actor actor, final Vector2 newScreenSizeInStageCoords) {
          if (actor != null) {
              updateActorPosition(actor, actor.getStage(), newScreenSizeInStageCoords);
          }
      }
docs: When called BEFORE resizing the stage, moves the actor to match the same aspect ratio as before. Useful for
  windows and dialogs in screen viewports.
  
  @param actor will be repositioned. Can be null, method invocation will be ignored.
  @param newScreenSizeInStageCoords screen coords processed by stage.