Libgdx Table's height is 0 after adding actors

Question

I want to add background to my Table with Labels, but Table's height = 0. For this reason background's height is 0 too.

tooltipGroup = new Table(skin);
tooltipGroup.setWidth(w/6);
Label.LabelStyle headerStyle = new Label.LabelStyle(headerFont, Color.GREEN);
Label headerLabel = new Label(effect.getName(), headerStyle);
headerLabel.setWidth(w/6);
headerLabel.setX(20);
headerLabel.setWrap(true);
headerLabel.setAlignment(Align.topLeft);
tooltipGroup.add(headerLabel).width(w/6).row();
Label.LabelStyle style = new Label.LabelStyle(font, Color.WHITE);
Label descriptionLabel = new Label(effect.getDescription(), style);
descriptionLabel.setWidth(w/6);
descriptionLabel.setWrap(true);
descriptionLabel.setAlignment(Align.topLeft);
descriptionLabel.setPosition(20, -descriptionLabel.getHeight());
tooltipGroup.add(descriptionLabel).width(w/6).fill(true);
tooltipGroup.setPosition(mouseX, h - mouseY);
Drawable background = skin.getDrawable("tooltip_background");
tooltipGroup.setBackground(background);
stage.addActor(tooltipGroup);

Answer 1

Try to use pack() method on Table before calling getHeight()

tooltipGroup.pack();
table_height = tooltipGroup.getHeight();