The Spacer class extends the Region class and provides functionality to create flexible spaces in layouts such as HBox and VBox. It is primarily used to push adjacent nodes apart or together by filling up available space.
The Spacer can be toggled between active and inactive states. When active, it tries to grow as much as possible within its parent container. When inactive, it collapses and doesn't take up any space.
The growth direction of the Spacer (horizontal or vertical) is determined based on its parent container. For instance, when placed inside an HBox, the Spacer will grow horizontally. Conversely, inside a VBox, it will grow vertically.
The active state of the Spacer can also be controlled through CSS with the "-fx-active" property.