Place a button in a TitledPane and it will fill the control.

Amy's comment on RT-17226 says "Buttons have a default max size clamped to their preferred size, which is why the buttons don't expand to fill the tiles by default. " However, in this case, the button does expand without unclamping the max size of the button.

As a workaround, you can put the button inside a StackPane inside the TitledPane and it won't resize itself.

Example attached.

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.TitledPane;
import javafx.scene.layout.StackPane;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class TitledPaneWidthTest extends Application {
  public static void main(String[] args) {
    Application.launch(args);
  }

  @Override
  public void start(Stage primaryStage) {
    primaryStage.setTitle("Titled Pane Width Test");

    final Button b1 = new Button("Hit Me B1");
    final TitledPane p1 = new TitledPane("Button should be skinnier", b1);

    final StackPane stackPane = new StackPane();
    final Button b2 = new Button("Hit Me B2");
    stackPane.getChildren().add(b2);
    final TitledPane p2 = new TitledPane("Button in StackPane so ok", stackPane);

    // display the scene.
    final VBox root = new VBox();
    root.setSpacing(10);
    root.getChildren().addAll(p1, p2);
    Scene scene = new Scene(root, 500, 300);
    primaryStage.setScene(scene);
    primaryStage.show();
  }
}