如果您没有特定的理由使用stackpane作为根，那么可以使用anchorpane。你可以为你的左，右堆叠窗格设置3个侧锚，并将堆叠窗格的宽度与你的根宽度绑定，所以如果你调整你的窗口大小，你的堆叠窗格将适应它

import javafx.application.Application;
import javafx.geometry.Insets;
import javafx.geometry.Pos;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.*;
import javafx.scene.paint.Color;
import javafx.stage.Stage;

public class NotesApplication extends Application {
    @Override
    public void start(Stage stage) {
        AnchorPane root = new AnchorPane(); // Changed stackpane to anchor pane
        root.setPrefSize(700,700);

        StackPane leftPane = new StackPane();
        leftPane.minWidthProperty().bind(root.widthProperty().divide(2)); // Binding leftPane's width with the half of your root

        Button button = new Button("Button 1");
        leftPane.setBackground(new Background(new BackgroundFill(Color.YELLOW, CornerRadii.EMPTY, Insets.EMPTY)));
        leftPane.getChildren().add(button);
        leftPane.setAlignment(Pos.CENTER_LEFT);

        StackPane rightPane = new StackPane();
        rightPane.minWidthProperty().bind(root.widthProperty().divide(2)); // Binding rightPane's width with the half of your root

        Button button1 = new Button("Button 2");
        rightPane.getChildren().add(button1);
        rightPane.setAlignment(Pos.CENTER_RIGHT);
        rightPane.setBackground(new Background(new BackgroundFill(Color.RED, CornerRadii.EMPTY, Insets.EMPTY)));

        root.getChildren().addAll(leftPane, rightPane);

        // Setting the anchors 
        root.setLeftAnchor(leftPane, 0.0);
        root.setTopAnchor(leftPane, 0.0);
        root.setBottomAnchor(leftPane, 0.0);

        root.setRightAnchor(rightPane, 0.0);
        root.setTopAnchor(rightPane, 0.0);
        root.setBottomAnchor(rightPane, 0.0);

        Scene scene = new Scene(root);
        stage.setScene(scene);
        stage.show();
    }

    public static void main(String[] args) {
        launch(args);
    }
}