How to use the @lumino/widgets.BoxLayout.setStretch function in @lumino/widgets

hsplitPanel.addWidget(dockPanel);
    hsplitPanel.addWidget(rightHandler.stackedPanel);

    hboxPanel.addWidget(leftHandler.sideBar);
    hboxPanel.addWidget(hsplitPanel);
    hboxPanel.addWidget(rightHandler.sideBar);

    rootLayout.direction = 'top-to-bottom';
    rootLayout.spacing = 0; // TODO make this configurable?
    // Use relative sizing to set the width of the side panels.
    // This will still respect the min-size of children widget in the stacked
    // panel.
    hsplitPanel.setRelativeSizes([1, 2.5, 1]);

    BoxLayout.setStretch(headerPanel, 0);
    BoxLayout.setStretch(topHandler.panel, 0);
    BoxLayout.setStretch(hboxPanel, 1);
    BoxLayout.setStretch(bottomPanel, 0);

    rootLayout.addWidget(headerPanel);
    rootLayout.addWidget(topHandler.panel);
    rootLayout.addWidget(hboxPanel);
    rootLayout.addWidget(bottomPanel);

    // initially hiding header and bottom panel when no elements inside
    this._headerPanel.hide();
    this._bottomPanel.hide();

    this.layout = rootLayout;

    // Connect change listeners.
    this._tracker.currentChanged.connect(this._onCurrentChanged, this);

hboxPanel.addWidget(leftHandler.sideBar);
    hboxPanel.addWidget(hsplitPanel);
    hboxPanel.addWidget(rightHandler.sideBar);

    rootLayout.direction = 'top-to-bottom';
    rootLayout.spacing = 0; // TODO make this configurable?
    // Use relative sizing to set the width of the side panels.
    // This will still respect the min-size of children widget in the stacked
    // panel.
    hsplitPanel.setRelativeSizes([1, 2.5, 1]);

    BoxLayout.setStretch(headerPanel, 0);
    BoxLayout.setStretch(topHandler.panel, 0);
    BoxLayout.setStretch(hboxPanel, 1);
    BoxLayout.setStretch(bottomPanel, 0);

    rootLayout.addWidget(headerPanel);
    rootLayout.addWidget(topHandler.panel);
    rootLayout.addWidget(hboxPanel);
    rootLayout.addWidget(bottomPanel);

    // initially hiding header and bottom panel when no elements inside
    this._headerPanel.hide();
    this._bottomPanel.hide();

    this.layout = rootLayout;

    // Connect change listeners.
    this._tracker.currentChanged.connect(this._onCurrentChanged, this);
    this._tracker.activeChanged.connect(this._onActiveChanged, this);

hsplitPanel.addWidget(leftHandler.stackedPanel);
    hsplitPanel.addWidget(dockPanel);
    hsplitPanel.addWidget(rightHandler.stackedPanel);

    hboxPanel.addWidget(leftHandler.sideBar);
    hboxPanel.addWidget(hsplitPanel);
    hboxPanel.addWidget(rightHandler.sideBar);

    rootLayout.direction = 'top-to-bottom';
    rootLayout.spacing = 0; // TODO make this configurable?
    // Use relative sizing to set the width of the side panels.
    // This will still respect the min-size of children widget in the stacked
    // panel.
    hsplitPanel.setRelativeSizes([1, 2.5, 1]);

    BoxLayout.setStretch(headerPanel, 0);
    BoxLayout.setStretch(topHandler.panel, 0);
    BoxLayout.setStretch(hboxPanel, 1);
    BoxLayout.setStretch(bottomPanel, 0);

    rootLayout.addWidget(headerPanel);
    rootLayout.addWidget(topHandler.panel);
    rootLayout.addWidget(hboxPanel);
    rootLayout.addWidget(bottomPanel);

    // initially hiding header and bottom panel when no elements inside
    this._headerPanel.hide();
    this._bottomPanel.hide();

    this.layout = rootLayout;

    // Connect change listeners.

hsplitPanel.addWidget(rightHandler.stackedPanel);

    hboxPanel.addWidget(leftHandler.sideBar);
    hboxPanel.addWidget(hsplitPanel);
    hboxPanel.addWidget(rightHandler.sideBar);

    rootLayout.direction = 'top-to-bottom';
    rootLayout.spacing = 0; // TODO make this configurable?
    // Use relative sizing to set the width of the side panels.
    // This will still respect the min-size of children widget in the stacked
    // panel.
    hsplitPanel.setRelativeSizes([1, 2.5, 1]);

    BoxLayout.setStretch(headerPanel, 0);
    BoxLayout.setStretch(topHandler.panel, 0);
    BoxLayout.setStretch(hboxPanel, 1);
    BoxLayout.setStretch(bottomPanel, 0);

    rootLayout.addWidget(headerPanel);
    rootLayout.addWidget(topHandler.panel);
    rootLayout.addWidget(hboxPanel);
    rootLayout.addWidget(bottomPanel);

    // initially hiding header and bottom panel when no elements inside
    this._headerPanel.hide();
    this._bottomPanel.hide();

    this.layout = rootLayout;

    // Connect change listeners.
    this._tracker.currentChanged.connect(this._onCurrentChanged, this);
    this._tracker.activeChanged.connect(this._onActiveChanged, this);

.catch(e => {
          // Show a revealed promise error.
          const error = new Widget();
          // Show the error to the user.
          const pre = document.createElement('pre');
          pre.textContent = String(e);
          error.node.appendChild(pre);
          BoxLayout.setStretch(error, 1);
          this.node.removeChild(spinner.node);
          spinner.dispose();
          content.dispose();
          this._content = null;
          toolbar.dispose();
          this._toolbar = null;
          layout.addWidget(error);
          this._isRevealed = true;
          throw error;
        });
    } else {

constructor(options: MainAreaWidget.IOptions) {
    super(options);
    this.addClass('jp-MainAreaWidget');
    this.id = DOMUtils.createDomID();

    const content = (this._content = options.content);
    const toolbar = (this._toolbar = options.toolbar || new Toolbar());
    const spinner = this._spinner;

    const layout = (this.layout = new BoxLayout({ spacing: 0 }));
    layout.direction = 'top-to-bottom';
    BoxLayout.setStretch(toolbar, 0);
    BoxLayout.setStretch(content, 1);
    layout.addWidget(toolbar);
    layout.addWidget(content);

    if (!content.id) {
      content.id = DOMUtils.createDomID();
    }
    content.node.tabIndex = -1;

    this._updateTitle();
    content.title.changed.connect(this._updateTitle, this);
    this.title.closable = true;
    this.title.changed.connect(this._updateContentTitle, this);

    if (options.reveal) {
      this.node.appendChild(spinner.node);
      this._revealed = options.reveal

constructor(options: MainAreaWidget.IOptions) {
    super(options);
    this.addClass('jp-MainAreaWidget');
    this.id = DOMUtils.createDomID();

    const content = (this._content = options.content);
    const toolbar = (this._toolbar = options.toolbar || new Toolbar());
    const spinner = this._spinner;

    const layout = (this.layout = new BoxLayout({ spacing: 0 }));
    layout.direction = 'top-to-bottom';
    BoxLayout.setStretch(toolbar, 0);
    BoxLayout.setStretch(content, 1);
    layout.addWidget(toolbar);
    layout.addWidget(content);

    if (!content.id) {
      content.id = DOMUtils.createDomID();
    }
    content.node.tabIndex = -1;

    this._updateTitle();
    content.title.changed.connect(this._updateTitle, this);
    this.title.closable = true;
    this.title.changed.connect(this._updateContentTitle, this);

    if (options.reveal) {
      this.node.appendChild(spinner.node);