Pane
   VBox
   StackPane
      WebView

AnchorPane
   VBox
   StackPane
      WebView

<AnchorPane fx:id="anchor" maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="600.0" prefWidth="800.0" xmlns="http://javafx.com/javafx/9.0.1" xmlns:fx="http://javafx.com/fxml/1" fx:controller="application.BrowserController">
   <children>
      <VBox fx:id="vbox" prefHeight="600.0" prefWidth="200.0" style="-fx-background-color: red;" />
<StackPane fx:id="subPane" alignment="CENTER_RIGHT" AnchorPane.bottomAnchor="0.0" AnchorPane.leftAnchor="0.0" AnchorPane.rightAnchor="0.0" AnchorPane.topAnchor="0.0">
         <children>
            <WebView fx:id="webview" prefWidth="600.0" />
         </children>
      </StackPane>
   </children>
</AnchorPane>