Overlap container hide header row on resize - fix height width of some container - resize

Question

I am having fxml, which have top row where all my update / delete / save buttons resides. Beneath it I am having one TreeTableView where my data display.
Problem is when I am resing my window from top to bottom my header row disappear/missing/not showing/hidden/ovelap. I am finding the solution where from certain widht/height user cannot resize the window and whatever component is essential those can be visible after resize from anywhere.
Here is my code:
<?xml version="1.0" encoding="UTF-8"?>
<?import javafx.scene.*?>
<?import javafx.collections.*?>
<?import javafx.scene.control.cell.*?>
<?import javafx.scene.shape.*?>
<?import javafx.scene.image.*?>
<?import javafx.scene.text.*?>
<?import javafx.geometry.*?>
<?import java.lang.*?>
<?import javafx.scene.control.*?>
<?import javafx.scene.layout.*?>
<?import javafx.scene.control.TreeTableView?>
<?import javafx.scene.control.Accordion?>
<?import javafx.scene.control.TitledPane?>
<?import javafx.scene.shape.*?>
<?import javafx.scene.image.*?>
<?import javafx.scene.text.*?>
<?import javafx.geometry.*?>
<?import java.lang.*?>
<?import javafx.scene.layout.*?>
<?import javafx.scene.layout.VBox?>
<?import javafx.scene.layout.HBox?>
<?import javafx.scene.control.TableColumn?>
<?import javafx.scene.control.cell.PropertyValueFactory?>
<?import javafx.collections.FXCollections ?>
<?import javafx.scene.control.CustomMenuItem?>
<VBox prefHeight="780.0" prefWidth="1120.0" stylesheets="#../Cls.css" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1">
<children>
<AnchorPane id="topAnchopane">
<children>
<HBox fillHeight="false" nodeOrientation="RIGHT_TO_LEFT" prefHeight="27.0" prefWidth="400.0" AnchorPane.bottomAnchor="0.0" AnchorPane.leftAnchor="0.0" AnchorPane.rightAnchor="0.0" AnchorPane.topAnchor="3.0" StackPane.alignment="TOP_RIGHT">
<children>
<Button fx:id="tmpl_reset" minHeight="16.0" minWidth="16.0" mnemonicParsing="false" onAction="#onResetColumnConfig" prefHeight="16.0" prefWidth="16.0" text="">
<cursor>
<Cursor fx:constant="HAND" />
</cursor>
<styleClass>
<String fx:value="icon" />
<String fx:value="clear" />
</styleClass>
<HBox.margin>
<Insets left="5.0" top="4.0" />
</HBox.margin>
</Button>
<Button fx:id="tmpl_clear" minHeight="16.0" minWidth="16.0" mnemonicParsing="false" onAction="#onClearTemplate" prefHeight="16.0" prefWidth="16.0" text="">
<cursor>
<Cursor fx:constant="HAND" />
</cursor>
<styleClass>
<String fx:value="icon" />
<String fx:value="reset" />
</styleClass>
<HBox.margin>
<Insets left="5.0" top="4.0" />
</HBox.margin>
</Button>
<Button fx:id="tmpl_delete" minHeight="16.0" minWidth="16.0" mnemonicParsing="false" onAction="#onDeleteTemplate" prefHeight="16.0" prefWidth="16.0" text="">
<cursor>
<Cursor fx:constant="HAND" />
</cursor>
<styleClass>
<String fx:value="icon" />
<String fx:value="delete" />
</styleClass>
<HBox.margin>
<Insets left="5.0" top="4.0" />
</HBox.margin>
</Button>
<Button fx:id="tmpl_save" minHeight="16.0" minWidth="16.0" mnemonicParsing="false" onAction="#onSaveTemplate" prefHeight="16.0" prefWidth="16.0" text="">
<cursor>
<Cursor fx:constant="HAND" />
</cursor>
<styleClass>
<String fx:value="icon" />
<String fx:value="save" />
</styleClass>
<HBox.margin>
<Insets left="5.0" top="4.0" />
</HBox.margin>
</Button>
<Button fx:id="tmpl_search" minHeight="16.0" minWidth="16.0" mnemonicParsing="false" onAction="#advancedSearch" prefHeight="16.0" prefWidth="16.0" text="">
<cursor>
<Cursor fx:constant="HAND" />
</cursor>
<styleClass>
<String fx:value="icon" />
<String fx:value="search" />
</styleClass>
<HBox.margin>
<Insets left="5.0" top="4.0" />
</HBox.margin>
</Button>
</children>
</HBox>
<HBox fillHeight="false" nodeOrientation="LEFT_TO_RIGHT" prefHeight="27.0" prefWidth="980.0" AnchorPane.bottomAnchor="0.0" AnchorPane.leftAnchor="0.0" AnchorPane.rightAnchor="140.0" AnchorPane.topAnchor="3.0" StackPane.alignment="TOP_LEFT">
<children>
<HBox prefHeight="24.0">
<children>
<Text text="Template Name">
<HBox.margin>
<Insets left="10.0" top="7.0" />
</HBox.margin></Text>
<ComboBox fx:id="templateNameComboBox" promptText="Select Template Name">
<HBox.margin>
<Insets left="5.0" />
</HBox.margin></ComboBox>
</children>
</HBox>
<HBox prefHeight="24.0">
<children>
<Label styleClass="seperator" text="">
<HBox.margin>
<Insets left="5.0" right="5.0" />
</HBox.margin></Label>
</children>
</HBox>
<HBox prefHeight="24.0">
<children>
<Text text="Template Type ">
<HBox.margin>
<Insets top="7.0" />
</HBox.margin></Text>
<Label fx:id="templateTypeComboBox">
<HBox.margin>
<Insets left="5.0" top="5.0" />
</HBox.margin></Label>
</children>
</HBox>
<HBox prefHeight="24.0">
<children>
<Label styleClass="seperator" text="">
<HBox.margin>
<Insets left="5.0" right="5.0" />
</HBox.margin></Label>
</children>
</HBox>
<HBox prefHeight="24.0">
<children>
<CheckBox fx:id="defaultTemplate" disable="true">
<HBox.margin>
<Insets top="4.0" />
</HBox.margin></CheckBox>
<Text text="Default Template">
<HBox.margin>
<Insets top="7.0" />
</HBox.margin></Text>
</children>
</HBox>
</children>
</HBox>
</children>
</AnchorPane>
<HBox prefWidth="850.0">
<children>
<AnchorPane minWidth="-Infinity" prefWidth="484.0" HBox.hgrow="ALWAYS">
<children>
<TreeTableView fx:id="treeTableView" layoutX="1.0" layoutY="1.0" minHeight="400.0" minWidth="754.0" prefHeight="752.0" prefWidth="754.0" styleClass="k-table-view" AnchorPane.bottomAnchor="43.0" AnchorPane.leftAnchor="0.0" AnchorPane.rightAnchor="0.0" AnchorPane.topAnchor="0.0" HBox.hgrow="ALWAYS" VBox.vgrow="ALWAYS">
<columns>
<TreeTableColumn style="-fx-font-family: Calibri; -fx-font-size: 11px;" text="Message Type">
<cellValueFactory>
<TreeItemPropertyValueFactory property="messageType" />
</cellValueFactory>
</TreeTableColumn>
<TreeTableColumn prefWidth="110.0" style="-fx-font-family: Calibri; -fx-font-size: 11px;" text="MessageId">
<cellValueFactory>
<TreeItemPropertyValueFactory property="messageId" />
</cellValueFactory>
</TreeTableColumn>
<TreeTableColumn prefWidth="115.0" style="-fx-font-family: Calibri; -fx-font-size: 11px;" text="Originator Reference">
<cellValueFactory>
<TreeItemPropertyValueFactory property="originatorRef" />
</cellValueFactory>
</TreeTableColumn>
<TreeTableColumn prefWidth="115.0" style="-fx-font-family: Calibri; -fx-font-size: 11px;" text="Search Key">
<cellValueFactory>
<TreeItemPropertyValueFactory property="searchKey" />
</cellValueFactory>
</TreeTableColumn>
<TreeTableColumn style="-fx-font-family: Calibri; -fx-font-size: 11px;" text="Message Description">
<cellValueFactory>
<TreeItemPropertyValueFactory property="guiMessageStatusDesc" />
</cellValueFactory>
</TreeTableColumn>
<TreeTableColumn style="-fx-font-family: Calibri; -fx-font-size: 11px;" text="Message Reason">
<cellValueFactory>
<TreeItemPropertyValueFactory property="guiMessageStatus" />
</cellValueFactory>
</TreeTableColumn>
<TreeTableColumn prefWidth="100.0" style="-fx-font-family: Calibri; -fx-font-size: 11px;" text="Updated Date Time">
<cellValueFactory>
<TreeItemPropertyValueFactory property="updateTime" />
</cellValueFactory>
</TreeTableColumn>
<TreeTableColumn prefWidth="100.0" style="-fx-font-family: Calibri; -fx-font-size: 11px;" text="XML Link">
<cellValueFactory>
<TreeItemPropertyValueFactory property="xmlMessage" />
</cellValueFactory>
</TreeTableColumn>
</columns>
<columnResizePolicy>
<TreeTableView fx:constant="CONSTRAINED_RESIZE_POLICY" />
</columnResizePolicy>
</TreeTableView>
<HBox layoutX="2.0" layoutY="745.0" prefHeight="50.0" prefWidth="474.0">
<children>
<Label text="Page Count">
<HBox.margin>
<Insets top="17.0" />
</HBox.margin>
</Label>
<TextField fx:id="itemsPerPage" maxWidth="-Infinity" minWidth="-Infinity" prefHeight="22.0" prefWidth="40.0" text="50">
<HBox.margin>
<Insets left="8.0" top="14.0" />
</HBox.margin>
</TextField>
<Pagination fx:id="pagination" maxWidth="-Infinity" minWidth="-Infinity" prefHeight="42.0" prefWidth="235.0" translateX="1.0" translateY="1.0">
<HBox.margin>
<Insets top="8.0" />
</HBox.margin>
</Pagination>
<Label fx:id="displayInfo">
<HBox.margin>
<Insets top="17.0" />
</HBox.margin>
</Label>
</children>
</HBox>
</children>
</AnchorPane>
</children>
</HBox>
</children>
<styleClass>
<!-- <String fx:value="Sapphire" /> -->
<String fx:value="defaultFont" />
</styleClass>
</VBox>
These are the images before and after
Before Resize:
After Resize