ADDITIONAL SYSTEM INFORMATION :
Ubuntu Linux 18.04
java version "1.8.0_201"
Java(TM) SE Runtime Environment (build 1.8.0_201-b09)
Java HotSpot(TM) 64-Bit Server VM (build 25.201-b09, mixed mode)

A DESCRIPTION OF THE PROBLEM :
If you configure the TreeTableView in single-selection mode and you set it so that cell-selection is enabled you get a bug when calling the selectPrevious() method on the selection-model when one of the cells in the last row is selected. If you repeatedly call selectPrevious() the selected cell will not wrap around to the second-last row as you would expect.

REGRESSION : Last worked in version 8u231

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1.Create a TreeTableView and populate with multiple row and column data.
2. Set selection-mode to SelectionMode.SINGLE.
3. Enable cell-selection mode.
4. Select a cell in the last row.
5. Have a button that calls selectionModel.selectPrevious() and click it repeatedly.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Selection should have wrapped around to the second-last row. This behavior can be seen by clicking a cell in any of the other rows and you'll see the desired behavior when selectPrevious() is called repeatedly.
ACTUAL -
Selection will be stuck in the last row and will not wrap around to second-last row.

---------- BEGIN SOURCE ----------
import java.util.List;

import javafx.application.Application;
import javafx.beans.property.IntegerProperty;
import javafx.beans.property.SimpleIntegerProperty;
import javafx.beans.property.SimpleStringProperty;
import javafx.beans.property.StringProperty;
import javafx.scene.Node;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.SelectionMode;
import javafx.scene.control.TreeItem;
import javafx.scene.control.TreeTableColumn;
import javafx.scene.control.TreeTableView;
import javafx.scene.control.TreeTableView.TreeTableViewSelectionModel;
import javafx.scene.control.cell.TextFieldTreeTableCell;
import javafx.scene.control.cell.TreeItemPropertyValueFactory;
import javafx.scene.layout.BorderPane;
import javafx.scene.layout.FlowPane;
import javafx.stage.Stage;
import javafx.util.converter.IntegerStringConverter;

public class TreeTableViewTest extends Application {
    
    public static class Person {
        
        private final StringProperty nameProperty;
        private final StringProperty surnameProperty;
        
        public Person() {
            this.nameProperty = new SimpleStringProperty();
            this.surnameProperty = new SimpleStringProperty();
        }
        
        public StringProperty nameProperty() {
            return this.nameProperty;
        }
        
        public void setName(String value) {
            this.nameProperty.set(value);
        }
        
        public String getName() {
            return this.nameProperty.get();
        }
        
        public StringProperty surnameProperty() {
            return this.surnameProperty;
        }
        
        public void setSurname(String value) {
            this.surnameProperty.set(value);
        }
        
        public String getSurname() {
            return this.surnameProperty.get();
        }
    }
    
    public static class Dog {
        
        private final StringProperty nameProperty;
        private final IntegerProperty ageProperty;
        private final StringProperty breedProperty;
        
        public Dog() {
            this.nameProperty = new SimpleStringProperty();
            this.ageProperty = new SimpleIntegerProperty();
            this.breedProperty = new SimpleStringProperty();
        }
        
        public StringProperty nameProperty() {
            return this.nameProperty;
        }
        
        public void setName(String value) {
            this.nameProperty.set(value);
        }
        
        public String getName() {
            return this.nameProperty.get();
        }
        
        public IntegerProperty ageProperty() {
            return this.ageProperty;
        }
        
        public void setAge(int value) {
            this.ageProperty.setValue(value);
        }
        
        public int getAge() {
            return this.ageProperty.get();
        }
        
        public StringProperty breedProperty() {
            return this.breedProperty;
        }
        
        public void setBreed(String breed) {
            this.breedProperty.set(breed);
        }
        
        public String getBreed() {
            return this.breedProperty.get();
        }
    }
    
    private TreeItem<Object> createTreeItems() {
        TreeItem<Object> rootItem = new TreeItem<>();
        
        List<TreeItem<Object>> rootChildren = rootItem.getChildren();
        
        Person john = new Person();
        john.setName("John");
        john.setSurname("Denver");
        TreeItem<Object> johnTreeItem = new TreeItem<>(john);
        
        rootChildren.add(johnTreeItem);
        
        List<TreeItem<Object>> johnChildren = johnTreeItem.getChildren();
        
        Dog charlie = new Dog();
        charlie.setName("Charlie");
        charlie.setAge(4);
        charlie.setBreed("Labrador");
        TreeItem<Object> charlieTreeItem = new TreeItem<>(charlie);
        johnChildren.add(charlieTreeItem);
        
        Dog daisy = new Dog();
        daisy.setName("Daisy");
        daisy.setAge(7);
        daisy.setBreed("Bulldog");
        TreeItem<Object> daisyTreeItem = new TreeItem<>(daisy);
        johnChildren.add(daisyTreeItem);
        
        Person alice = new Person();
        alice.setName("Alice");
        alice.setSurname("Goodhead");
        TreeItem<Object> aliceTreeItem = new TreeItem<>(alice);
        
        rootChildren.add(aliceTreeItem);
        
        List<TreeItem<Object>> aliceChildren = aliceTreeItem.getChildren();
        
        Dog buck = new Dog();
        buck.setName("Buck");
        buck.setAge(1);
        buck.setBreed("Border Collie");
        TreeItem<Object> buckTreeItem = new TreeItem<>(buck);
        aliceChildren.add(buckTreeItem);
        
        return rootItem;
    }
    
    @Override
    public void start(Stage primaryStage) throws Exception {
        TreeTableView<Object> treeTableView = new TreeTableView<>();
        
        TreeTableViewSelectionModel<Object> selectionModel = treeTableView.getSelectionModel();
        selectionModel.setSelectionMode(SelectionMode.SINGLE);
        selectionModel.setCellSelectionEnabled(true);
        
        treeTableView.setEditable(true);
        
        List<TreeTableColumn<Object, ?>> columns = treeTableView.getColumns();
        
        TreeTableColumn<Object, String> nameColumn = new TreeTableColumn<>("Name");
        nameColumn.setCellValueFactory(new TreeItemPropertyValueFactory<>("name"));
        nameColumn.setCellFactory(TextFieldTreeTableCell.forTreeTableColumn());
        columns.add(nameColumn);
        
        TreeTableColumn<Object, String> surnameColumn = new TreeTableColumn<>("Surname");
        surnameColumn.setCellFactory(TextFieldTreeTableCell.forTreeTableColumn());
        surnameColumn.setCellValueFactory(new TreeItemPropertyValueFactory<>("surname"));
        columns.add(surnameColumn);
        
        TreeTableColumn<Object, Integer> ageColumn = new TreeTableColumn<>("Age");
        ageColumn.setCellFactory(TextFieldTreeTableCell.forTreeTableColumn(new IntegerStringConverter()));
        ageColumn.setCellValueFactory(new TreeItemPropertyValueFactory<>("age"));
        columns.add(ageColumn);
        
        TreeTableColumn<Object, String> breedColumn = new TreeTableColumn<>("Breed");
        breedColumn.setCellFactory(TextFieldTreeTableCell.forTreeTableColumn());
        breedColumn.setCellValueFactory(new TreeItemPropertyValueFactory<>("breed"));
        columns.add(breedColumn);
        
        TreeItem<Object> rootTreeItem = createTreeItems();
        treeTableView.setRoot(rootTreeItem);
        treeTableView.setShowRoot(false);
        
        TreeTableViewSelectionModel<Object> currentSelectionModel = treeTableView.getSelectionModel();
        currentSelectionModel.clearSelection();
        
        BorderPane mainPane = new BorderPane();
        mainPane.setCenter(treeTableView);
        
        FlowPane buttonPane = new FlowPane();
        List<Node> buttons = buttonPane.getChildren();
        
        Button nextButton = new Button("Next");
        nextButton.setOnAction(e -> currentSelectionModel.selectNext());
        buttons.add(nextButton);
        
        Button previousButton = new Button("Previous");
        previousButton.setOnAction(e -> currentSelectionModel.selectPrevious());
        buttons.add(previousButton);
        
        mainPane.setBottom(buttonPane);
        
        Scene scene = new Scene(mainPane, 800, 600);
        primaryStage.setScene(scene);
        
        primaryStage.show();
    }
    
    public static void main(String[] args) {
        Application.launch(args);
    }
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
There is no easy workaround for this bug. The bug is actually located in the TreeTableViewArrayListSelectionModel class' selectPrevious() method (line 3034). The line there:

} else if (pos.getRow() < getRowCount() - 1) {

should change to:

} else if (pos.getRow() > 0) {

to correctly check that the position is not on the first row.
I suspect this issue resulted from a copy-paste of the selectNext() method.

FREQUENCY : always