With the combination of multiple, cell selection and all cells selected the row is un-/selected for Table-/TreeTableRow (seen in the example below).

Steps to reproduce:
   - make sure there is a visible part of the row at the right
   - click into a cell of the first column
   - extend the selection to the right, such that both columns are selected
   - expected: same appearance in Table and TreeTable
   - actual: different appearance - for table the row part is not selected,
        for TreeTable the row part is selected

The technical reason is the different implementation of updateSelection in the corresponding row:

// tableRow:
selected = !sm.isCellSelectionEnabled() && sm.isSelected(int);

// treeTableRow:
selected = sm.isSelected(int);
 
The behavior in TableRow seems to be what's expected (there's a code comment to that effect) and TreeTableRow probably should be implemented the same way .. wondering why the different behavior doesn't show up in any test ..

Note: this turned up in review of JDK-8235491 but is unrelated: same behavior before/after that fix.

Example:

public class SelectionVisuals extends Application {

    private Parent createContent() {

        ObservableList<Locale> data = FXCollections.observableArrayList(
                Arrays.stream(Locale.getAvailableLocales(), 10, 20).collect(Collectors.toList()));

        TableView<Locale> table = new TableView<>(data);
        table.getColumns().addAll(createTableColumn("displayLanguage"), createTableColumn("displayCountry"));
        table.getSelectionModel().setCellSelectionEnabled(true);
        table.getSelectionModel().setSelectionMode(SelectionMode.MULTIPLE);

        TreeTableView<Locale> treeTable = new TreeTableView<>(createTreeData(data));
        treeTable.setShowRoot(false);
        treeTable.getColumns().addAll(createTreeTableColumn("displayLanguage"), createTreeTableColumn("displayCountry"));
        treeTable.getSelectionModel().setCellSelectionEnabled(true);
        treeTable.getSelectionModel().setSelectionMode(SelectionMode.MULTIPLE);

        HBox compare = new HBox(10, table, treeTable);
        BorderPane content = new BorderPane(compare);
        return content;
    }

    private <T> TableColumn<T, String> createTableColumn(String property) {
        TableColumn<T, String> column = new TableColumn<>(property);
        column.setCellValueFactory(new PropertyValueFactory<>(property));
        return column;
    }

    private TreeItem<Locale> createTreeData(ObservableList<Locale> data) {
        TreeItem<Locale> root = new TreeItem<>(Locale.ENGLISH);
        ObservableList<TreeItem<Locale>> children = root.getChildren();
        data.stream().map(TreeItem::new).forEach(children::add);
        return root;
    }

    private <T> TreeTableColumn<T, String> createTreeTableColumn(String property) {
        TreeTableColumn<T, String> column = new TreeTableColumn<>(property);
        column.setCellValueFactory(new TreeItemPropertyValueFactory<>(property));
        return column;
    }


    @Override
    public void start(Stage stage) throws Exception {
        stage.setScene(new Scene(createContent()));
        stage.show();
    }

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

}