To reproduce, run the example below

- open popup
- type anything in the textfield, including a space
- expected: popup remains open
- actual: popup hidden on space

The example:

public class ComboTextFieldSpace extends Application {
    
    @Override
    public void start(Stage stage) {
        HBox root = new HBox();

        ObservableList<String> items = FXCollections.observableArrayList(
                "One", "Two", "Three", "Four", "Five", "Six",
                "Seven", "Eight", "Nine", "Ten");
        ComboBox<String> cb = new ComboBox<String>(items);
        cb.setEditable(true);
        
        root.getChildren().addAll(cb);

        Scene scene = new Scene(root);
        stage.setScene(scene);
        stage.show();
    }

    public static void main(String[] args) {
        launch();
    }
    
    @SuppressWarnings("unused")
    private static final Logger LOG = Logger
            .getLogger(ComboTextFieldSpace.class.getName());
}


Culprit seems to be the keyPressed handler installed on the listView in ComboBoxListViewSkin.createList:

private ListView<T> createList() {
  ...
        _listView.setOnKeyPressed(t -> {
            // TODO move to behavior, when (or if) this class becomes a SkinBase
            if (t.getCode() == KeyCode.ENTER ||
                    t.getCode() == KeyCode.SPACE ||
                    t.getCode() == KeyCode.ESCAPE) {
                comboBox.hide();
            }
        });
     ...
}

Might be a reasonable thingy to do for not-editable combos but annoys users if editable. A quick hack for application code:

    public void disableHidingBySpace(ComboBox<String> cb) {
        ComboBoxListViewSkin<?> skin = (ComboBoxListViewSkin<?>) cb.getSkin();
        ListView<?> list = (ListView<?>) skin.getPopupContent();
        EventHandler<? super KeyEvent> old = list.getOnKeyPressed();
        if (old != null) {
            list.setOnKeyPressed(key -> {
                if (key.getCode() == KeyCode.SPACE && cb.isEditable()) {
                    return;
                }
                old.handle(key);
            });
        }
    }

A clean solution is probably related to a clean solution of JDK-8209788 - both could be handled by formally specify (in listViewBehavior) keyMappings for a ListView in the role of popupContent for a combo.