ComboBox: uncontained value not shown in buttonCell

Happens with an not-editable comboBox when setting a value that's not contained in the items "later" (than on startup) in its lifetime.

To reproduce, compile and run the example below
- click the button to set an uncontained value
- expected and actual: uncontained value shown
- open the dropdown and select a contained value
- expected and actual: selected item shown
- again click the button
- expected: uncontained value shown
- actual: buttonCell showing nothing

The misbehavior turned up on SO https://stackoverflow.com/q/55378762/203657 and feels like a regression: the (very) old JDK-8127575 which exhibited the not-showing always, it was fixed some years ago.

It might have been re-introduced by fixing JDK-8145588 (make sure that cell's selected property is updated in updateItem to allow for custom styles for that state) - at least removing that fix resulted in correct showing.

The flow of control for updating a uncontained value in the skins is:

1. listener to valueProperty calls - implicitely - comboBoxListViewSkin.updateDisplayNode

2. if uncontained it reaches the snippet:

   // RT-21336 Show the ComboBox value even though it doesn't
   // exist in the ComboBox items list (part two of fix)
   buttonCell.updateIndex(-1);
   boolean empty = updateDisplayText(buttonCell, value, false);

3. the updateIndex(-1) leads to marking the cell as dirty (bug fix JDK-8145588, done in cell.updateSelected)

        if (wasSelected != selected) {
            markCellDirty();
        }

    // itemDirty and markCellDirty introduced as a solution for JDK-8145588.
    // In the fullness of time, a more fully developed solution can be developed
    // that offers a public API around this lazy-dirty impl.
    private boolean itemDirty = false;
    private final void markCellDirty() {
        itemDirty = true;
        requestLayout();
    }


4. updateDisplayText changes the cell's text/empty properties under its feet, that is after doing this, cell.item != cell.text:

    cell.setText(s);
    cell.setGraphic(null);

5. (last act ;) at layout time of the cell, the dirty marker triggers an updateItem with the cell's item (which at this time is != cell.text, see 4)

    if (itemDirty) {
        updateItem(getItem(), isEmpty());
        itemDirty = false;
    }

On the whole there are two fishy parts:

- changing cell properties under its feet
- the dirty hot fix to guarantee valid selected in updateItem

Not entirely certain how this can be fixed: the former might require a cell that can handle uncontained values on its own, the latter has deeper roots, see the discussion in the bug report

Finally, the example to reproduce:

public class ComboUncontainedBug extends Application {

    private Parent createContent() {
        ComboBox<String> combo = new ComboBox<>(
                FXCollections.observableArrayList("Option 1", "Option 2", "Option 3")) ;
        Button uncontained = new Button("set value to uncontained");
        uncontained.setOnAction(e -> combo.setValue("uncontained value"));
        VBox content = new VBox(10, combo, uncontained);
        return content;
    }

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

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

    @SuppressWarnings("unused")
    private static final Logger LOG = Logger
            .getLogger(ComboUncontainedBug.class.getName());

}