the index of the startEditEvent must be the index where editing is started. As demonstrated in the failing test below, it is -1.

The reason seems to be in ListCell.startEdit where the event is created using list.editingIndex which still is -1 when the start is triggered from the cell (vs. triggered by someone else doing a list.editCell(someIndex)):

        super.startEdit();

         // Inform the ListView of the edit starting.
        if (list != null) {
            list.fireEvent(new ListView.EditEvent<T>(list,
                    ListView.<T>editStartEvent(),
                    null,
                    list.getEditingIndex()));
            list.edit(getIndex());
            list.requestFocus();
        }

A fix would be to use cell's index in the event.

The failing test:

    @Test
    public void testListEditStartOnCellStandalone() {
        ListView<String> control = new ListView<>(FXCollections
                .observableArrayList("Item1", "Item2", "Item3", "Item4"));
        control.setEditable(true);
        control.setCellFactory(TextFieldListCell.forListView());
        new StageLoader(control);
        int editIndex = 1;
        // cell retrieved via VirtualFlowTestUtils
        IndexedCell cell = getCell(control, editIndex);
        ObjectProperty<ListView.EditEvent> editEvent = new SimpleObjectProperty<>();
        control.addEventHandler(ListView.editStartEvent(), e -> editEvent.set(e));
        // start edit on cell
        cell.startEdit();
        // test cell state
        assertTrue(cell.isEditing());
        assertEquals(editIndex, cell.getIndex());
        // test editEvent
        assertNotNull(editEvent.get());
        assertEquals("type is startEdit", ListView.editStartEvent(), editEvent.get().getEventType());
        assertEquals("index on start event", editIndex, editEvent.get().getIndex());
    }