happens when cell index is update from table's editing index to -1, failing test method:

    @Test
    public void testOffEditingIndex() {
        int editingIndex = 2;
        int cellIndex = -1;
        cell.updateIndex(editingIndex);
        table.edit(editingIndex, editingColumn);
        cell.updateIndex(cellIndex);
        assertEquals("sanity: cell index changed", cellIndex, cell.getIndex());
        assertFalse("cell must not be editing on update from editingIndex" + editingIndex
                + " to cellIndex " + cellIndex, cell.isEditing());
    }

Looks like a left-over from fixing JDK-8150525 - bubbled up in review for JDK-8264127

The technical reason seems to be updateEditing which backs out if index == -1

    private void updateEditing() {
        if (getIndex() == -1 || getTableView() == null) return;

should make certain to not be in editing state in this case.

Attaching complete tests, that is also for the fixed state transitions (there are no tests for the former fix).