public int getKeyboardSelectedSubRow() {
        return KeyboardSelectionPolicy.DISABLED == getKeyboardSelectionPolicy()
                ? -1
                : keyboardSelectedSubrow;
    }
Get the index of the sub row that is currently selected via the keyboard.
If the row value maps to one rendered row element, the subrow is 0.

@return the currently selected subrow, or -1 if none selected
stroom-core-client-widget/src/main/java/com/google/gwt/user/cellview/client/AbstractCellTable.java