public weak CellRenderer? get_focus_from_sibling (CellRenderer renderer)
Gets the CellRenderer which is expected to be focusable for which
renderer is, or may be a sibling.
This is handy for CellArea subclasses when handling events, after determining the renderer at the event location it can then chose to activate the focus cell for which the event cell may have been a sibling.
the CellRenderer for which