A CAD view always starts with (and returns to) 'object selection mode'. The view maintains a current set
of selected design objects, shown highlighted. Selecting an object with the mouse adds it to the selection
set, or removes it if already selected. A set of special commands exist to assist with selection/deselection.
Many other commands are sensitive to the current object selection set and grey in and out as and when a
relevant object set is selected. Some commands require a single object to be selected, others require
more than just a single object or they conform to some other criteria.