A single-click with the left mouse button while the pointer is positioned
    over an unhighlighted entity will select (or highlight) that entity.

    A single-click with  the left mouse button while the pointer is positioned
    over an entity that has been highlighted previously will cause the
    expansion of that entity if that entity is not already in the expanded
    state. ("Expanding" the entity means that its subentities will become
    visible.)

    A double-click with the left mouse button while the pointer is positioned
    over any entity or subentity will cause the expansion or collapse of that
    entity.  For example, double-clicking the left mouse button on the Session
    Control entity after it has already expanded would cause that entity to
    be collapsed, and vice versa.