A press with the middle (or any other non-left) mouse button never reaches
the Studio's drag, place or select handlers: dragging pans the view even
on a selected object or while placing, and a click selects nothing. A
middle click no longer starts the browser's autoscroll.
check_studio covers a middle-drag on the selected truck (it stays put,
the view moves), a middle click on another object (the selection is
kept), and a middle-drag while placing (nothing is placed).
grabowski