Renders nothing when nothing is targeted, or on any page but the box's own, so a consumer can
mount it on every page unconditionally.
z-index: 3 is deliberate: below DrawFieldLayer's armed surface at 4, so a drag that starts
over an existing box while drawing draws rather than moves; above the FieldOverlay boxes,
which carry no z-index, so the move surface — not the box's select button — receives the drag.
Renders nothing when nothing is targeted, or on any page but the box's own, so a consumer can mount it on every page unconditionally.
z-index: 3is deliberate: belowDrawFieldLayer's armed surface at 4, so a drag that starts over an existing box while drawing draws rather than moves; above theFieldOverlayboxes, which carry noz-index, so the move surface — not the box's select button — receives the drag.