perspective.move takes constrain: true instead of a plane-space
direction. For each object, the command snaps the move to the nearest
axis or 45-degree diagonal of that object's own plane, so a selection on
several planes keeps every object on a line of its plane. The tool sends
the raw pointer with the flag.
On a diagonal, Snap to Grid now moves along the diagonal by the nearer
per-axis correction, so that edge lands on its gridline.