Holding Shift while dragging objects with the Perspective Selection tool
snaps the move, in the plane's own coordinates, to the nearest plane axis
or 45 degree diagonal. This applies to plain moves and to Alt and
Ctrl+Alt copies. The preview, the final move and the measurement label
use the snapped point. Pressing or releasing Shift, pressing 5 and
pressing 1 to 4 during the drag recompute the preview from the pointer;
a move perpendicular to the plane is not constrained.
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