Which boundary a drag off two coincident handles is moving.
When b0 === b1 the clamps leave exactly one degree of freedom in each
direction: b0 is pinned to ≤ b1 so it can only go anticlockwise, and b1
to ≥ b0 so it can only go clockwise. The direction of travel therefore names
the handle — the only reading that can't dead-end.
Which boundary a drag off two coincident handles is moving.
When
b0 === b1the clamps leave exactly one degree of freedom in each direction:b0is pinned to≤ b1so it can only go anticlockwise, andb1to≥ b0so it can only go clockwise. The direction of travel therefore names the handle — the only reading that can't dead-end.