fn grab_radius(pointer: PointerKind) -> WorldPxExpand description
How near a press must land to grab an anchor. A cursor aims at the drawn
ring, having watched it grow out under the pointer, so PORT_RADIUS is
exactly what it is aiming at. A fingertip has no hover phase to aim by and
covers several times that, so touch grabs anywhere the ring would have
grown — the region the cursor only gets to see is armed.