Selecting a piece

Clicking a turret's barrel selects the turret, not the barrel. A library model is a tree — a hull with a barrel with a muzzle, or a glTF import split into _primitive0, _primitive1 — so the mesh under the pointer is almost never the thing an author means.

So picking walks up from whatever was hit until it reaches a node the ensemble actually placed, and selects that piece. If the walk runs out of parents without finding one, the pick was scenery: the ground, the sky, the water. That is a miss, not a selection of something arbitrary.

Why a map rather than a search

Every pick would otherwise compare the hit against every piece's body. The map is built once per rebuild, keyed by the node objects themselves, so a pick is a walk up a short parent chain with a hash lookup at each step — and it stays that way with a thousand pieces.