The parent link (#122/#183) is meant to be the inverse of collection
membership, but only append() maintained it.
insert(), extend() and setitem() called setParent(owner.lock())
unconditionally. A scene collection has no owner drawable, so that
resolved to null -- and setParent(nullptr) also clears parent_scene. The
drawable landed in the scene's UI vector, rendered fine, and reported a
.parent of None.
The slice arms never touched the link at all. A slice-delete left the
removed child pointing at a parent that no longer contained it; a
slice-assign never parented the incoming items, nor unparented the ones
they displaced. A stale parent is a live weak_ptr to a frame the child
has left, and it is what get_global_position() walks.
Route every mutator through link_child()/unlink_child(), which are now
the single place that knows a scene parents via setParentScene() and a
drawable via setParent().
Three latent traps in the same functions, found while fixing this:
* collection[-1] on an EMPTY collection hung the engine forever:
`while (index < 0) index += size()` adds zero on every pass.
* getitem's `index > size() - 1` check underflowed to SIZE_MAX when
size() was 0, so the bounds check passed and it indexed an empty
vector.
* insert() and setitem() resolved the index before detaching the
incoming drawable from its old parent. If it was already a member of
the same collection, detaching erased it and shifted everything
after -- so an index validated against the old size could write past
the end. Slice-assign now builds its result as an independent vector
for the same reason.
closes#377
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
John McCardle
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