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fix(ui): maintain the parent link in every collection mutator

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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>
This commit is contained in:
John McCardle 2026-07-14 07:53:21 -04:00
commit 53234288ad
3 changed files with 370 additions and 56 deletions
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