Code review flagged silent corruption when dragging a mixed selection
of a parent + one of its children: the child's ids stayed in
pendingSnap even though nodeBox excluded it from the bbox. On drag
stop we shifted the child's parent-relative position by the same delta
the parent already moves by — double-snapping the child off-screen.
Restrict the pendingSnap id set to nodes that contributed a box
(top-level only). Children follow their parent's move automatically;
no extra shift is needed.
Pouzor
062a08f116