Scene loads add each asset to the scene as soon as it finishes. When a
load was cancelled (superseded by a newer load, or the editor unmounted)
the loader returned after its tasks settled, leaving whatever had
finished in the scene. On unmount that leaked the objects; on a
superseded load they appeared in the newer load's scene.
A cancelled load now removes and frees the objects it created. Lights
are left alone because Scene tracks its base lights in fields a newer
load may have re-pointed at them.
Tanish Manku
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