[Bug] Video disappears from editor after exporting (only background remains)

[joaothaira]

Harvested from archived upstream: siddharthvaddem#585 (4 comments there with more reports).

After export completes, the editor preview shows only the wallpaper background — source video gone, can't continue editing. Likely the exporter tears down or invalidates a decoder/object URL shared with the preview. Verify against current merged pipeline; the source-copy fast path (1f5afa9) may also sidestep one trigger.