Replace the flat opacity crossfade with a soft, feathered left→right wipe of the
trailer in (mask via @property, falls back to crossfade) — "a rollover and a
fade". The wipe now fires ONLY when YouTube reports PLAYING (handshake + state
events), not on a blind timer, with a 4.5s safety net. When the trailer ENDS the
original backdrop fades back in and the iframe is torn down.
5b6ef295f8