Unlike the content opacity it replaced, the dim quad also darkens the
backdrop showing through around the rows, so its rectangle edge read as
a hard line across the spotlight artwork. Ramp the dim in over narrow
top/bottom gradient bands (flat quad interior, non-AA to avoid seams);
band area keeps the gradient-shader cost negligible on weak TV GPUs.
Browse pass on the Mali-G31 box unchanged: draw p50 7.2ms, 2% janky.
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