The game reduces the scene to 8x8 on the GPU, block transfers that into RAM,
and the CPU sets its brightness adaptation from it. With BlockTransferAllowCreateFB
the copy went into a GPU-side buffer instead, the CPU read stale RAM, and the
game kept getting darker on the hardware backends (#15914).
New compat flag BlockTransferReadbackSmall: transfers of up to 64 pixels into
RAM are still read back, a frame late, so it doesn't stall. Brightness from
the issue's savestate now follows the software renderer's (72 vs 84 at 6 s,
84 vs 95 at 20 s, from 25 and falling), at the same speed as before.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
