MAME isn't rendering the full output of the Sega Pico and crops the borders that can be seen on real hardware.
再現手順
A year at Pooh's Corner (poohcorn) will show a colored border right at the intro screen:
追加情報
MAME already renders the full display output for systems such as the PC Engine or ZX Spectrum, so it would be consistent to not crop them for the Pico and other related systems (Mega Drive, etc).
グラフィック
megadriv - メガドライブ (EU版, PAL)
segapico - (不明セット)
sms - Sega Master System II