Constant SAVE_STATE_DEVICE_HEADROOM 

pub const SAVE_STATE_DEVICE_HEADROOM: usize = _; // 26usize

How much a snapshot can grow after it is first measured.

Devices attach lazily: a port reads “unplugged” (1 byte) until the host plugs a device in, and the largest device then takes EXPANSION_DEVICE_MAX_LEN. Two ports, so twice the difference.

v2.8.0 (libretro audit §2.1): the libretro core adds this to the retro_serialize_size it reports at load, because the frontend sizes its save-state, rewind and run-ahead buffers from that one answer, and a Zapper plugged in mid-game otherwise made every later save fail. The padding it leaves is zeroed and read back as padding (save_state::SectionIter).