TFT: horizontal noise lines on some animations

[wsclx]

Symptom

Reported by @wsclx on 2026-05-14 after the chunk-header magic fix (0x064F → 0x0650) landed in 382d59c and made the panel render full-screen instead of just the corner:

"Teilweise gehen die Animationen auch über den gesamten Bildschirm, haben aber teilweise horizontale Störlinien."

What we know

• Affects some but not all animations. Which presets show it, and where on the panel, is the data we need next.
• Encode geometry looks correct on paper: 128 × 128 × 2 = 32 768 bytes per frame + 256-byte frame-delay header. See crates/ak820-protocol/src/commands/tft.rs lines 30-176.
• Chunked upload: 4096-byte payload chunks with an 8-byte custom header. See crates/ak820-protocol/src/device.rs:608-660.
• Chunk boundary math: chunk 0 carries the 256-byte frame-delay header + first 3840 px-bytes (= 15 rows). Subsequent chunks carry 4096 bytes = 16 rows each. So chunk N's first byte does not necessarily land on a row boundary for chunks 1+ relative to the panel's scan order.

Hypotheses (in order of likelihood)

1. Chunk-boundary stride bug. The firmware may be writing each chunk's payload starting at a different VRAM offset than we assume — manifesting as a band of corrupted pixels at the seam between chunks. Test: render a preset with a flat solid colour (e.g. solid-magenta). If the noise lines are at row offsets that match chunk boundaries (rows 15, 31, 47, …), this is it.
2. Missing inter-chunk ack. The web driver's setTftUserAnimation may wait for a SET_LED_USER_ANIMATION = 65 ack between chunks. We don't wait. Test: insert a small delay (~5ms) between device.write calls and see if the noise vanishes.
3. Trailing-pad bug. Our last chunk is zero-filled to 4096 bytes. Maybe the firmware reads past the declared payload length. Test: pad with 0xFF or with a repeated last-byte instead.

What's needed to triage

• Phone photo of the AK820 Pro's TFT showing the artefact on one named preset (so we can re-run with the same byte stream).
• Approximate band positions (row counts from top, full-width vs partial).

Repro

1. pnpm tauri dev
2. TFT view → apply a preset that shows the bug.
3. Photo.

Workaround

None.