terminal: clip Sixel and iTerm payloads to final viewport geometry

[subinium]

Priority: P2
Evidence: Source-verified; real decoder/viewport reproduction required
Baseline: v0.23.0 source 37933f9, audited 2026-09-05. Native execution/compile probes used Rust 1.98.0 on aarch64 macOS unless explicitly stated otherwise.

Problem and Reproduction

Sixel/iTerm images are encoded before layout. Partial viewport clipping changes the recorded rect/footprint but does not crop or resize the encoded payload. A20x4 image in a 2-row scroller can therefore emit more image area than tracked as visible.

src/context/widgets_display/rich_output.rs:368 · src/buffer.rs:631

Scope

• Carry source data/geometry to final placement, then crop or encode for the actual visible area.
• If a protocol cannot represent the required crop safely, use a defined fallback instead of emitting the full payload.
• Keep damage footprints and exposed-cell clearing equal to what the terminal actually receives.

Expected Effect

Prevent images from painting over neighboring panes and leaving stale pixels after scrolling.

Acceptance Criteria

• Top/left/right/bottom and two-axis nested clipping cases match the intended visible image.
• Move/resize/remove partially visible images without painting outside viewport or leaving stale damage.
• Use a decoder-aware fixture and actual supported terminal; envelope-only escape assertions are insufficient.
• Follow the repository quality gates and the evidence rules in tracking: post-v0.23 correctness, resource efficiency and WASM completion #366; record actual results rather than treating a proposed test as passed.

Boundaries and Dependencies

Do not claim all terminals implement the same crop controls; protocol-specific handling is part of scope.

No blocking implementation dependency identified; coordinate shared code with the tracker.

Related closed work: #265. This is a scoped follow-up at the audit baseline, not a claim that every prior fix is absent.

Tracking: #366. This issue records unresolved work, not an implemented fix.

[subinium]

v0.24.0 Disposition

The implementation uses final clip/source geometry, valid Sixel crop/scale paths and a safe fallback for clipped iTerm output. Decoded protocol and viewport regressions pass. Actual Kitty/Sixel/iTerm GUI rendering at resize/scroll boundaries has not been observed, so the physical terminal acceptance remains open.

Implementation and evidence mapping: PR #433. Release v0.24.0 and its 20/20 release workflow are verified. This issue stays open only for the remaining acceptance/investigation described above; publication alone does not establish it.