# US-023 Nvim input frame branch label

## Status

implemented

## Lane

normal (2 risk flags: existing behavior — the nvim statusline chrome ladder and its pinned ladder strings change deliberately; public contracts — `renderInputLineBorder`/`renderInputBoxFrame` gain an optional parameter, other presets must not change one byte). Stronger validation required.

## Product Contract

The nvim preset's `line` input frame embeds the git branch name into the TOP border rule of the input box, and the `⎇ branch` segment is REMOVED from the statusline right cluster (a value must never appear twice in the user zone).

Approved mockup (user-approved 2026-08-25):

```
──────────────────────────────── ⎇ main ─
 ❯  …input…
──────────────────────────────────────────
 HIGH  anthropic · claude-sonnet-4        84k/200k 42% · CH 91%  MCP: …
```

- Top rule = `─`×N + ` ⎇ branch ` + `─`×1, total exactly the terminal width. Label carries 1 space on each side and exactly 1 trailing `─` anchors the right edge.
- Colour (FINAL): the rule's dashes keep the rule's own colour (`inputRuleColorizer`); `⎇` and the branch name take the `muted` tone — the same tier the model id uses on the bar, because the branch is identity like the model id; the LOC renders BARE (`+N -M`, no brackets, gitsigns/lualine convention) keeping the semantic `success`/`error` tones. Format (order/spaces/signs) is owned by the shared `buildBranchBadge` (`style: "bare"`; the legacy call sites default to `"brackets"` and stay byte-identical).
- No branch (no git / empty) → plain rule, byte-identical to today. Bottom rule unchanged in every case.
- Non-`line` frames (user `inputBoxStyle` override to `solid`/`outline`/`halfblock`) have no rule to embed into → no label, no error. The other three presets pass no label and stay byte-identical.
- Branch comes from the SAME provider the statusline already uses (`getBranch`); no new data source. Normalized like status text (`normalizeSingleLine`) and hard-capped at source: `truncatePlain(branch, NVIM_BRANCH_MAX=24, "…")` — the same source-cap pattern as `NVIM_MODEL_ID_MAX`.
- Long branch: the label needs at least 2 leading `─` + label block + 1 trailing `─`; below that threshold the label is DROPPED entirely (plain rule, no dangling `…`-trơ). Appearance by width is MONOTONIC: once shown, widening never hides it.

Statusline (companion change):

- The `⎇ branch` segment is deleted from the right cluster: old rung `⎇ main · 84k/200k 42% · CH 91%` becomes `84k/200k 42% · CH 91%`. Freed columns flow to the extension status.
- P3 loses its `branch` seen-flag and gains a NEGATIVE assert: no `⎇` may appear in the statusline at any width/fixture.
- The pinned degradation ladder changes DELIBERATELY (baseline change recorded in docs/TEST_MATRIX.md, not a regression).

## Verification Contract

- P7 (new, red-first greenfield: written and run BEFORE implementation, red because the label never appears, then green): sentinel branches (ASCII + CJK), width 1..200 — label appears monotonic, appears exactly once, truncates with `…` at the 24-col cap, below threshold a plain rule with no dangling ellipsis, the rule row carries no `\x1b[0m` and `visibleWidth === width`.
- P7 mutation-proof (ritual, same as P6 round 5b): glued label (M1) and removed drop law (M2) must turn P7 red.
- FINAL verification (muted + bare): the `⎇ <name>` glyphs must carry the MUTED SGR (asserted via the suite's tone source, each segment closed with `\x1b[39m`) — mutation M6-final (name switched to ruleFg OR unstyled) must turn P7 red; M7 (source cap removed) red via the two-way `truncatePlain` pin; M8 (bare→brackets) red. Stage detection is formatter-owned (classified against `buildBranchBadge`'s own plain output, anchored by rule spaces + trailing dash) with a `fix+2-retry` fixture pinning that git-legal `+`/`-` in names is never mistaken for LOC; each sign+number is one coloured segment; zeros self-hide; unknown labels fail outright.
- P3 negative assert mutation: re-adding `⎇` to the statusline must turn it red.
- P1/P2/P4/P6 must stay green unchanged.

## Implementation Notes

- Seam: `renderInputLineBorder(width, topLabel?)`; `renderInputBoxFrame(inputLines, width, topLabel?)` passes it ONLY to the top rule of the `line` frame; only `renderNvimLayout` supplies a label.
- Deliverable: uncommitted diff + gallery {no-git, `main`, `feature/rename-用戶-flow`, 40-char branch} × {30, 60, 100}; Mark review → user eyeball → Biscuit.
- Base commit: `e37a3d2`.