feat(tui): render fenced code on a surface, sized to the frame

[BunsDev]

What

Fenced code in the transcript was drawn as a fixed 50-dash rule plus a yellow │ gutter, with the raw source line pushed verbatim (crates/tui/src/messages/markdown.rs:23). Three problems followed.

The block had no edges. Code inherited the app background, so a fence bled into the prose around it and read as indented text rather than as a block.

The rules ignored the frame. Hardcoded 50 dashes regardless of terminal width, so they under- or overshot on every terminal that wasn't exactly that wide.

Long lines were clipped silently. render_markdown already receives the frame width and the code path ignored it, so ratatui cut the line at the buffer edge with no marker. A truncated line was indistinguishable from a complete one — which matters most for exactly the content a reader is most likely to copy.

After

  ┌─ RUST ──────────────────────────────────────────
  │ fn main() {
  │     println!("hello");
  │ }
  └─────────────────────────────────────────────────

Every row sits on COVEN_CODE_PANEL_BG and is fitted to the frame — clipped when it overruns, padded when it falls short — so the block reads as one continuous slab with a square right edge. Rules span the real width. Body lines truncate with …. A tagged fence gets an accent-colored chip.

Review attention

• The edge glyph and the chip are independent. An untagged fence still opens with ┌; it just has no chip. My first version keyed the glyph off the label and opened untagged fences with └ — the visual check caught it, and the test that should have caught it had an || that accepted either edge. Both are fixed.
• The fence tag is model output, so it is validated against a conservative identifier set plus a length cap and rejected outright rather than mangled into a chip.
• Colors move off Color::Yellow/Color::White onto the COVEN_CODE_* tokens, per follow-up 4 of docs/tui-ux-accessibility-review.md — the named ANSI colors are terminal-theme dependent, so contrast through them cannot be reasoned about.

The first commit is not mine to own

cargo fmt --all -- --check (rust-ci.yml:52) fails on a pristine checkout of main — render_response_reader landed unformatted in #181:

$ git checkout origin/main && cd src-rust && cargo fmt --all -- --check
Diff in .../crates/tui/src/response_reader.rs:179

The gate is red on main and every new PR inherits it, so the fix is here as its own commit. Pure formatting, no behavior change.

Tests

The live render_markdown fence path had no tests before this — only the unreachable messages::render_code_block is snapshotted (tests/render_snapshots.rs:214). Added 8 cases: surface fill, frame-width fitting, the chip, matching open/close edges, untagged fences, tag rejection, truncation, and narrow frames down to zero width (which is how I found a rule-overflow bug at widths below the chip).

Verification

cd src-rust
cargo fmt --all                                          # clean
cargo clippy --workspace --all-targets -- -D warnings    # clean, no warnings
cargo test --workspace                                   # 32 suites, 0 failures
git diff --check                                         # clean

Manual TUI validation per AGENTS.md, tmux 80x24:

tmux new-session -d -s cc-md-test -x 80 -y 24
tmux send-keys -t cc-md-test "./target/debug/coven-code" Enter
tmux capture-pane -t cc-md-test -p

App starts clean, welcome box intact. The strings scripts/tui-tests/cases/02_startup.sh pins — Coven v, Tips for getting started, What's new, and the ❯ prompt — all still render, so the startup case is unaffected.

Known gaps

• No syntax highlighting for fences. syntect is already a dependency and already wired for diffs (diff_viewer.rs:977); that is the obvious next step and is deliberately out of scope here to keep this diff reviewable.
• messages::render_code_block remains dead code, kept alive only by its snapshot test. Not touched.

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Actions	Updated
docs	Ready	Preview	Sep 15, 2026 8:36am UTC