feat(shell): paint a rule row under the fullscreen status line

[carolitascl]

Closes #1249

PR Type

New feature (type:feature)

Summary

• Paint a full-width ─ rule directly under the fullscreen status bar in the editor frame's border role, so the status row and the prompt read as one panel.
• Let the fullscreen layout measure the header leaf (basis: "auto") instead of pinning one row, so the extra line is not clipped.
• Keep the usage click affordance bound to the status row; the rule row is decorative.
• Responsive by construction: the rule renders only while the fullscreen sidebar is active and disappears with it below the sidebar breakpoint.

Changes

File	Change
lib/shell-bar.ts	Add renderShellHeaderRule(theme, width) — full-width border-role rule.
extensions/gentle-shell.ts	Render the rule as the header rail's second line; gate the usage click to row 0.
lib/shell-sidebar-layout.ts	Measure the header leaf (basis: 1 → "auto"); refresh stale comments.
tests/shell-bar.test.ts	Unit tests for the rule (border role, full width, zero/negative clamps).
tests/shell-sidebar-layout.test.ts	Measured-basis assertion, two-line header passthrough, below-breakpoint removal, header width minus the rail inset.

Test Plan

• node --experimental-strip-types --test tests/shell-bar.test.ts tests/shell-sidebar-layout.test.ts tests/shell-sidebar.test.ts tests/gentle-shell.test.ts — 162 tests pass
• pnpm run typecheck — exit 0
• No shell scripts modified — shellcheck N/A
• Visual check in a ≥140-column terminal (pi -e ./extensions/gentle-shell.ts): rule under the status bar, gone below the breakpoint

Contributor Checklist

• Linked an issue (Closes #1249) — awaiting maintainer status:approved on the issue
• Added exactly one type:* label — requires triage permission; please add type:feature
• Conventional commit format (feat(shell): paint a rule row under the fullscreen status line)
• No Co-Authored-By trailers

Summary by CodeRabbit

• New Features

  • Added a full-width decorative divider beneath the shell status line when the fullscreen sidebar is active.
  • The divider adapts to the available width and displays correctly across one- and two-line header layouts.

• Bug Fixes

  • Prevented the decorative divider from responding to mouse clicks.
  • Improved header sizing so both status and divider rows render without clipping or layout issues.
  • Ensured the divider remains hidden below the sidebar breakpoint.

[coderabbitai]

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: ASSERTIVE

Plan: Advanced

Run ID: 5b183f70-7757-4be0-ad74-c67ad08f7b59

📥 Commits

Reviewing files that changed from the base of the PR and between 5454832 and f39f304.

📒 Files selected for processing (5)

• extensions/gentle-shell.ts
• lib/shell-bar.ts
• lib/shell-sidebar-layout.ts
• tests/shell-bar.test.ts
• tests/shell-sidebar-layout.test.ts

Included review availability: Your plan provides up to 4 included reviews per hour; 3 remain after this review.

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📝 Walkthrough

Walkthrough

The fullscreen shell header now renders a border-colored horizontal rule beneath the status line. The layout measures the header content automatically, and clicks on the rule row are ignored. Below the sidebar breakpoint, the rule is not rendered.

Changes

Fullscreen header rule

Layer / File(s)	Summary
Rule rendering lib/shell-bar.ts, tests/shell-bar.test.ts	Adds renderShellHeaderRule, which paints a width-clamped ─ rule with the border theme role. Tests cover themed, plain, zero-width, and negative-width output.
Header integration extensions/gentle-shell.ts, lib/shell-sidebar-layout.ts	Adds the rule as a second fullscreen header row, ignores clicks outside the status row, and changes the header basis to "auto".
Responsive layout validation tests/shell-sidebar-layout.test.ts	Updates layout types and expectations for automatic sizing. Tests cover two-line fullscreen rendering and no rule below the sidebar breakpoint.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~12 minutes

Change: Feature · Severity of issue fixed: Low

Sequence Diagram(s)

sequenceDiagram
  participant ShellHeader
  participant ShellRuleRenderer
  participant SidebarLayout
  ShellHeader->>ShellRuleRenderer: Render rule for theme and width
  ShellRuleRenderer-->>ShellHeader: Return border-colored rule row
  ShellHeader->>SidebarLayout: Provide status and rule rows
  SidebarLayout-->>ShellHeader: Measure header with auto basis

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Suggested reviewers: alan-thegentleman

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 14.29% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 7 functions across 5 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the primary change: adding a decorative rule row beneath the fullscreen status line.
Linked Issues check	✅ Passed	Issue #1249 has coding requirements, and the reviewed head meets them. renderShellHeaderRule returns a ─ line with Math.floor(width) width, clamps negative widths to zero, and applies the `borde…
Out of Scope Changes check	✅ Passed	The changes stay within Issue #1249. They add the rule renderer, integrate it into the fullscreen header rail, change header measurement to support the second line, preserve status-row click handling,…

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✨ Finishing Touches

🧪 Generate unit tests (beta)

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[Alan-TheGentleman]

Approved. The change is well-contained and the hard parts are done right.

I verified the layout and mouse behavior against pi-tui 0.85.1:

• basis: "auto" is the right fix: pi-tui measures the leaf by rendering it, so the two-line header sizes itself instead of being clipped by the old hard basis: 1.
• The event.y !== 0 gate is correct: mouse events arrive box-relative, so row 0 keeps the usage affordance and the rule row is inert, exactly as intended.
• Both header lines render at the same width (terminal width minus the header right inset), and the Math.max(0, Math.floor(width)) clamp covers the degenerate cases.

Spot checks on my side: 162 tests pass across the four touched suites, and pnpm run typecheck reports no regressions.

One non-blocking nit for context: headerActive now keys off any non-blank header line, and the rule is always non-blank, so the header stays active even if renderShellHeaderBar ever returned empty text. That only happens below the width where even the bare brand fits, which is unreachable at fullscreen widths, so no change needed. Just worth knowing the rule now keeps the header alive on its own.

Nice work, the tests cover exactly the behavior that could regress.