fix(printer): use @page size:auto so thermal receipts print full-size

[kilbot]

Problem

After updating to a 1.9.x build, printing to a thermal receipt printer via the system print dialog produces a tiny, unreadable receipt floated at the top of the page instead of one that fills the 80mm roll.

Root cause

buildPrintableReceiptHtml emitted @page { size: 80mm } (or 58mm) for thermal templates. A single length in the CSS size descriptor sets both page dimensions → an 80mm × 80mm square page (verified in headless Chromium: the generated HTML renders to an 80.1×80.1mm PDF). Electron prints through the interactive window.print() dialog (apps/electron/src/main/print-external-url.ts, via a data: URL from system-print-adapter.electron.ts), which honours @page; the browser then shrinks that square onto the printer's real roll paper (80 (72) × 297mm) → the tiny result.

Why the previous value wasn't the fix either

Before a499ded9c, this emitted ${paperWidth} auto (80mm auto). But the @page size grammar is <length>{1,2} | auto | <page-size> — there is no "length + auto" form. 80mm auto is invalid; browsers ignore it and fall back to Letter/A4 (confirmed: 215.9×279.4mm). That was the P1 Codex finding on #429, which a499ded9c "fixed" by switching to the valid-but-square 80mm. So:

Value	Valid?	Result
80mm auto (pre-a499ded9c)	❌ invalid	ignored → prints on Letter/A4 (tiny)
80mm (shipped v1.9.0–v1.9.6)	✅ valid	80mm square → tiny/chopped
auto (this PR)	✅ valid	uses the printer's roll paper, content shrink-fits to the roll width

CSS @page size cannot express a fixed-width, content-height page, so we defer to the printer's own paper.

Fix

Emit @page { size: auto; margin: 0; } for thermal templates (A4 unchanged). The browser uses the thermal printer's selected roll paper (e.g. 80×297mm) and shrink-fits the receipt content to the roll width.

Tradeoff: this relies on the user's printer being set to its thermal roll paper (which it is for a thermal printer). It does not force an 80mm width if a thermal template is sent to an A4 printer — an uncommon case. A width-forcing alternative would require measuring the rendered height and emitting a valid two-length size: 80mm <height>mm, which needs changes in the Electron main print flow (submodule) and was deliberately deferred.

Testing

• packages/printer vitest: 409 passed, incl. updated assertions that thermal emits size: auto and guards against both broken forms (80mm square and invalid 80mm auto); added 58mm coverage.
• pnpm run lint — 0 errors; pnpm typecheck --force — passes.
• Reproduced the square page box (80.1×80.1mm) and the invalid-auto Letter fallback (215.9×279.4mm) in headless Chromium.

⚠️ Remaining validation

Headless/PDF backends don't model a thermal printer's roll paper, so the size: auto render must be confirmed on a real thermal printer (e.g. the PRP-250C in the original report) before release. This is the one step I can't do here.

Merchant workaround (no update needed)

• Settings → Printing → switch the receipt template back to the standard/A4 template, or
• configure the thermal printer as a direct ESC/POS connection (Network / USB / Bluetooth), which bypasses this HTML print path entirely.

[greptile-apps]

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[coderabbitai]

📝 Walkthrough

Walkthrough

The pageSize computation for thermal (non-a4) receipt widths in buildPrintableReceiptHtml was changed to append "auto" to the width value, allowing flowing height instead of a fixed square page size. Corresponding tests were updated to reflect this new @page sizing string.

Changes

Thermal page sizing fix

Layer / File(s)	Summary
Page sizing logic and test updates packages/printer/src/print-html.ts, packages/printer/src/__tests__/print-html.test.ts	pageSize for thermal widths now uses "<width> auto" instead of a fixed width, with a comment explaining the fix; tests for 80mm/58mm and prepareSystemPrintHtml updated to expect the new @page sizing string.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Poem

A rabbit hopped to print a bill,
The paper squished, undersized still.
"Auto height!" the rabbit cried,
Now receipts flow long and wide.
Tests confirm the fix held tight — 🐇🧾

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly matches the main fix: changing thermal receipt printing to use @page size:auto for full-size output.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/receipt-print-page-size

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_{Comment @coderabbitai help to get the list of available commands.}

[kilbot]

⚠️ Hold — do not merge as-is. This restores @page { size: 80mm auto }, but per CSS Paged Media the size descriptor has no <length> auto form — it's invalid syntax that browsers ignore, falling back to Letter/A4 (confirmed: headless Chromium renders the generated HTML at 215.9×279.4mm). That's exactly the bug Codex flagged as P1 on #429, which a499ded9c fixed by switching to a valid single length 80mm.

The real problem: CSS @page size cannot express "fixed width, content-driven height". 80mm (valid) → 80mm square → the tiny-receipt bug; 80mm auto (invalid) → ignored → Letter. Both wrong.

Reworking toward a spec-valid fix (either size: auto to honour the printer's selected roll paper, or a two-length size with a real/measured height). Converting to draft.

[github-actions]

📊 Test Coverage Report

Package	Statements	Branches	Functions	Lines
@wcpos/core	🔴 23.1%	🔴 26.9%	🔴 22.3%	🔴 22.6%
@wcpos/components	🔴 24.6%	🔴 27.3%	🔴 16.8%	🔴 24.3%
@wcpos/database	🟡 60.2%	🔴 55.0%	🟡 62.7%	🔴 59.7%
@wcpos/hooks	🔴 52.4%	🔴 50.1%	🔴 45.0%	🔴 52.4%
@wcpos/utils	🔴 55.9%	🟡 61.5%	🟡 71.4%	🔴 53.8%
@wcpos/query	🟢 83.9%	🟡 76.9%	🟡 77.7%	🟢 84.0%
Average	🔴 50.0%	🔴 49.6%	🔴 49.3%	🔴 49.5%

Coverage Legend

• 🟢 Good (≥80%)
• 🟡 Moderate (60-79%)
• 🔴 Needs improvement (<60%)
• ⚪ No coverage data

--- _{🤖 Updated by GitHub Actions • [View full report](https://github.com/wcpos/monorepo/actions/runs/28787520448)}

[github-actions]

🚀 Deployment Summary

Item	Status
Preview URL	❌ Failed to deploy
E2E Tests	⏭️ Skipped
Commit	a0d6d41

---

_{🤖 Updated by GitHub Actions}