perf(shorthandCss): merge each distinct style value only once

[cossssmin]

shorthandCss ran a full postcss + postcss-merge-longhand pass for every single style attribute. Utility-generated emails repeat the same inline styles a lot, so most of those runs were redoing the same work.

It now caches the merged result per distinct style value, for the duration of one call (no module-level state, so nothing builds up in long-running processes).

Numbers

shorthandCss alone:

Email	style attributes (distinct)	Before	After
Typical order email (12KB)	107 (33)	3.6ms	1.3ms
Long email (48KB)	627 (33)	18.3ms	1.2ms

Full warm render with the renderer reused: ~41ms → ~36ms for the typical email, ~94ms → ~76ms for the long one.

Output is byte-identical on both emails, and there's a new test for repeated identical style values across elements and MSO conditional comments.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • CSS processing now consistently combines identical padding values into shorthand styles in paragraphs and table cells, including table cells within MSO conditional comments. This helps produce more compact, consistent inline styles across supported content.

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Advanced

Run ID: cca64a02-5ef1-4c1c-9c9e-eda76627da74

📥 Commits

Reviewing files that changed from the base of the PR and between 3a1003a and 6ae0b89.

📒 Files selected for processing (2)

• src/tests/transformers/shorthandCss.test.ts
• src/transformers/shorthandCss.ts

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📝 Walkthrough

Walkthrough

The transformer now caches merged inline-style results for each call and returns cached values for repeated styles. A test checks padding shorthand output on a paragraph, a table cell, and a table cell inside an MSO conditional comment.

Changes

CSS shorthand merging

Layer / File(s)	Summary
Cache merged inline styles and verify output src/transformers/shorthandCss.ts, src/tests/transformers/shorthandCss.test.ts	shorthandCssDom caches results by original style value during each call. It preserves and caches the original value when parsing fails or yields no declaration text. The test checks that the three elements produce padding: 4px 8px without a remaining padding-top declaration.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to 6ae0b

The supplied context identifies no material merge risk from this per-call optimization; proceed with normal checks.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 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 main performance change: caching shorthand CSS merges so each distinct style value is processed only once.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 2 files.
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.

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