fix(shared): truncate never splits an emoji

[ValeraZSD]

Thread titles could end in � when the truncation boundary split an emoji's UTF-16 surrogate pair. The shared helper now selects a safe boundary before adding the ellipsis, preserving joined emoji, modifiers, combining marks, and paired flag indicators.

The implementation uses a small forward code-point scan and avoids Intl.Segmenter, which shared mobile code cannot use under the repository's Hermes constraint. Ordinary short-text trimming remains unchanged.

Two focused tests replace six overlapping cases. They fail with the original helper, and their cluster assertions also fail with a surrogate-only fix. The helper and thread-title suites pass all 7 tests; targeted lint, formatting, and shared-package typecheck pass. No live client verification was performed during this takeover.

Original fix by @ValeraZSD in #13413. Maintainer takeover by Claude Opus 5.5 and Claude Fable 5.1 through T3 Code, independently reviewed by Claude Opus 5.5 and GPT-6.1-Sol through the Codex harness.

[macroscopeapp]

Approvability

Verdict: Approved at 1ebde9f

Macroscope's review found this PR approvable — This is a small, self-contained shared-string bug fix that preserves existing behavior for ordinary text while preventing emoji and grapheme clusters from being split during truncation. Production impact is limited to safer formatting of over-limit titles and labels, with focused regression coverage.

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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• packages/shared/src/String.test.ts
• packages/shared/src/String.ts

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📝 Walkthrough

The truncate function now selects a grapheme-cluster boundary before appending .... Tests cover truncation boundaries for emoji, combining marks, and regional-indicator pairs.

Changes: String truncation

Layer / File(s)	Summary
Truncation cutoff and tests packages/shared/src/String.ts, packages/shared/src/String.test.ts	truncate scans code points and adjusts its cutoff to avoid splitting supported grapheme clusters. Tests cover family and skin-tone emoji, combining marks, and paired regional indicators.

Priority: ⬇️ Low

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

Change: Bug fix

Suggested reviewers: bentbrain

Merge Risk: ⚪ Minimal · up to 1ebde

Emoji truncation preserves the checked joined-emoji boundary. No actionable merge-blocking risk remains beyond normal checks.

Security architecture risk: ⚪ Minimal · up to 1ebde

The change preserves supported emoji clusters when shortening text. The existing title inputs, caller relationships, and privileges remain unchanged. No material security risk was identified in the changed behavior.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — For the evidenced title consumer, user-influenced text and attachment names can change the resulting shortened title, but the PR does not expand their reachability or grant additional authority. The helper remains a formatter, not an authorization or sanitization control.

Pre-merge checks: 4

Check name	Status	Explanation
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.
Title check		The title clearly and concisely describes the main change: preventing the shared truncation helper from splitting emojis.
Description check		The description explains the problem, implementation, scope, approval context, focused verification, limitations, and the models and harness used. It does not use the template headings, but it provide…

Full details: Description check

Explanation

The description explains the problem, implementation, scope, approval context, focused verification, limitations, and the models and harness used. It does not use the template headings, but it provides the required information in a complete, focused format.

✨ Finishing Touches: 💡 1

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[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@packages/shared/src/String.ts`:
- Line 9: Update truncate to find a grapheme-cluster boundary at or before
maxLength, rather than only avoiding a split UTF-16 surrogate pair; preserve the
existing UTF-16 length limit and suffix behavior, and add a test for truncating
a multi-code-point emoji such as a ZWJ sequence.

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• packages/shared/src/String.test.ts
• packages/shared/src/String.ts

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Dismissing prior approval to re-evaluate e4e34ce

[maria-rcks]

Note

Written by claude-opus-5-5 on behalf of Maria

Hi! We are cleaning up open PRs, and this one does not say which model or harness was used to create it. If this change is really important, we recommend rebuilding the PR with a newer model and noting the model and harness in the PR description.

[maria-rcks]

Note

Written by claude-opus-5-5 on behalf of Maria

Reopening, this was closed by mistake. Sorry for the noise!

[juliusmarminge]

Taking over this one. I’ll reduce the tests to the useful coverage, update the implementation against current main, and verify it before publishing the changes.

Dismissing prior approval to re-evaluate 1ebde9f