fix(web): preserve Mermaid source when copying selected text

[NikitaMGrimm]

Problem

Selecting prose across a rendered Mermaid diagram drops the diagram from copied Markdown in assistant and sent-user messages. Drag from the text above a diagram to the text below it, copy, and paste: only the prose remains.

Change

Give rendered diagrams the existing Markdown copy override containing fenced Mermaid source. Crossing selections preserve the selected surrounding prose; selections wholly inside a diagram copy its complete source. Partial “Show code” selections remain partial. Diagram-source links are excluded when choosing structured context records to export.

Scope and approval

Submitted under the very small, focused obvious-bug exception: the established selection-to-Markdown workflow loses rendered message content. The fix stays in the shared web/desktop renderer and adds no dependency or setting. No prior maintainer approval is claimed.

Verification

Previously recorded on this PR head; not rerun in this audit:

• vp test run apps/web/src/components/chat/MessagesTimeline.test.tsx apps/web/src/markdown-clipboard.mermaid.test.ts apps/web/src/markdown-clipboard.test.ts: passed. Regressions cover crossing, inside, start/end-inside selections, source backticks, partial source-view copying, and exporting an image record only when its chip is selected, even when its link appears in a Mermaid comment.
• Web typecheck, targeted lint, formatting, and diff checks passed; lint reported existing timeline warnings.
• Isolated T3 Browser checks used mouse selection and native copy/paste shortcuts in both message roles. Selection-range checks also covered collapsed/expanded user messages and partial source copying.

Dragging from a diagram label still does not start a text selection. Packaged Electron and native mobile were not exercised.

Before is left; after is right. The recording copies the first three blocks of an expanded user message, leaving its final instruction unselected.

mermaid-selection-copy-before-after.mp4

Note

🤖 Agent assistance: gpt-6.1-sol via Codex

[macroscopeapp]

Approvability

Verdict: Approved at 7c7a2d1

Macroscope's review found this PR approvable — This is a small, well-contained clipboard bug fix that preserves Mermaid source while keeping structured context exports limited to actually selected chips. The production changes are localized to existing chat rendering and copy handling, with targeted regression coverage and no product-default or static-analysis configuration changes.

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[coderabbitai]

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ℹ️ Recent review info

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• Configuration used: Repository: pingdotgg/t3code/.coderabbit.yaml
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• Plan: Advanced
• Run ID: 131a7c28-9f63-4789-858a-92763cadc1bd

📥 Commits

Reviewing files that changed from the base of the PR and between 6446708 and 7c7a2d1.

📒 Files selected for processing (2)

• apps/web/src/components/chat/MessagesTimeline.test.tsx
• apps/web/src/components/chat/MessagesTimeline.tsx

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📝 Walkthrough

Walkthrough

The change adds an exported Markdown code-block serializer and uses it to preserve Mermaid source in clipboard payloads for selections within rendered diagrams. The copied-Markdown scan excludes Mermaid elements when identifying structured context records.

Changes

Mermaid Clipboard

Layer / File(s)	Summary
Markdown code-block serialization apps/web/src/markdown-clipboard.ts, apps/web/src/markdown-clipboard.mermaid.test.ts	The exported serializeCodeBlockToMarkdown helper trims one trailing newline, selects a fence, adds an optional language label, and formats the closing fence and spacing. The existing serializer delegates to this helper. A test checks that the fence is longer than a backtick run in the source.
Diagram clipboard payload apps/web/src/components/chat/MermaidDiagram.tsx, apps/web/src/markdown-clipboard.ts, apps/web/src/markdown-clipboard.mermaid.test.ts	The diagram wrapper stores its source as fenced Mermaid Markdown. Clipboard handling uses that source and sanitized selected HTML when the selection is within the wrapper. Tests cover selections across and inside a diagram, and partial selections in Mermaid source view.
Context-record copy handling apps/web/src/components/chat/MessagesTimeline.tsx, apps/web/src/components/chat/MessagesTimeline.test.tsx	The copied-Markdown scan excludes Mermaid elements when identifying structured context records. Tests check clipboard behavior when a selected context chip is present or absent.

Priority: ⬇️ Low

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

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant MermaidDiagram
  participant DOMSelection
  participant chatMarkdownClipboardPayload
  MermaidDiagram->>chatMarkdownClipboardPayload: store serialized Mermaid Markdown
  DOMSelection->>chatMarkdownClipboardPayload: provide selected range
  chatMarkdownClipboardPayload->>MermaidDiagram: read stored Markdown source

Loading

Suggested reviewers: t3dotgg

Merge Risk: ⚪ Minimal · up to 7c7a2

Mermaid source remains available for copying without being treated as a context record. No issue identified here requires a change before merging.

Security Architecture Review

Security architecture risk: ⚪ Minimal · up to 7c7a2

Copying a selected diagram now preserves its source without granting additional access or automatically attaching related context. No material security risk was found in the changed behavior.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The introduced exposure is the complete source of a selected rendered diagram, including source not visible in its labels, entering the user's clipboard. In the inspected path, copying that source alone does not authorize context-record export or import.

Trust Boundaries and Controls

• observed — Mermaid copy attributes are excluded from structured-record discovery. Remaining selected references must match message records before export. On paste, structured clipboard data is size-bounded and schema-decoded, then records are filtered against references in the pasted text. The regression test includes a context-looking link inside Mermaid source and verifies that it does not independently trigger record export.

Resilience and Maintainability Implications

• observed — Missing clipboard data and collapsed selections return without a custom write; selections without matching context records leave structured capture inactive. Multiple ranges use a fresh collection and deduplicated reference IDs. Sequential, non-atomic clipboard writes predate this PR; the selector change does not alter their ordering or introduce a new persistent recovery state.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 12.50% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 8 functions across 5 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
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.
Title check	✅ Passed	The title clearly identifies the main change: preserving Mermaid source when copying selected text.
Description check	✅ Passed	The description covers the problem, change, scope exception, and focused verification results. It also notes limitations and includes UI evidence and agent details.

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[NikitaMGrimm]

Note

🤖 gpt-6.1-sol responding on behalf of NikitaMGrimm

Verified the structured-context concern and fixed it in 7c7a2d1. The user-message copy handler now excludes Mermaid source overrides when choosing context records; the source still copies as fenced Markdown. The regression failed before the fix and now confirms that an image link hidden in a Mermaid comment exports no image record unless its chip is selected. All 115 tests across the timeline and clipboard suites, web typecheck, and targeted lint/format checks passed (existing lint warnings only).

[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!

Dismissing prior approval to re-evaluate 7c7a2d1