fix(ui): preserve syntax state across folded hunks

[benvinegar]

Summary

• highlight partial VCS diffs against validated old/new source prefixes so hidden multiline openers preserve Shiki grammar state
• remap source-backed tokens onto the existing Pierre rows while retaining per-side syntax and word-diff styling
• fall back to patch-fragment highlighting when source is unavailable, stale, or rejected
• invalidate highlight caches when the source provider changes
• add unit and PTY regression coverage for the Elixir heredoc case in Highlighting mis-tokenization when a hunk starts inside an Elixir heredoc #664

Screenshots

Before

Patch-only highlighting incorrectly keeps def hello do in the hidden heredoc's string state.

After

Source-backed highlighting restores def to the Elixir keyword state.

Testing

• bun run typecheck
• bun run lint
• bun run format:check
• focused unit tests: 26 passed
• bun run test:integration: 87 passed
• bun run test:tty-smoke: 9 passed
• real Git/PTY Elixir reproduction

Notes

This corrects source-backed reviews such as diff, show, and stash show. Arbitrary source-less hunk patch input remains best-effort because omitted lexical context cannot be reconstructed reliably.

bun run test has one unrelated existing failure in src/extensions/hostRuntimeModules.test.ts, reproduced on untouched origin/main.

Addresses #664.

This PR description was generated by Pi using OpenAI GPT-5.6

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Project	Deployment	Actions	Updated (UTC)
hunk-web	Ready	Preview	Aug 5, 2026 4:34pm

[greptile-apps]

Greptile Summary

This PR preserves lexical state for partial source-backed diffs by validating full-source prefixes, highlighting them through Pierre, and remapping tokens onto the visible patch rows.

• Adds source-prefix validation and old/new line-index remapping with patch-fragment fallback
• Extends highlight-cache invalidation to account for source providers
• Adds unit and PTY regression coverage for folded multiline syntax

Confidence Score: 4/5

The PR appears safe to merge, with a non-blocking watch-mode cache-efficiency issue for unchanged partial files.

Source-prefix validation and token remapping have guarded fallbacks and focused regression coverage, but cache invalidation follows newly allocated fetcher objects rather than stable source revisions, forcing unnecessary source loading and highlighting after reloads.

Files Needing Attention: src/ui/diff/useHighlightedDiff.ts

Important Files Changed

Filename	Overview
src/ui/diff/sourceBackedHighlight.ts	Adds validation, prefix-metadata construction, and deterministic remapping of full-source highlight lines to partial-diff indexes.
src/ui/diff/pierre.ts	Integrates source-backed highlighting with guarded fallback to the existing patch-fragment and plain-text paths.
src/ui/diff/useHighlightedDiff.ts	Invalidates highlights by source-fetcher identity, which prevents stale reuse but also defeats cache reuse across equivalent reload-created providers.
src/ui/diff/sourceBackedHighlight.test.ts	Covers source-prefix grafting, stale-source rejection, remapping, and absent sides for added or deleted files.
test/pty/highlighting.test.ts	Adds an end-to-end terminal regression test for preserving Elixir heredoc grammar state.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart LR
  A[Partial diff metadata] --> B{Source fetcher available?}
  B -- No --> F[Highlight patch fragment]
  B -- Yes --> C[Fetch old and new source]
  C --> D{Visible lines and gaps validate?}
  D -- No --> F
  D -- Yes --> E[Highlight validated source prefixes]
  E --> G[Remap tokens to partial line indexes]
  F --> H[Build split or stack rows]
  G --> H

Loading

Prompt To Fix All With AI

### Issue 1
src/ui/diff/useHighlightedDiff.ts:89-94
**Fetcher identity defeats cache reuse**

Each session reload rebuilds `DiffFile` and its source fetcher, so assigning cache IDs by fetcher object identity gives an unchanged partial file a new highlight key. This discards reusable highlights and prefetched work, forcing another source fetch and highlight pass and causing an avoidable plain-text-to-highlighted repaint in watch mode.

---

For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

_{Reviews (1): Last reviewed commit: "fix(ui): preserve syntax state across fo..." | Re-trigger Greptile}

[greptile-apps]

Fetcher identity defeats cache reuse

Each session reload rebuilds DiffFile and its source fetcher, so assigning cache IDs by fetcher object identity gives an unchanged partial file a new highlight key. This discards reusable highlights and prefetched work, forcing another source fetch and highlight pass and causing an avoidable plain-text-to-highlighted repaint in watch mode.

Knowledge Base Used: Diff Rendering Pipeline

Prompt To Fix With AI

This is a comment left during a code review.
Path: src/ui/diff/useHighlightedDiff.ts
Line: 89-94

Comment:
**Fetcher identity defeats cache reuse**

Each session reload rebuilds `DiffFile` and its source fetcher, so assigning cache IDs by fetcher object identity gives an unchanged partial file a new highlight key. This discards reusable highlights and prefetched work, forcing another source fetch and highlight pass and causing an avoidable plain-text-to-highlighted repaint in watch mode.

**Knowledge Base Used:** [Diff Rendering Pipeline](https://app.greptile.com/modem/-/custom-context/knowledge-base/modem-dev/hunk/-/docs/ui-diff-rendering.md)

---

For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

Note: If this suggestion doesn't match your team's coding style, reply to this and let me know. I'll remember it for next time!

[benvinegar]

Fixed in 42c76fe. Source readers can now expose a stable sourceCacheKey, which is combined with the per-file patch fingerprint. The Git adapter derives it from resolved refs and semantic index entries, so equivalent reload-created fetchers reuse highlighted output while changed source bases still invalidate safely. Added cache, adapter-boundary, and Git regression coverage plus extension documentation.

_{Responded by Pi using OpenAI GPT-5.6.}

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