[kev1n77]

Summary

• Disable Prism's default JSON worker message handler before Prism initializes in the custom highlight worker.
• Keep the application's structured { id, content, language } worker protocol unchanged.
• Add a real Vite-bundle worker regression test that verifies structured messages return highlighted output without the [object Object] JSON parse failure.

Fixes #

Type and Areas

Type: Regression fix / test

Areas: Web UI

Motivation / Impact

Prism automatically installs a JSON-based worker handler when it detects a worker. The application already sends structured objects to the same worker, so Prism attempted to run JSON.parse(event.data) on an object converted to "[object Object]", producing the frontend crash seen in the recent #185 run. The setup module now sets Prism.disableWorkerMessageHandler before Prism is imported, leaving the application's handler as the only message handler.

No direct user-facing copy change. ResizeObserver and Peer workspace issues were intentionally not modified.

Verification

• pnpm run test:run src/flow_chat/timeline/highlight.worker.test.ts — passed (1 test).
• pnpm exec tsc --noEmit --target ES2020 --module ESNext --moduleResolution bundler --lib 'es2020,dom,dom.iterable' --strict --skipLibCheck --esModuleInterop --allowSyntheticDefaultImports src/flow_chat/timeline/prismWorkerSetup.ts src/flow_chat/timeline/highlight.worker.ts src/flow_chat/timeline/highlight.worker.test.ts — passed.
• pnpm run desktop:dev with the requested profile-dev environment — Rust/Tauri build completed and the dev desktop started successfully.
• Manual dev-browser Worker check using the Vite-generated highlight.worker.ts?worker_file&type=module bundle — TypeScript, JSON, Rust, and unknown-language fallback messages all returned; monitored structured-object request returned one response with zero Worker errors.
• Current profile-dev webview log inspection — no highlight.worker, Prism SyntaxError, or "[object Object]" is not valid JSON entry after the fix.

Reviewer Notes

• The rebase was completed onto upstream/main before pushing.
• The change is limited to the highlight worker setup and its focused regression test.
• Remote workspace, Peer Device Mode, and Detached Dispatch scenarios were not exercised; this is a local Web UI Worker fix.
• Full workspace checks were not run because the focused Worker test and TypeScript verification cover the changed files and the desktop dev build was separately validated.
• AI-assisted change; focused runtime and regression verification completed.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable.

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