* fix(frontend): default to Webpack over Turbopack in dev to avoid PostCSS worker leak on macOS
On macOS arm64, Turbopack + Next.js 16.2.11 + Tailwind CSS v4 causes an
unbounded spawn of PostCSS evaluator processes that consume high CPU and
memory and never return a response. Webpack is unaffected.
Change the no-override default in getDevBundler() from platform-dependent
Turbopack (all non-Windows) to Webpack. DEER_FLOW_DEV_BUNDLER=turbo
continues to work as an explicit opt-in for local diagnosis.
Fixes#5132
* docs(frontend): address webpack default review feedback
* docs(frontend): clarify webpack default rationale
* Stabilize Windows frontend development startup
Next.js Turbopack can infer a user-level workspace root and hit an internal Rust task panic on Windows. Keep the frontend root explicit and select Webpack on Windows while preserving Turbopack elsewhere.
Constraint: The existing non-Windows development workflow should keep Turbopack.
Rejected: Disable Turbopack on every platform | unnecessary performance regression.
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: Revisit the Windows Webpack fallback after the upstream Turbopack panic is resolved.
Tested: Prettier, ESLint, TypeScript, 139 frontend test files with 1060 tests, and Windows pnpm dev smoke test on port 3010.
Not-tested: Production build and browser E2E suite.
Related: #4957
* Make the frontend bundler fallback diagnosable
Keep the Windows Webpack fallback while allowing local diagnosis to select either supported Next.js bundler. Resolve the Turbopack root through the Node 20-compatible URL API so the explicit workspace boundary remains effective across the declared Next.js engine range.
Constraint: Windows defaults to Webpack because of the observed Turbopack panic, while non-Windows defaults to Turbopack.
Rejected: Accept arbitrary bundler values | Next.js only supports the two explicit flags and invalid values should fail early.
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: Revisit the default Windows fallback when the upstream TaskGuard issue is resolved; the override is intentionally retained for diagnosis.
Tested: Frontend format, ESLint, TypeScript, 139-file/1062-test Rstest suite, target launcher tests, and next.config.js absolute-root load check.
Not-tested: Production build and browser E2E suite.
