Vitest browser workspace packages under Yarn PnP may need packageExtensions for vite

What happened

After moving slate-browser from a nested npm island into a real workspace package, the Bun lane still passed and the Playwright lanes still passed.

The Vitest browser lane broke under Yarn PnP with:

• @vitest/mocker tried to access vite

This was not the package forgetting to depend on Vitest. It was the browser stack relying on vite through a chain Yarn PnP considered undeclared.

What fixed it

The minimal fix was a Yarn package extension in .tmp/slate-v2/.yarnrc.yml:

packageExtensions:
  "@vitest/mocker@*":
    dependencies:
      vite: "*"

After that:

• yarn workspace slate-browser test worked as a real workspace command
• the browser contract lane stopped needing the old nested npm escape hatch

Reusable rule

If a Vitest browser lane moves from a nested npm package into a Yarn PnP workspace package, expect hidden undeclared dependency assumptions to surface.

Do not immediately retreat back to a sidecar npm island.

First check whether the failure is really a PnP package-extension problem and fix that at the Yarn boundary.