Browser mode: `vi.mock` has no effect on `packages/app-utils` when the root is `packages/app`

[saryn17]

Describe the bug

With the Playwright provider and the root at packages/app, vi.mock('../../app-utils/src/value') has no effect. A mock for packages/lib works.

The module is served as /@fs/.../packages/app-utils/src/value.ts. resolveMockPath() strips the root with path.startsWith(fsRoot), which also matches app-utils. The mock ends up registered as -utils/src/value.ts, and the Playwright route never matches the request.

vitest/packages/mocker/src/browser/mocker.ts

Lines 258 to 272 in 428e2e5

	private resolveMockPath(path: string) {
	const config = this.config
	const fsRoot = join('/@fs/', config.root)
	// URL can be /file/path.js, but path is resolved to /file/path
	if (path.startsWith(config.root)) {
	return path.slice(config.root.length)
	}
	if (path.startsWith(fsRoot)) {
	return path.slice(fsRoot.length)
	}
	return path
	}

packages/mocker/src/node/resolver.ts checks the boundary with withTrailingSlash(root). I have a fix with a test that does the same in resolveMockPath() and will open a PR.

I used Claude Code to help track this down.

Reproduction

https://github.com/sary8/vitest-browser-mock-sibling-root

npm install
npx playwright install chromium
npm test

The module in packages/app-utils test fails with expected 'real' to be 'mocked'.

System Info

System:
    OS: Windows 11 10.0.26200
    CPU: (16) x64 AMD Ryzen AI 7 350 w/ Radeon 860M
    Memory: 4.19 GB / 31.29 GB
  Binaries:
    Node: 25.2.1 - C:\Program Files\nodejs\node.EXE
    npm: 11.6.2 - C:\Program Files\nodejs\npm.CMD
    pnpm: 10.33.0 - C:\Users\sarys\AppData\Roaming\npm\pnpm.CMD
    bun: 1.4.2 - C:\Users\sarys\AppData\Roaming\npm\bun.CMD
  Browsers:
    Chrome: 154.0.8037.57
    Edge: Chromium (153.0.4234.32)
  npmPackages:
    @vitest/browser-playwright: 5.0.2 => 5.0.2
    playwright: 1.63.0 => 1.63.0
    vitest: 5.0.2 => 5.0.2

Used Package Manager

npm

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