App Router: misc correctness stragglers (root-layout redirect, global-not-found default, esm `.js`, react@experimental, middleware-base-path)

[james-elicx]

This issue was created by an agent auditing CI failures from the Next.js Deploy Suite (vinext main vs Next.js v16.2.6, 2026-06-08).

Problem

A handful of small, independent App Router correctness failures with no shared root cause and no existing tracking issue. Grouped here for triage; split out if any turns out to be substantial.

Affected Test Suites

• test/e2e/app-dir/root-layout-redirect/root-layout-redirect.test.ts (1 — "should work using browser"; redirect issued from the root layout)
• test/e2e/app-dir/global-not-found/not-present/not-present.test.ts (1 — "should render default 404 when global-not-found is not defined but enabled")
• test/e2e/app-dir/app-esm-js/index.test.ts (1 — "should be able to render nextjs api in app router"; .js ESM extension)
• test/e2e/app-dir/rsc-basic/rsc-basic-react-experimental.test.ts (1 — "should opt into the react@experimental when enabling $flag")
• test/e2e/middleware-base-path/test/index.test.ts (1 — "should execute from absolute paths")

Estimated impact

~5 e2e failures across 5 suites.

Recommendation

Reproduce and triage each independently; promote any non-trivial one to its own issue. middleware-base-path is adjacent to the closed basePath issues (#1329 / #1333).