test: setOffline() persists across navigations

[Dmitry Gozman (dgozman)]

References #42174.

[github-actions]

Test results for "tests 1"

3 flaky

⚠️ [chromium-library] › library/video.spec.ts:664 › screencast › should capture full viewport `@chromium-ubuntu-22.04-arm-node20`
⚠️ [chromium-library] › library/chromium/chromium.spec.ts:301 › should report intercepted service worker requests in HAR `@realtime-time-library-chromium-linux`
⚠️ [chromium-library] › library/video.spec.ts:664 › screencast › should capture full viewport `@chromium-ubuntu-22.04-node20`

51124 passed, 1214 skipped

---

Merge workflow run.

[github-actions]

Test results for "MCP"

3 failed
❌ [chromium] › mcp/cli-session.spec.ts:56 › close-all @mcp-macos-latest-chromium
❌ [firefox] › mcp/cli-core.spec.ts:57 › click link @mcp-windows-latest-firefox
❌ [webkit] › mcp/cli-killall.spec.ts:42 › kill-all kills filtered dashboard pid @mcp-windows-latest-webkit

8098 passed, 1311 skipped

---

Merge workflow run.

[github-actions]

Hi, I'm the Playwright bot and I took a first look at the CI failures.

🟢 CI is clear — all three failures are pre-existing flakes

This PR only adds a test to browsercontext-basic.spec.ts and touches no MCP CLI code. The three failing tests are unrelated MCP CLI tests, and each one flips verdict on unrelated SHAs across the results DB.

Details

The tests 1 job had no failures (3 flaky, rescued on retry). The three real failures are all in the MCP job, in files this PR doesn't touch.

Pre-existing flake / infra

• [firefox] › mcp/cli-core.spec.ts:57 › click link — flake. Across the DB this test failed 13 of 726 firefox runs (~2%) on unrelated SHAs.
• [chromium] › mcp/cli-session.spec.ts:56 › close-all — flake. Failed 2 of 732 chromium runs and passed the rest, on unrelated SHAs.
• [webkit] › mcp/cli-killall.spec.ts:42 › kill-all kills filtered dashboard pid — flake. Failed 7 of 739 webkit runs (also flaky on chromium and chrome), on unrelated SHAs.

The PR's diff is confined to a navigator.onLine browser-context test — it doesn't exercise the MCP CLI (close-all, click link, kill-all) paths, so it can't be the cause.

_{Triaged by the Playwright bot - agent run}