fix(chromium): sync navigator.platform with userAgentData when custom UA is set

[manimovassagh]

When a custom userAgent is configured (e.g. via device presets like Desktop Chrome),
navigator.platform leaks the host OS instead of matching the emulated user agent:

• navigator.userAgentData.platform → "Windows" (from the preset UA)
• navigator.platform → "MacIntel" (host OS leaked)

Libraries like React Aria check userAgentData.platform first, get "Windows", and
then expect Windows keyboard behavior — breaking tests that use ControlOrMeta.

Fix across all three browsers:

• Chromium: passes platform parameter in Emulation.setUserAgentOverride, derived from the same UserAgentMetadata already computed for userAgentData
• Firefox: calls Browser.setPlatformOverride alongside Browser.setUserAgentOverride
• WebKit: calls Page.overridePlatform alongside Page.overrideUserAgent

A shared navigatorPlatformFromUA() helper in browserContext.ts derives navigator.platform from the UA string for Firefox and WebKit. Chromium uses its own metadata-driven navigatorPlatform() in crPage.ts for higher precision.

When no custom user agent is set, behavior is unchanged across all browsers.

Setting PLAYWRIGHT_NO_UA_PLATFORM=1 disables the new behavior as an opt-out escape hatch.

Fixes #39568

[pavelfeldman]

Since this change is potentially breaking, please introduce env variable PLAYWRIGHT_NO_UA_PLATFORM that opts in from this behavior. That way we can recommend it to those affected.

While you are here, please explore the feasibility of this fix for WebKit and Firefox.

[manimovassagh]

Just to confirm — should PLAYWRIGHT_NO_UA_PLATFORM enable the platform override (opt-in, off by default), or disable it (opt-out, on by default)? Happy to flip the logic either way.

[manimovassagh]

Updated the PR with both changes:

1. PLAYWRIGHT_NO_UA_PLATFORM env variable
The platform override is active by default. Setting PLAYWRIGHT_NO_UA_PLATFORM=1 disables it, reverting to the previous behavior where navigator.platform reflects the host OS.

2. Firefox and WebKit support
Both browsers already had the protocol commands but weren't using them:

• Firefox: Browser.setPlatformOverride
• WebKit: Page.overridePlatform

Added a shared navigatorPlatformFromUA() helper in browserContext.ts that derives navigator.platform from the user agent string. All three browsers now use it when a custom userAgent is set.

Tested locally on Chromium, Firefox, and WebKit — all passing.

[github-actions]

Test results for "MCP"

2 failed
❌ [chrome] › mcp/autowait.spec.ts:19 › racy navigation destroys context @mcp-windows-latest
❌ [chrome] › mcp/cli-session.spec.ts:176 › list --all lists sessions from all workspaces @mcp-windows-latest

5337 passed, 186 skipped

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Merge workflow run.

[github-actions]

Test results for "tests 1"

2 failed
❌ [webkit-page] › page/page-basic.spec.ts:88 › page.title should not throw during navigation @webkit-ubuntu-22.04-node20
❌ [playwright-test] › ui-mode-test-network-tab.spec.ts:397 › should not preserve selection across test runs @macos-latest-node20

5 flaky

⚠️ [chromium-page] › page/page-request-continue.spec.ts:754 › propagate headers cross origin redirect after interception `@ubuntu-22.04-chromium-tip-of-tree`
⚠️ [chromium-library] › library/video.spec.ts:358 › screencast › should capture navigation `@chromium-ubuntu-22.04-arm-node20`
⚠️ [chromium-library] › library/trace-viewer.spec.ts:1223 › should display language-specific locators `@chromium-ubuntu-22.04-node22`
⚠️ [firefox-library] › library/inspector/cli-codegen-1.spec.ts:1080 › cli codegen › should not throw csp directive violation errors `@firefox-ubuntu-22.04-node20`
⚠️ [webkit-library] › library/browsertype-connect.spec.ts:758 › run-server › should upload a folder `@webkit-ubuntu-22.04-node20`

38776 passed, 845 skipped

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Merge workflow run.