Current limitations blocking Playwright's WebDriver BiDi adoption

We've spent a couple weeks adding support for Bidi to Playwright and running Playwright tests with the new protocol. The good news is there has been a large progress in Bidi support in Firefox and Chromium, many Playwright tests are now passing. However, there is still a long list of issues that are blocking Playwright from adopting Bidi as the browser automation protocol. At this point the number of bugs we identified is too large to file individual reports for each of them, instead we'll use this issue to describe some of the major problems that we've discovered.

🎉 Current stats are the following:
- Chromium: 2368 / 3877 passed (3.2m)
- Firefox: 1469 / 3877 passed (4.1m)

## Spec

- [ ] Provide a way to configure the viewport before a popup or new tab starts loading content. Currently, the `browsingContext.SetViewport` command can only be sent to an existing page, and there is no way to pause the load of a new page (e.g., opened via ctrl+click or a popup); these pages always load with the default viewport first.
- [ ] Allow to configure options(aka capabilities) such as `proxy` and `acceptInsecureCerts` per user context rather than per `session`, so that same browser process can be reused for testing with different options.
- [ ] Add support for `isMobile` emulation to enable fixed layout mode.
- [ ] Allow specifying a screen size that is different from the viewport size.
- [ ] Include support for landscape mode emulation.
- [ ] Add support for getting content quads to ensure accurate clicking inside transformed elements. This is particularly relevant for tests involving transformed elements, as seen in `tests/page/page-click.spec.ts`.
- [ ] Implement `DOM.getFrameOwner` and `DOM.describeNode` to enable converting between DOM node handles and Frame handles. This would improve the implementation of `Locator.contentFrame()` and `FrameLocator.owner()`, as the current workaround using `contentWindow` does not work with shadow DOM, as seen in the `page/frame-frame-element.spec.ts` tests.
- [ ] Introduce an event or method to track when a navigation is committed, beyond the current `DOMContentLoaded` and `load` events, which would be useful for some client requirements (e.g., 'should return when navigation is committed if commit is specified'). We need to know immediately if the navigation has failed or succeeded.
- [ ] Provide access to the request body (see issue: https://github.com/w3c/webdriver-bidi/issues/748).
- [ ] Provide access to the response body (see issue: https://github.com/w3c/webdriver-bidi/issues/747).
- [ ] Include `request.resourceType` support (see issue: https://github.com/w3c/webdriver-bidi/issues/704).
- [ ] Add `suggestedFileName` to `browsingContext.downloadWillBegin`.
- [ ] Enable retrieval of the body of a download.
- [ ] Add locale emulation (https://github.com/w3c/webdriver-bidi/issues/774), including:
- `accept-language` headers
- JS Date and Intl APIs
- `navigator.language`
- [ ] Add support for timezone emulation (see issue: https://github.com/w3c/webdriver-bidi/issues/749).
- [ ] Improve user agent emulation to be per user context (see https://github.com/w3c/webdriver-bidi/issues/448#issuecomment-1944294296)
- Interception is prohibitively expensive and therefore impractical.
- Also not all requests are intercepted, e.g. WebSocket.
- [ ] Add support for setting extra HTTP headers per user context, same as user agent https://github.com/w3c/webdriver-bidi/issues/777
- [ ] Allow to call `script.addPreloadScript` per context, so that the script is always evaluated before navigation in the page. See e.g. `'init script should run only once in popup'` [test](https://github.com/microsoft/playwright/blob/cd4dabef8bd0c80cf91c93fd170be1aaf87cc7a9/tests/page/page-add-init-script.spec.ts#L102-L111) in `page-add-init-script.spec.ts`.
- [ ] Support offline mode emulation https://github.com/w3c/webdriver-bidi/issues/776
- [ ] Support emulation of disabled JavaScript https://github.com/w3c/webdriver-bidi/issues/746

These are just some of the issue that we discovered while looking at the first cut. There are many other issues that are currently blocked by the ones above.

## Implementation

- [x] Firefox: `browsingContext.setViewport` fails intermittently with `{"type":"error","id":14,"error":"unknown error","message":"AbortError: Actor 'MessageHandlerFrame' destroyed before query 'MessageHandlerFrameParent:sendCommand' was resolved","stacktrace":""}`. See this [bug report](https://bugzilla.mozilla.org/show_bug.cgi?id=1918287).
- [x] Firefox: `page.setContent` doesn't work because of 'Error: page.setContent: SecurityError: The operation is insecure.' in utility context. [Bug report](https://bugzilla.mozilla.org/show_bug.cgi?id=1918288)
- [ ] Firefox `pointerDown/pointerUp/pointerDown/pointerUp` in separate `input.performActions` commands don't generate `dblclick` event, see 'should dblclick the div' in page-mouse.spec.ts
- [ ] Firefox: `script.realmDestroyed` comes after `browsingContext.contextDestroyed`
- [ ] Firefox: `script.evaluate` exception have no details/message
- [ ] Firefox: `browsingContext.contextDestroyed` comes before input.performActions response, see 'should not throw UnhandledPromiseRejection when page closes'. I.e. some events can come after browsingContext was destroyed.
- [ ] Firefox: navigation id stays the same for different navigations, see 'should return from goto if new navigation is started'
- [x] Intermittent Error: Protocol error (script.evaluate): unknown error
Message: AbortError: Actor 'MessageHandlerFrame' destroyed before query 'MessageHandlerFrameParent:sendCommand' was resolved - apparently fails if a command is sent before about:blank navigation finishes.
- [x] Firefox: lazy loading iframes are not reported (can be disabled in settings I think)
- [ ] Firefox: `setViewport` does not affect `window.screen.width/height`, `matchMedia`, see 'should emulate device width'
- [ ] Firefox: CSP tests are failing
- [x] Firefox: \"url\" not supported yet in network.continueRequest
- [ ] Firefox: no requestfailed event when it's cancelled by the page, see 'should not throw if request was cancelled by the page' in
- [ ] Firefox: 'should amend utf8 post data' - doesn't work with non-latin post data
- [ ] Firefox: 'redirected requests should report overridden headers' - does not allow to override headers on redirects
- [ ] Firefox: `browsingContext.create` sometimes hangs in parallel tests
- [ ] Firefox: `browsingContext.downloadWillBegin` is not fired
- [ ] Firefox: "Blocked request with id 15 not found" when sending network.continueWithAuth, authentication not working
- [x] Firefox: `about:blank` page is required for firefox to not close even in headless! (--silent didn't work)
- [x] Firefox: crash reporter dialog, can probably be fixed with settings.
- [ ] Firefox: need canonical settings for testing browser
- [x] Chromium: Method 'network.setCacheBehavior' is not implemented.

Start by selecting one unchecked limitation and reading its referenced Playwright test, such as tests/page/page-click.spec.ts, tests/page/page-frame-frame-element.spec.ts, page-add-init-script.spec.ts, or page-mouse.spec.ts. Compare the expected behavior with the relevant WebDriver BiDi capability, then run the named test and its surrounding suite. Done means the selected limitation is implemented and the affected test passes without regressing the broader BiDi suite.

Written by the indexing model from the issue text.