Restore urlOpener / auth-browser customization for signInWithRedirect on React Native (v6)

Is this related to a new or existing framework?

React Native

Is this related to a new or existing API?

Authentication

Is this related to another service?

Amazon Cognito (Hosted UI / Managed Login)

Describe the feature you'd like to request

On React Native, signInWithRedirect() (Amplify v6) opens the Cognito Hosted UI inside a system auth session — ASWebAuthenticationSession on iOS and Custom Tabs on Android — via @aws-amplify/rtn-web-browser. There is no supported way to customize how that browser is opened or presented.

In Amplify v5 the Auth OAuth config accepted a urlOpener option, which gave apps control over the browser step. That hook was removed for React Native in v6, so this customization is no longer possible.

Concrete problem this blocks: The Cognito Hosted UI renders its form inputs with a font-size below 16px. On iOS, Safari/WebKit auto-zooms the viewport whenever a focused input's font-size is < 16px (see https://css-tricks.com/16px-or-larger-text-prevents-ios-form-zoom/). The result is a jarring zoom-in every time the user taps the email/password field on the Hosted UI, and there is currently no way for developers to fix it:

• We can't inject CSS / a <meta viewport maximum-scale=1> into ASWebAuthenticationSession — it's a system-provided browser, not a WebView we own.
• Cognito classic Hosted UI CSS customization does not allow font-size on inputField-customizable (only width/height/color/background-color/border).
• Cognito Managed Login branding has no fontSize field anywhere in its settings schema (the input class exposes only borderRadius / backgroundColor / borderColor / placeholderColor).
• v6 removed the urlOpener hook, so we can't route the OAuth flow through a browser we control either.

So the app is left with no supported path to prevent the iOS auto-zoom on the Hosted UI sign-in form.

Describe the solution you'd like

Restore a supported browser-customization hook for signInWithRedirect() on React Native — e.g. bring back a urlOpener (or equivalent) option so apps can control how the Cognito Hosted UI is opened and, if needed, present it in a WebView they own where standard workarounds (injected CSS / viewport meta) are possible.

Describe alternatives you've considered

• Cognito classic Hosted UI CSS customization — font-size is not an allowed property on inputField-customizable, so it's stripped.
• Cognito Managed Login branding editor / API — no font-size control exists in the branding settings schema.
• App-side viewport/CSS injection — impossible against ASWebAuthenticationSession.
• Building a fully custom sign-in screen with the non-hosted Amplify APIs (signIn, confirmSignIn, etc.) — works, but requires abandoning the Hosted UI and re-implementing every flow, which is a large workaround for a presentation bug.

Additional context

• iOS auto-zoom threshold: input font-size < 16px triggers a viewport zoom on focus.
• Related: #3410 (Ability to customize pop ups when calling signInWithRedirect on iOS).
• Environment: aws-amplify ^6.16.4, @aws-amplify/react-native ^1.3.3, @aws-amplify/rtn-web-browser ^1.4.0, Expo, iOS.