feat(android): opt-in Partial Custom Tab via androidCustomTabPartialHeightFraction

[omeratt]

Fixes #1118

Description

Adds an opt-in androidCustomTabPartialHeightFraction AuthConfiguration
field that, on Chrome 107+, renders the authorization Chrome Custom Tab as
a user-resizable bottom sheet at the caller-specified fraction of the
screen height — matching the modal feel of iOS's
ASWebAuthenticationSession. See #1118 for the UX motivation and
side-by-side screenshots.

The feature is gracefully degrading and zero-regression:

• Unset (default) → no behavior change. Chrome renders the full-screen
  Custom Tab as today.
• Set to a number in (0, 1] → between
  authService.createCustomTabsIntentBuilder() and .build(), the
  library calls
  setInitialActivityHeightPx(displayHeight * fraction, CustomTabsIntent.ACTIVITY_HEIGHT_ADJUSTABLE).
  Older Chrome silently ignores the extra and falls back to the
  full-screen Custom Tab — no crash, no warning, no behavior surprise.
• Skipped when androidTrustedWebActivity is true (TWAs intentionally
  render full-screen).

What changed

File	Change
packages/react-native-app-auth/index.d.ts	New optional androidCustomTabPartialHeightFraction?: number on AuthConfiguration.
packages/react-native-app-auth/index.js	Validation (must be a number in (0, 1], otherwise invariant throws); Android-only positional argument to the native bridge. iOS path is untouched (Platform.OS === 'android' guard).
packages/react-native-app-auth/android/build.gradle	Bumps androidx.browser:browser 1.4.0 → 1.5.0 for the setInitialActivityHeightPx API.
packages/react-native-app-auth/android/src/main/java/com/rnappauth/RNAppAuthModule.java	Threads the fraction through authorize and authorizeWithConfiguration; conditionally calls setInitialActivityHeightPx between builder creation and .build().
packages/react-native-app-auth/index.spec.js	5 new tests on the Android-specific describe block (valid fraction passthrough; default null; throw on non-number; throw on ≤ 0; throw on > 1). Updates the existing 5 native-bridge assertions to include the new positional arg.
docs/docs/usage/config.md	Documents the new option, the Chrome 107+ floor, and the prefetchConfiguration warmup recommendation needed for the bottom sheet to render on the first invocation.
.changeset/android-partial-custom-tabs.md	Minor bump changeset (additive, opt-in, backward compatible).

Usage

import { authorize, prefetchConfiguration } from 'react-native-app-auth';

const config = {
  issuer: 'https://accounts.google.com',
  clientId: '<YOUR_CLIENT_ID>',
  redirectUrl: '<YOUR_REDIRECT_URL>',
  scopes: ['openid', 'email'],
  androidCustomTabPartialHeightFraction: 0.85, // 85% of screen height on Chrome 107+
};

// On the screen that will invoke authorize(), call prefetchConfiguration on
// mount so the bottom sheet renders on the first tap (otherwise Chrome's
// first launch falls back to full-screen; subsequent launches get the sheet).
useEffect(() => {
  prefetchConfiguration({ warmAndPrefetchChrome: true, ...config });
}, []);

// Later, on user tap:
const result = await authorize(config);

Steps to verify

1. yarn install at the repo root, then in packages/react-native-app-auth/:
   • yarn lint → passes (no new warnings introduced by this PR).
   • yarn test → 78 tests pass, including 5 new tests for
     androidCustomTabPartialHeightFraction validation + native-bridge
     passthrough.
2. Build the example app on an Android emulator running Chrome 107+ (the
   repo's CI emulator at API 35 ships Chrome 147, which works). Add
   androidCustomTabPartialHeightFraction: 0.85 to the example config and
   call prefetchConfiguration on screen mount, then tap the authorize
   button → the Chrome Custom Tab renders as a bottom sheet at ~85% of the
   screen height; the host activity is visible behind it; the user can
   drag the sheet up to full-screen via the handle. Cancel via the X
   button → returns to the host app.
3. Smoke the fallback path: with androidCustomTabPartialHeightFraction
   unset, the flow renders full-screen exactly as before this PR.
4. Smoke a Trusted Web Activity flow with the option set → the option is
   ignored (TWA renders full-screen as before).
5. TypeScript: import { AuthConfiguration } and try
   { androidCustomTabPartialHeightFraction: 0.85 } satisfies AuthConfiguration
   → typechecks; { androidCustomTabPartialHeightFraction: 'half' } →
   type error. Confirms the index.d.ts change is correct.

Tests, README, and TypeScript definitions are updated.

[changeset-bot]

🦋 Changeset detected

Latest commit: 14239ab

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
react-native-app-auth	Minor

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Pull request overview

This PR adds an Android-only, opt-in configuration option (androidCustomTabPartialHeightFraction) to present the authorization Chrome Custom Tab as a user-resizable bottom sheet (Partial Custom Tabs on Chrome 107+), improving UX parity with iOS while keeping default behavior unchanged when unset.

Changes:

• Adds androidCustomTabPartialHeightFraction?: number to the public AuthConfiguration TypeScript type and documents its behavior and caveats.
• Validates the fraction in JS ((0, 1]) and forwards it as an additional Android-only positional argument over the native bridge.
• Wires the fraction through the Android native module and sets the Partial Custom Tab initial height via setInitialActivityHeightPx, plus updates tests and bumps androidx.browser:browser to 1.5.0.

Reviewed changes

Copilot reviewed 6 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
packages/react-native-app-auth/index.d.ts	Adds the new optional config field to the public TS API.
packages/react-native-app-auth/index.js	Validates the fraction and forwards it to the Android native bridge.
packages/react-native-app-auth/android/build.gradle	Bumps androidx.browser:browser to enable the Partial Custom Tabs API.
packages/react-native-app-auth/android/src/main/java/com/rnappauth/RNAppAuthModule.java	Applies the partial height fraction to the Custom Tab builder (when applicable).
packages/react-native-app-auth/index.spec.js	Updates bridge argument assertions and adds Android-specific validation/forwarding tests.
docs/docs/usage/config.md	Documents the new option, platform/version constraints, and prefetch recommendation.
.changeset/android-partial-custom-tabs.md	Declares a minor version bump for the additive feature.

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