Document how platform detection works from 4.7

resources/views/docs/mobile/4/testing/advanced.md

@@ -29,6 +29,24 @@ it('omits the android translucency on ios', function () {
 `assertElement($type, $matcher)` finds an element of a wire type, optionally narrowed by a closure receiving the wire
 node; `assertMissingElement()` is its inverse.
 
+`platform` also answers [platform detection](../the-basics/system#platform-detection) inside the screen
+<x-docs.version-badge changed="4.7" />: `System::isIos()` and friends, the `isIos()` helpers, and the `@ios`,
+`@android`, `@mobile` and `@web` Blade directives all report the platform you passed.
+
+In a test that doesn't render a screen, such as an HTTP test for a Blade view, set the `NATIVEPHP_PLATFORM`
+environment variable instead <x-docs.version-badge since="4.7" />. It's what the app sets on a real device. Clear it
+after each test so it doesn't leak into the next one:
+
+```php
+afterEach(fn () => putenv('NATIVEPHP_PLATFORM'));
+
+it('offers Sign in with Apple on iOS', function () {
+    putenv('NATIVEPHP_PLATFORM=ios');
+
+    $this->get('/login')->assertSee('Sign in with Apple');
+});
+```
+
 To audit a tree for screen-reader accessibility rather than a specific element, see
 [Accessibility](accessibility).
 

resources/views/docs/mobile/4/the-basics/device.md

@@ -76,8 +76,8 @@ $info['model'];      // e.g. 'iPhone15,3'
 
 <aside>
 
-`System::isIos()`, `isAndroid()`, and `isMobile()` are thin conveniences over `getInfo()['platform']` — reach
-for those when you only need platform detection. See [System](system).
+When you only need the platform, reach for `System::isIos()`, `isAndroid()`, `isMobile()` or `platform()` instead.
+On the device they don't call `getInfo()` at all. See [System](system).
 
 </aside>
 

resources/views/docs/mobile/4/the-basics/system.md

@@ -19,9 +19,27 @@ use Native\Mobile\Facades\System;
 System::isIos();      // true on iOS
 System::isAndroid();  // true on Android
 System::isMobile();   // true on either platform
+System::platform();   // 'ios', 'android', or null outside the app
 ```
 
-Use these to branch behavior or conditionally render UI for a specific platform.
+Use these to branch behavior or conditionally render UI for a specific platform. `System::platform()`
+<x-docs.version-badge since="4.7" /> gives you the answer as a string, which saves checking both.
+
+On the device <x-docs.version-badge changed="4.7" />, the platform comes from an environment variable the app sets
+before PHP starts, so these checks are cheap enough to call as often as you like, even once per row of a long list.
+Before 4.7 every check asked the native side, which was far slower.
+
+Under [Jump](jump), PHP runs on your computer instead of the phone, so that variable isn't set. The first check asks
+the connected device, and the answer is remembered after that.
+
+To render a screen as one platform in a test, see [Platform variants](../testing/advanced#platform-variants).
+
+<aside>
+
+If you've been using the undocumented `Native\Mobile\Platform` class, switch to these methods. It's
+<x-docs.version-badge deprecated="4.7" /> and passes straight through to `System` until it's removed.
+
+</aside>
 
 Each is also available as a global helper function — `isIos()`, `isAndroid()`, `isMobile()` — for terse use in
 Blade and components: