Fix iOS channel crash from unbounded scroll depth (#8407)

### Summary

Opening a channel on iOS 27.0.1 could terminate Buzz when a reversed,
lazily loaded timeline reported an unbounded scroll extent. Flutter
forwarded `Infinity` to the native navigation bar, and UIKit threw
`CALayerInvalidGeometry` while setting the scroll view offset.

Use the finite viewport depth for that unbounded case, preserving the
scrolled-under appearance. Reject non-finite offsets at the native
platform boundary as a final safety check.

### Validation

- Added tests: a Flutter widget regression for an unbounded reversed
timeline, and an iOS native regression for a non-finite platform-channel
offset.
- Reproduced the original crash by tapping `#general` on an iPhone SE
running iOS 27.0.1. With this change, the channel rendered and survived
repeated back-and-reopen cycles. Native logging confirmed the channel
now sends finite offsets where it previously sent `Infinity`.
- The native regression passed on iOS 26.5 and 27.0 simulators with
Xcode 27.0. Flutter widget tests and `flutter analyze` passed.
- A full channel-flow simulator run was not available: the isolated
simulator Profile build could not resolve `BuzzPushKit`. The iOS 27.0.1
physical run exercised the full channel flow.
- The full Flutter suite had eight notification and push test failures,
all reproduced on the unchanged 0.24.0 source.

---------

Signed-off-by: Tom Brow <tomb@block.xyz>

mobile/ios/Runner/IosNavigationBar.swift

@@ -530,6 +530,7 @@ private final class IosNavigationBarView: NSObject, FlutterPlatformView {
   }
 
   private func setScrollOffset(_ value: CGFloat) {
+    guard value.isFinite else { return }
     offset = max(0, value)
     material.alpha = alwaysFrosted ? 1 : min(1, offset / 12)
     applyScroll()

mobile/ios/RunnerTests/RunnerTests.swift

@@ -520,6 +520,29 @@ class RunnerTests: XCTestCase {
     }
   }
 
+  @MainActor
+  func testNavigationIgnoresNonFiniteScrollOffsets() throws {
+    let parent = UIViewController()
+    let window = UIWindow(frame: UIScreen.main.bounds)
+    window.rootViewController = parent
+    window.makeKeyAndVisible()
+    defer { window.isHidden = true }
+    let messenger = NavigationTestMessenger()
+    let bar = IosNavigationBarFactory(messenger: messenger, parent: parent).create(
+      withFrame: CGRect(x: 0, y: 0, width: window.bounds.width, height: 120),
+      viewIdentifier: 999990,
+      arguments: ["title": "general", "alwaysFrosted": true])
+    parent.view.addSubview(bar.view())
+    bar.view().layoutIfNeeded()
+    let navigation = try XCTUnwrap(parent.children.first as? UINavigationController)
+    let scroll = try XCTUnwrap(navigation.topViewController?.view as? UIScrollView)
+    messenger.scroll(to: 52)
+    let finiteOffset = scroll.contentOffset.y
+    XCTAssertTrue(finiteOffset.isFinite)
+    messenger.scroll(to: .infinity)
+    XCTAssertEqual(scroll.contentOffset.y, finiteOffset, accuracy: 0.5)
+  }
+
   @MainActor
   func testSubtitlelessThreadKeepsFullFallbackBehindControls() throws {
     let parent = UIViewController()

mobile/lib/shared/widgets/frosted_scaffold.dart

@@ -175,8 +175,14 @@ class FrostedScaffold extends HookWidget {
         if (!nativePinned && depth == 0 && metrics.axis == Axis.vertical) {
           // Keep real depth so a later UIKit metrics update (Dynamic Type or
           // rotation) can apply its new collapse range without another scroll.
-          final next = topContentDepth.clamp(0.0, double.infinity);
-          if ((scrollOffset.value - next).abs() > 0.1) {
+          // Lazy reversed lists can report an unbounded extentAfter. The
+          // visible viewport is enough depth for UIKit's collapsed state,
+          // while infinity is not a valid UIScrollView content offset.
+          final finiteDepth = topContentDepth.isFinite
+              ? topContentDepth
+              : metrics.viewportDimension;
+          final next = finiteDepth.clamp(0.0, double.infinity);
+          if (next.isFinite && (scrollOffset.value - next).abs() > 0.1) {
             WidgetsBinding.instance.addPostFrameCallback((_) {
               if (context.mounted) scrollOffset.value = next;
             });

mobile/test/shared/widgets/frosted_scaffold_test.dart

@@ -1,6 +1,7 @@
 import 'package:buzz/shared/theme/theme.dart';
 import 'package:buzz/shared/widgets/frosted_app_bar.dart';
 import 'package:buzz/shared/widgets/frosted_scaffold.dart';
+import 'package:buzz/shared/widgets/ios_navigation_bar.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 
@@ -12,6 +13,35 @@ Border? _appBarBorder(WidgetTester tester) {
 }
 
 void main() {
+  testWidgets('unbounded reversed scroll depth stays finite for native bar', (
+    tester,
+  ) async {
+    await tester.pumpWidget(
+      const MaterialApp(
+        home: FrostedScaffold(
+          appBar: FrostedAppBar(title: Text('Conversation')),
+          body: SizedBox(key: ValueKey('timeline')),
+        ),
+      ),
+    );
+    final source = tester.element(find.byKey(const ValueKey('timeline')));
+    final offset = IosNavigationScrollScope.maybeOf(source)!;
+    ScrollUpdateNotification(
+      metrics: FixedScrollMetrics(
+        minScrollExtent: 0,
+        maxScrollExtent: double.infinity,
+        pixels: 0,
+        viewportDimension: 600,
+        axisDirection: AxisDirection.up,
+        devicePixelRatio: 1,
+      ),
+      context: source,
+      scrollDelta: 0,
+    ).dispatch(source);
+    await tester.pump();
+    expect(offset.value, 600);
+  });
+
   testWidgets('foreground paints above header and leaves body interactive', (
     tester,
   ) async {