This PR

• Fixes Scroll "back" on web causes browser back action (unless perfectly horizontal) #152588

Some history and reasoning

Root cause

The bug was introduced in PR #145500, which fixed issue #139263 by ensuring that the Web engine knows when a scroll event is not handled by Flutter. This allows the browser to perform default actions like page scrolling or back/forward navigation. Without this, Flutter might "swallow" events it doesn't use, leading to a "dead" feel on the web.

Why reverting is safe and won't regress Issue #139263

Reverting the manual respond(true) calls in Scrollable is safe because PointerSignalResolver (the centralized mediator for these events) already has the logic to notify the engine. More specifically, PointerSignalResolver.resolve calls event.respond(allowPlatformDefault: true) if and only if no widget in the hit-test tree registered interest in the event.

Moving this responsibility back to the PointerSignalResolver ensures that:

1. If any widget handles the event: It registers with the resolver, and respond(true) is not called.
2. If no widget handles the event: No one registers, and the resolver calls respond(true) at the end of the dispatch cycle.

The current bug exists because Scrollable was trying to be too helpful by calling respond(true) based only on its local state, which overrides any handling done by other widgets in the hierarchy.

Testing and verifying

A new regression test was created that highlight how the respond calls should work in nested Scrollable elements.

The "fallback" nature of PointerSignalResolver is already

1. documented in pointer_signal_resolver.dart:

  void resolve(PointerSignalEvent event) {
    if (_firstRegisteredCallback == null) {
      assert(_currentEvent == null);
      // Nothing in the framework/app wants to handle the `event`. Allow the
      // platform to trigger any default native actions.
      event.respond(allowPlatformDefault: true);
      return;
    }
    // ... handles the registered callback ...
  }

2. tested in pointer_signal_resolver_test.dart:

  test('Resolving with no entries should notify engine of no-op', () {
    var allowedPlatformDefault = false;
    final tester = PointerSignalTester();
    tester.event = PointerScrollEvent(
      onRespond: ({required bool allowPlatformDefault}) {
        allowedPlatformDefault = allowPlatformDefault;
      },
    );
    tester.resolver.resolve(tester.event);
    expect(
      allowedPlatformDefault,
      isTrue,
      reason: 'Should have called respond with allowPlatformDefault: true',
    );
  });

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• I followed the [breaking change policy] and added [Data Driven Fixes] where supported.
• All existing and new tests are passing.