…eSemantics

Description

Issue

If a scrolling widget such as ListView or GridView (with shrinkWrap: true) is nested under another scrolling widget such as SingleChildScrollView, then new "beyond the fold" content which is revealed by swiping off screen cannot be selected by tap, and continued swiping beyond the fold will eventually make a11y focus jump to a random element on iOS.

import 'package:flutter/material.dart';

void main() => runApp(A11yApp());

class A11yApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final appBar =
    AppBar(title: const Icon(Icons.archive));

    return MaterialApp(
      home: Scaffold(
        appBar: appBar,
        body: SingleChildScrollView(
          child: Column(
            children: [_buildGridView(), _buildGridView(), _buildGridView()],
          ),
        ),
      ),
    );
  }

  Widget _buildGridView() {
    return Column(
      children: [
        const Text('Grid header'),
        GridView.count(
          shrinkWrap: true,
          crossAxisSpacing: 10,
          mainAxisSpacing: 10,
          crossAxisCount: 2,
          children: <Widget>[
            for (var i = 0; i < 50; ++i)
              Container(
                padding: const EdgeInsets.all(8),
                color: Colors.green[100],
                child: Text('Number $i'),
              ),
          ],
        ),
      ],
    );
  }
}

Fix

The inner semantics pane of the child ListView/GridView needs to have its position updated to match the outer semantics pane when the parent SingleChildScrollView scrolls.

This comment on issue #61631 explains the technical details of the fix.

Related Issues

Fixes #61631

Tests

I added the following tests:

• widgets/scrollable_semantics_test.dart
  • "transform of inner node from useTwoPaneSemantics scrolls correctly with nested scrollables"
    • Ensures the global rect of the inner and outer semantics panes of a ListView match when nested under a SingleChildScrollView. The parent SingleChildScrollView is scrolled to the end -> beginning -> middle of the shrink-wrapped ListView's hidden elements to test both scroll directions.

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I signed the CLA.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Did any tests fail when you ran them? Please read Handling breaking changes.

• No, no existing tests failed, so this is not a breaking change.