Flutter Web Scroll Event Handling Fixes

This directory documents the fixes for three related GitHub issues dealing with scroll event handling in Flutter web applications.

Issues Overview

Issue	Problem	Key Fix
#156985	Scroll events bubble to parent page when Flutter is in iframe	`preventDefault()` in iframe + explicit parent scroll via `postMessage`
#157435	Touch scroll doesn't propagate to host page (embedded mode)	Don't `preventDefault()` on touch + platform channel for scroll propagation
#113196	Mouse scroll blocked over cross-origin iframe in HtmlElementView	Transparent overlay captures wheel events and forwards to Flutter

Architecture

┌─────────────────────────────────────────────────────────────────┐
│                         Host/Parent Page                         │
│  ┌──────────────────────────────────────────────────────────┐   │
│  │                    Flutter Web App                        │   │
│  │  ┌─────────────────────────────────────────────────────┐ │   │
│  │  │              Flutter Scrollables                     │ │   │
│  │  │  ┌───────────────────────────────────────────────┐  │ │   │
│  │  │  │   HtmlElementView (cross-origin iframe)       │  │ │   │
│  │  │  │   + Wheel Overlay (Issue #113196)             │  │ │   │
│  │  │  └───────────────────────────────────────────────┘  │ │   │
│  │  └─────────────────────────────────────────────────────┘ │   │
│  │                                                          │   │
│  │  pointer_binding.dart:                                   │   │
│  │  - Detects iframe embedding (Issue #156985)              │   │
│  │  - preventDefault() to block native scroll chaining       │   │
│  │  - Explicit parent scroll via postMessage                │   │
│  │  - Touch event handling (Issue #157435)                  │   │
│  └──────────────────────────────────────────────────────────┘   │
│                              │                                   │
│                    postMessage('flutter-scroll')                 │
│                              ▼                                   │
│           window.addEventListener('message', scrollBy)           │
└─────────────────────────────────────────────────────────────────┘

Files Modified

Engine (`engine/src/flutter/lib/web_ui/lib/src/engine/`)

File	Changes
`pointer_binding.dart`	Iframe detection, preventDefault in iframe, touch event handling, parent scroll
`dom.dart`	`scrollParentWindow()` function, `parent` property on DomWindow
`platform_dispatcher.dart`	`flutter/scroll` platform channel handler
`platform_views/content_manager.dart`	Wheel overlay for HtmlElementView
`view_embedder/embedding_strategy/*.dart`	`overscroll-behavior: contain` CSS

Framework (`packages/flutter/lib/src/widgets/`)

File	Changes
`scrollable.dart`	`respond(allowPlatformDefault: false)` when scroll handled
`scroll_position_with_single_context.dart`	Touch scroll propagation via platform channel

Demo Apps

Issue	Before	After
#156985	https://issue-156985-before.web.app	https://issue-156985-after.web.app
#157435	https://issue-157435-before.web.app	https://issue-157435-after.web.app
#113196	https://issue-113196-before.web.app	https://issue-113196-after.web.app

Host Page Requirements

For iframe embedding (Issue #156985), the host page must listen for scroll messages:

<script>
  window.addEventListener('message', function(event) {
    if (event.data && event.data.type === 'flutter-scroll') {
      window.scrollBy(event.data.deltaX, event.data.deltaY);
    }
  });
</script>

Testing

# Build engine with changes
cd /Users/zhongliu/dev/flutter/engine/src/flutter/lib/web_ui
felt build

# Test demo app
cd /Users/zhongliu/dev/flutter-apps/issue_156985_after
flutter run -d chrome --local-web-sdk=wasm_release

# Verify:
# 1. Scroll inside Flutter - parent page should NOT scroll
# 2. Scroll to boundary - parent page SHOULD scroll
# 3. Nested scrollables - inner scrolls first, then outer, then parent

Key Design Decisions

postMessage for cross-origin safety: Using postMessage instead of direct window.parent.scrollBy() ensures the solution works for both same-origin and cross-origin iframes.
Two-flag system for nested scrollables: The _lastWheelEventAllowedDefault and _lastWheelEventHandledByWidget flags work together to ensure correct behavior with nested scrollables.
Overlay for cross-origin iframes: Since cross-origin iframes completely isolate events, an overlay is the only way to capture wheel events without breaking iframe functionality.
Don't block touch events: Touch scrolling uses browser native behavior, so we don't preventDefault() on touch to allow smooth scrolling experience.