core: Implement cross-context LocalConnection via BroadcastChannel

[Bubby4j]

(note: LLM generated code by a Rust newbie)

Overview

This PR implements complete cross-context LocalConnection support for the Ruffle web player, enabling communication between different movies on a page, across iframes, and between different tabs/windows on the same origin.

Previously, LocalConnection only worked within a single Ruffle instance. This implementation introduces a BroadcastChannel-based backend for transport and a localStorage-based registry for listener discovery and health tracking.

Key Features

• Cross-Tab Messaging: Uses the BroadcastChannel API to deliver AMF-serialized messages between Ruffle instances.
• Listener Registry: Uses localStorage to track active LocalConnection listeners across the origin, implementing the "heartbeat" pattern used by Flash to prune stale connections.
• Latency Minimized: Implemented an instantaneous player wake-up mechanism. The backend now holds a weak reference to the Player and triggers an immediate local connection update upon receiving a channel event, bypassing the standard frame-rate-bound polling loop.
• Spec Compliance: Enforces the 40KB message size limit and handles superdomain/IP address logic correctly via get_superdomain.
• Robustness & Safety:
  • Cleanup logic during beforeunload to ensure localStorage locks are released even if a tab is closed abruptly.

Limitations

• Flash supported cross-origin LocalConnection in certain cases, but this is not easily implemented via BroadcastChannel, and is a far rarer use case. Same-origin only.

Testing

Automated Tests

• WebdriverIO Integration Tests: Added a suite of cross-tab integration tests in web/packages/selfhosted/test/integration_tests/local_connection_cross_tab/:
  • connect/send between tabs.
  • Message size limit (40KB) enforcement.
  • Stale listener pruning via heartbeat timeouts.
  • Cleanup on tab close/beforeunload.
• Unit Tests: Added tests for get_superdomain to verify IP address and domain split logic.

Manual Verification

• Tested with a real-time chat app written in AS2 which has a separate swf loaded in an iframe for chat logs.

Related

#259

#2410
#10493
#10068