WebExtensions group related threads

[fregante]

• Event listeners in Content Scripts and communication between main and isolated CS worlds w3c/webextensions#241
• Proposal: Provide a secure drop-in replacement for window.postMessage() w3c/webextensions#77
• Difficulty broadcasting messages due to inconsistent messaging behaviour w3c/webextensions#35
• Inconsistency: runtime.onMessage listening for contentScript messages within extension popups w3c/webextensions#92
• request: allow to retrieve a frameID from an <iframe> element w3c/webextensions#12
• Proposal: deprecate window.postMessage(message, '*') for use with extensions w3c/webextensions#78 (comment)
• New API proposal : extension.getContexts() w3c/webextensions#334
• A multi-tab broadcast version of chrome.tabs.sendMessage w3c/webextensions#722