TamperMonkey - message between scripts on different subdomains

You can use GM_getValue, GM_setValue & GM_addValueChangeListener to achieve cross-tab user script communication.

Add the following lines in your user script header.

// @grant        GM_setValue
// @grant        GM_getValue
// @grant        GM_addValueChangeListener

The following lines of rough code will simplify the cross-tab user script communication.

function GM_onMessage(label, callback) {
  GM_addValueChangeListener(label, function() {
    callback.apply(undefined, arguments[2]);
  });
}

function GM_sendMessage(label) {
  GM_setValue(label, Array.from(arguments).slice(1));
}

So all you'll need to do is the following to send and receive messages.

GM_onMessage('_.unique.name.greetings', function(src, message) {
  console.log('[onMessage]', src, '=>', message);
});
GM_sendMessage('_.unique.name.greetings', 'hello', window.location.href);

NOTE Sending messages may not trigger your callback if the message sent is the same as before. This is due to GM_addValueChangeListener not firing because the value has not changed, i.e. same value as before even though GM_setValue is called.

Using @grant enables the sandbox, which can sometimes result in difficulties when trying to interact with complicated page objects on Greasemonkey.

If you do not want to enable the sandbox with @grant, another option is to have the userscript create an iframe to the other domain, and then post a message to it. On the other domain, in the iframe, listen for messages. When a message is received, use BroadcastChannel to send the message to every other tab on that other domain, and your other tabs with the userscript running can have the same BroadcastChannel open and listen for messages.

For example, to create a userscript on stackoverflow.com that can send a message to a userscript running in a different tab on example.com:

// ==UserScript==
// @name             0 Cross-tab example
// @include          /^https://example\.com\/$/
// @include          /^https://stackoverflow\.com\/$/
// @grant            none
// ==/UserScript==

if (window.location.href === 'https://example.com/') {
  const broadcastChannel = new BroadcastChannel('exampleUserscript');
  if (window.top !== window) {
    // We're in the iframe:
    window.addEventListener('message', (e) => {
      if (e.origin === 'https://stackoverflow.com') {
        broadcastChannel.postMessage(e.data);
      }
    });
  } else {
    // We're on a top-level tab:
    broadcastChannel.addEventListener('message', (e) => {
      console.log('Got message', e.data);
    });
  }
} else {
  // We're on Stack Overflow:
  const iframe = document.body.appendChild(document.createElement('iframe'));
  iframe.style.display = 'none';
  iframe.src = 'https://example.com';

  setTimeout(() => {
    iframe.contentWindow.postMessage('Sending message from Stack Overflow', '*');
  }, 2000);
}

This results in:

If you want two-way communication, not just one-way communication, have both parent pages create a child iframe to a single target domain (say, to example.com). To communicate to other tabs, post a message to the child iframe. Have the child iframe listen for messages, and when seen, post a BroadcastChannel message to communicate with all other iframes. When an iframe receives a BroadcastChannel message, relay it to the parent window with postMessage.

// ==UserScript==
// @name             0 Cross-tab example
// @include          /^https://example\.com\/$/
// @include          /^https://(?:stackoverflow|stackexchange)\.com\/$/
// @grant            none
// ==/UserScript==

if (window.location.href === 'https://example.com/') {
  const broadcastChannel = new BroadcastChannel('exampleUserscript');
  if (window.top !== window) {
    // We're in an iframe:
    window.addEventListener('message', (e) => {
      console.log('iframe received message from top window');
      if (e.origin === 'https://stackoverflow.com' || e.origin === 'https://stackexchange.com') {
        broadcastChannel.postMessage(e.data);
      }
    });
    broadcastChannel.addEventListener('message', (e) => {
      console.log('iframe received message from BroadcastChannel');
      window.top.postMessage(e.data, '*');
    });
  }
} else {
  // We're on Stack Overflow or Stack Exchange
  const iframe = document.body.appendChild(document.createElement('iframe'));
  iframe.style.display = 'none';
  iframe.src = 'https://example.com';
  window.addEventListener('message', (e) => {
    if (e.origin === 'https://example.com') {
      console.log(`Top window ${window.origin} received message from iframe:`, e.data);
    }
  });
  if (window.location.href === 'https://stackoverflow.com/') {
    setTimeout(() => {
      console.log('stackoverflow posting message to iframe');
      iframe.contentWindow.postMessage('Message from stackoverflow', '*');
    }, 2000);
  }
}

In the above code, a tab on Stack Overflow sends a message to a tab on Stack Exchange. Result screenshot: