background.js

chrome.tabs.onUpdated.addListener(function (tabId, changeInfo, tab) {
    // changeInfo object: https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/onUpdated#changeInfo
    // status is more reliable (in my case)
    // use "alert(JSON.stringify(changeInfo))" to check what's available and works in your case
    if (changeInfo.status === 'complete') {
      chrome.tabs.sendMessage(tabId, {
        message: 'TabUpdated',
        url: tab.url
      });
    }
  })

  /*
  var uid = null
  
  chrome.runtime.onMessage.addListener(
    function(message, sender, sendResponse) {
        switch(message.type) {
            case "setUID":
                uid = message.uid;
                break;
            case "getUID":
              if(uid){
                sendResponse(uid);
              }
              break;
            default:
                console.error("Unrecognised message: ", message);
        }
    }
  );*/