Statement after sending message keeps active in chrome extension

Question

I am sending a messages back and forth from background.js to the content script when a certain condition has been met String(historyItems.length) == "0". It works fine, but I noted that once the message has been sent once (not before that), the function displayMe();at the other end is active, even when the when the condition has not been met.

background.js:

chrome.extension.onMessage.addListener(function (request, sender, sendResponse) {
  chrome.history.search({
    text: request.text,
    maxResults: 10
  }, function(historyItems) {
    if(String(historyItems.length) == "0"){
        chrome.tabs.query({currentWindow: true}, function(tabs) {
          chrome.tabs.sendMessage(tabs[0].id, {greeting: "hello"}, function(response) {
          });
        });
    }
  });

contentscript.js:

chrome.runtime.onMessage.addListener(
  function(request, sender, sendResponse) {
    console.log(sender.tab ?
                "from a content script:" + sender.tab.url :
                "from the extension");
    if (request.greeting == "hello"){
      displayMe();
      sendResponse({farewell: "goodbye"});
    }
});

It is all triggered with this function in contentscript.js:

function buildUrlList(){
  document.addEventListener('mouseover', function(event) {
    var hoveredEl = event.target;
    if (hoveredEl.tagName !== 'A') {
      return;
    } 
    chrome.runtime.sendMessage({text: "Trigger Search"});
  });
}

I just want the function in the content script activate when the background.js send the message (once). How can I make this work (javascript only)?