First installation of the extension - sending messages to the background script from the event handler in the content script does not work

After the first installation (out of AppStore) of the extension in the browser, the content script is correctly inserted into the page (twice for some reason) and a message is sent from the root of the content script to the background script, which responds correctly.

However, if an event handler is registered within the content script, within which the message is also sent to the background script, it will never reach the background script.

window.addEventListener("message", function (event) {
         // We only accept messages from ourselves
         if (event.source !== window) {
             return;
         }
         if (event.data.source && event.data.source === appIdentification) {
                 browser.runtime.sendMessage(event.data);
         }
     }, false);

It does not matter with what delay the event handler is called (i.e. the background script is not asleep). If I refresh the page or close and reopen the browser and reload the page, everything works correctly and the message sent from the event handler is already delivered to the background script.

The event handler is used so that the extension code is uniform for all browsers (Chrome, Safari, Edge, Opera, Firefox), i.e. it is not intended to use externally_connectable for sending messages from the webpage directly to the background script, which Safari should support. The expected behavior is that the extension will work even after the first installation, as is the case with other browsers.

Procedure:

1. Enter the test website: https://www.mssf.cz/testapp/check_client.aspx

2. Do the initial installation of the extension (could be downloaded from here: https://1drv.ms/f/c/76f4c93826df41a0/Ej5MQX9ctyhHv_P9_t_6uAwB05ET-nzXuMhPeu56nOgkWg?e=cudqRJ)

3. Set a breakpoint in the event handler for "message" within the content script, open the background script and set a breakpoint in the event handler for onMessage

4. Click on the "Validate certificate" button on the page loaded in point 1

5. Step through the content script to the point where the message is sent to the background script, the breakpoint within the background script is never hit, which is an error, the message should come to the background script