Question

This problem seems to have been sort of resolved, as long as the URL of the page you’re injecting your javascript into starts with www. What do you do if it doesn’t? Here’s the relevant part of my manifest:

"content_scripts": [
  {
  "run_at": "document_start",
  "matches": ["https://groups.google.com/forum/?fromgroups=#!newtopic/opencomments-site-discussions"],
  "js": ["postMsg.js"]
  }
],

The problem, according to another stackoverflow post, is because the URL of the page doesn’t begin with ‘www’. Does that mean that you can’t inject javascript into secure pages whose URL doesn’t begin with ‘www’, or is there another way? This had never been a problem in the past, because my extension had run with Version 1 manifests.

Forgot to add the content script:

var subject = document.getElementById("p-s-0");

subject.setAttribute("value", "foo");   

The element with ID “p-s-0” is the Subject field in the Google Groups Post page, so the field should display “foo”.

Answer 1

A few issues:

1. That is a not valid match pattern because they only specify up to the URL path (the part before the ?).

   Change the matches to:

   "matches": ["https://groups.google.com/forum/*"],
   

2. The overall URL (https://groups.google.com/forum/?fromgroups=#!newtopic/opencomments-site-discussions) is not practical because Google changes the URL parameters willy nilly. For example, fromgroups is not often present, and may not have the = if it is. Additional parameters, like hl=en come and go. (This is the reason why my earlier answer worked for me, but not for you.)

   So, using include_globs in the manifest would be a messy, error-prone exercise.
   The solution is to checklocation.hash within the content script.

3. The script is set to "run_at": "document_start", so the content script is running before there is any node with id p-s-0.

   Change the manifest to "run_at": "document_end".

4. The new Google groups is heavily AJAX driven. So, The “New Topic” page is usually “loaded” without actually loading a whole new page. This means the content script will not rerun. It needs to monitor for “new” AJAX-loaded pages.

   Check for “new” pages by monitoring the hashchange event.

5. Additionally, the p-s-0 element is added by AJAX, and is not immediately available on a “new” page. Check for this element within a setInterval.

---

Putting it all together,
The manifest.json becomes:

{
    "manifest_version": 2,
    "content_scripts": [ {
        "run_at":           "document_end",
        "js":               [ "postMsg.js" ],
        "matches":          [ "https://groups.google.com/forum/*" ]
    } ],
    "description":  "Fills in subject when posting a new topic in select google groups",
    "name":         "Google groups, Topic-subject filler",
    "version":      "1"
}

The content script(postMsg.js) becomes:

fireOnNewTopic ();  // Initial run on cold start or full reload.

window.addEventListener ("hashchange", fireOnNewTopic,  false);

function fireOnNewTopic () {
    /*-- For the pages we want, location.hash will contain values
        like: "#!newtopic/{group title}"
    */
    if (location.hash) {
        var locHashParts    = location.hash.split ('/');
        if (locHashParts.length > 1  &&  locHashParts[0] == '#!newtopic') {
            var subjectStr  = '';
            switch (locHashParts[1]) {
                case 'opencomments-site-discussions':
                    subjectStr  = 'Site discussion truth';
                    break;
                case 'greasemonkey-users':
                    subjectStr  = 'GM wisdom';
                    break;
                default:
                    break;
            }

            if (subjectStr) {
                runPayloadCode (subjectStr);
            }
        }
    }
}

function runPayloadCode (subjectStr) {
    var targetID        = 'p-s-0'
    var failsafeCount   = 0;
    var subjectInpTimer = setInterval ( function() {
            var subject = document.getElementById (targetID);
            if (subject) {
                clearInterval (subjectInpTimer);
                subject.setAttribute ("value", subjectStr);
            }
            else {
                failsafeCount++;
                //console.log ("failsafeCount: ", failsafeCount);
                if (failsafeCount > 300) {
                    clearInterval (subjectInpTimer);
                    alert ('Node id ' + targetID + ' not found!');
                }
            }
        },
        200
    );
}