Seems that something has changed in Ajax-land; a process that worked well for years to let the user know the server was actually chewing through its work has stopped working.

I've distilled it down as best I can to the following:

Client-side:

function start () {
        var jsonResponse = '', lastResponseLen = false;

        $.ajax({
            url: getAjaxUrl(),
            method: 'POST',
            data: {
                REQ_TYPE: 'Misc_Action',
                SUB_TYPE: 'Misc_TestComms'
            },
            xhrFields: {
                onprogress: function(e) {
                    var thisResponse, response = e.currentTarget.response;
                    if(lastResponseLen === false) {
                        thisResponse = response;
                        lastResponseLen = response.length;
                    } else {
                        thisResponse = response.substring(lastResponseLen);
                        lastResponseLen = response.length;
                    }

                    jsonResponse = JSON.parse(thisResponse);

                    document.getElementById('messages').innerHTML = 'Processed '+jsonResponse.count+' of '+jsonResponse.total;
                }
            },
            success: function(text) {
                console.log('done!');
                document.getElementById('messages').innerHTML = 'Process completed successfully';
            }
        });
    }

(note that the url is correctly formed)

Server-side:

function Misc_TestComms () {
    $runs = 25;
    $i = 0;
    echo json_encode(array('progress' => 0, 'count' => $i);
    flush();
    ob_flush();

    while ($i < $runs) {
        $i++;
        echo json_encode(array('progress' => (($i/$runs)*100), 'count' => $i);
        flush();
        ob_flush();
        sleep(1);
    }
}

The client-side receives no progress messages until the script has completed, at which point both the onprogress and success sections are triggered.

Suggestions?