Hi,

I managed to do that the following way:

1. What you want is to modify the function _fetchDocumentMessages in JS file chat_manager.js from native 'mail' addon, in ChatManager class. Instead of sort by message id, you want to sort by date :

_fetchDocumentMessages : function (ids, options) {
    var loaded_msgs = _.filter(messages, function (message) {
        return _.contains(ids, message.id);
    });
    var loaded_msg_ids = _.pluck(loaded_msgs, 'id');

    options = options || {};
    if (options.force_fetch || _.difference(ids.slice(0, LIMIT), loaded_msg_ids).length) {
        var ids_to_load = _.difference(ids, loaded_msg_ids).slice(0, LIMIT);

        return this._rpc({
                model: 'mail.message',
                method: 'message_format',
                args: [ids_to_load],
                context: session.user_context,
            })
            .then(function (msgs) {
                var processed_msgs = [];
                _.each(msgs, function (msg) {
                    processed_msgs.push(add_message(msg, {silent: true}));
                });
                return _.sortBy(loaded_msgs.concat(processed_msgs), function (msg) {
                    //CUSTOM HERE: sort by date instead of id in native
                    return msg.date;
                });
            });
    } else {
        return $.when(loaded_msgs);
    }
},

2. Thing is, the way ChatManager is defined, you cannot properly inherit it in your custom module (I can't explain exactly why, i am not JS expert)... In this case I found out it is not possible to use a "include" as usually done to overwrite only a JS class method.

3. So I copied / paste the whole chat_manager.js file from 'mail' module to my custom module, and made the previous modification in my pasted file

4. Then i told odoo to replace the native file by mine, by putting in the xml (note the expr in xpath and 'replace' position):

<template id="assets_backend" name="sort message by date assets" inherit_id="web.assets_backend">
    <!--unable to inherit javascript properly. So whole script is replaced here-->
    <!--see https://www.odoo.com/fr_FR/forum/aide-1/question/how-inheritance-of-a-js-mail-chat-manager-130963-->
    <xpath expr="//script[@src='/mail/static/src/js/chat_manager.js']" position="replace">
        <script src="/your_custom_module/static/src/js/chat_manager.js" type="text/javascript"/>
    </xpath>
    
</template>