website/web/static/js/inline-replies.js

// nonshy inline "Quote" and "Reply" buttons that activate the comment field
// on the current page. Common logic between forum threads and photo comments.

document.addEventListener('DOMContentLoaded', function() {
    const $message = document.querySelector("#message");

    // Enhance the in-post Quote and Reply buttons to activate the reply field
    // at the page header instead of going to the dedicated comment page.
    (document.querySelectorAll(".nonshy-quote-button") || []).forEach(node => {
        const message = node.dataset.quoteBody,
            replyTo = node.dataset.replyTo,
            commentID = node.dataset.commentId;

        // If we have a comment ID, have the at-mention link to it.
        let atMention = "@" + replyTo;
        if (commentID) {
            atMention = `[@${replyTo}](/go/comment?id=${commentID})`;
        }

        node.addEventListener("click", (e) => {
            e.preventDefault();

            if (replyTo) {
                $message.value += atMention + "\n\n";
            }

            // Prepare the quoted message.
            var lines = [];
            for (let line of message.split("\n")) {
                lines.push("> " + line);
            }

            $message.value += lines.join("\n") + "\n\n";
            $message.scrollIntoView();
            $message.focus();
        });
    });

    (document.querySelectorAll(".nonshy-reply-button") || []).forEach(node => {
        const replyTo = node.dataset.replyTo,
            commentID = node.dataset.commentId;

        // If we have a comment ID, have the at-mention link to it.
        let atMention = "@" + replyTo;
        if (commentID) {
            atMention = `[@${replyTo}](/go/comment?id=${commentID})`;
        }

        node.addEventListener("click", (e) => {
            e.preventDefault();
            $message.value += atMention + "\n\n";
            $message.scrollIntoView();
            $message.focus();
        });
    });
});