Adds a button to Inbox by Gmail's interface, allowing you to easily insert a signature at the current cursor position

inbox_signatures.user.js

// ==UserScript==
// @name         Inbox Signatures
// @namespace    http://gerritbirkeland.com/
// @version      1.0
// @description  Adds a button to Inbox by Gmail's compose dialog to add a signature.
// @author       Gerrit0
// @match        https://inbox.google.com/*
// @grant        none
// ==/UserScript==

/*jshint
    esnext: true
*/

(function() {
    let config = {
        interval: 500,
        signature: `Sincerely,<br><br>Gerrit<br><a href="mailto:gerrit@gerritbirkeland.com">gerrit@gerritbirkeland.com</a><br><br>`,
    };

    document.head.innerHTML += `<style>
        .sig_pic:after { content: '✎'; }
        .sig_pic { height: 20px; width: 20px; }
    </style>`;

    let addSig = (event) => {
        console.log(event);

        let range = document.getSelection().getRangeAt(0);
        let nnode = document.createElement('span');
        range.surroundContents(nnode);
        nnode.innerHTML = config.signature;

        event.stopPropagation();
    };

    let checkNeeded = () => {
        Array.from(document.querySelectorAll('.h3')).forEach((bar) => {
            if (bar.querySelector('.sig_pic') === null) {
                bar.querySelector('.n').innerHTML += `<div class="g-W g-W-DKlKme" tabindex="-1" style="-webkit-user-select: none;">
                    <div class="g-W-Cw g-W-Cw-mD g-mu-sE" aria-disabled="true" role="separator" style="-webkit-user-select: none;">&nbsp;</div>
                    <div class="g-mu-sE g-W-K" title="Append signature" role="button" aria-pressed="false" id="+link" tabindex="0" style="-webkit-user-select: none;">
                        <div class="g-mu-sE g-W-K-Ig-lc" style="-webkit-user-select: none;">
                            <div class="g-mu-sE g-W-K-s1-lc" style="-webkit-user-select: none;">
                                <div class="sig_pic" style="-webkit-user-select: none;"></div>
                            </div>
                        </div>
                    </div>
                </div>`;
                bar.querySelector('.sig_pic').addEventListener('click', addSig);
            }
        });
    };

    setInterval(checkNeeded, config.interval);
}());

To do: Add support for inline replies. Currently this only works in the popup box.