Ajouter un bouton JavaScript en utilisant Greasemonkey ou Tampermonkey?

Ok, voici un script complet qui ajoute un bouton live aux pages de questions ainsi¹:

// ==UserScript==
// @name        _Adding a live button
// @description Adds live example button, with styling.
// @include     http://stackoverflow.com/questions/*
// @grant       GM_addStyle
// ==/UserScript==

/*--- Create a button in a container div.  It will be styled and
    positioned with CSS.
*/
var zNode       = document.createElement ('div');
zNode.innerHTML = '<button id="myButton" type="button">'
                + 'For Pete\'s sake, don\'t click me!</button>'
                ;
zNode.setAttribute ('id', 'myContainer');
document.body.appendChild (zNode);

//--- Activate the newly added button.
document.getElementById ("myButton").addEventListener (
    "click", ButtonClickAction, false
);

function ButtonClickAction (zEvent) {
    /*--- For our dummy action, we'll just add a line of text to the top
        of the screen.
    */
    var zNode       = document.createElement ('p');
    zNode.innerHTML = 'The button was clicked.';
    document.getElementById ("myContainer").appendChild (zNode);
}

//--- Style our newly added elements using CSS.
GM_addStyle ( multilineStr ( function () {/*!
    #myContainer {
        position:               absolute;
        top:                    0;
        left:                   0;
        font-size:              20px;
        background:             orange;
        border:                 3px outset black;
        margin:                 5px;
        opacity:                0.9;
        z-index:                1100;
        padding:                5px 20px;
    }
    #myButton {
        cursor:                 pointer;
    }
    #myContainer p {
        color:                  red;
        background:             white;
    }
*/} ) );

function multilineStr (dummyFunc) {
    var str = dummyFunc.toString ();
    str     = str.replace (/^[^\/]+\/\*!?/, '') // Strip function () { /*!
            .replace (/\s*\*\/\s*\}\s*$/, '')   // Strip */ }
            .replace (/\/\/.+$/gm, '') // Double-slash comments wreck CSS. Strip them.
            ;
    return str;
}

¹ étonnamment, cette question ne semble pas avoir été posée tout à fait de cette façon avant.

si vous me demandez, vous pouvez le faire beaucoup plus petit (avec HTML5 n es6) comme:

function addButton(text, onclick, cssObj) {
    cssObj = cssObj || {position: 'absolute', bottom: '7%', left:'4%', 'z-index': 3}
    let button = document.createElement('button'), btnStyle = button.style
    document.body.appendChild(button)
    button.innerHTML = text
    button.onclick = onclick
    btnStyle.position = 'absolute'
    Object.keys(cssObj).forEach(key => btnStyle[key] = cssObj[key])
    return button
}

exemple de script (pour sélectionner tous les e-mails lus dans google inbox):

// ==UserScript==
// @name        mark unread
// @namespace   all
// @include     https://inbox.google.com/*
// @version     1
// @grant       none
// ==/UserScript==

(function(){
    'use strict'

  window.addEventListener('load', () => {
    addButton('select read', selectReadFn)
    })

    function addButton(text, onclick, cssObj) {
        cssObj = cssObj || {position: 'absolute', bottom: '7%', left:'4%', 'z-index': 3}
        let button = document.createElement('button'), btnStyle = button.style
        document.body.appendChild(button)
        button.innerHTML = text
        button.onclick = onclick
        Object.keys(cssObj).forEach(key => btnStyle[key] = cssObj[key])
        return button
    }

    function selectReadFn() {
        [...document.getElementsByClassName('MN')].filter(isRead).forEach(element => element.click())
    }

    function isRead(element) {
        childs = element.parentElement.parentElement.parentElement.getElementsByClassName('G3')
        return ![...childs].some(e => e.innerText.search(/unread/i)!==-1)
    }

}())