A userscript for StackExchange that adds a "copy to clipboard" button to every code block. Hovering over the bottom right corner of a code block makes the button appear.

SE_copy_code_to_clipboard.user.js

// ==UserScript==
// @name         StackExchange copy code to clipboard
// @description  Adds a "copy to clipboard" button to each code block
// @version      1.0.10
// @author       Paul Pinterits
// @include      *://*.stackexchange.com/questions/*
// @include      *://meta.serverfault.com/questions/*
// @include      *://meta.stackoverflow.com/questions/*
// @include      *://meta.superuser.com/questions/*
// @include      *://serverfault.com/questions/*
// @include      *://stackoverflow.com/questions/*
// @include      *://superuser.com/questions/*
// @exclude      *://*/questions/tagged/*
// @exclude      *://*/questions/originals/*
// @require      https://github.com/Aran-Fey/userscript-lib/raw/ca6999d1bac2494421b70286f74d7a9a9ba636e7/userscript_lib.js
// @require      https://github.com/Aran-Fey/SE-userscript-lib/raw/bf77f40b25d7fa88a6c3f474390c858446154ec2/SE_userscript_lib.js
// @grant        GM_setClipboard
// @updateURL    https://gist.github.com/Aran-Fey/9b0855a1cec5a93a2157b3fd742785af/raw/SE_copy_code_to_clipboard.user.js
// @downloadURL  https://gist.github.com/Aran-Fey/9b0855a1cec5a93a2157b3fd742785af/raw/SE_copy_code_to_clipboard.user.js
// ==/UserScript==


function copy_code(codeblock){
    let code = codeblock.textContent;
    GM_setClipboard(code, 'text');
}

function add_copy_code_buttons(post){
    for (let codeblock of post.element.getElementsByTagName('PRE'))
        add_overlay_button(codeblock, '⎘ Copy', copy_code.bind(null, codeblock));
}

page.transform_posts(add_copy_code_buttons, Rerun.AFTER_CHANGE);