Greasemonkey script. In JIRA, insert a summary line next to the project/case number path. This is what I usually want to copy when composing a mail or a refer to a case.

JiraCopyKeyAndSummary.user.js

// ==UserScript==
// @name        JiraCopyKeyAndSummary
// @author      Daniel Sundberg
// @namespace   https://github.com/DanielSundberg
// @include     https://jira.*/browse/*
// @version     0.2
// @grant    		GM.setClipboard
// ==/UserScript==

// v0.2 2019-05-29
// -Updated for JIRA 7.12
// -Replaced key and summary label with copy button

var keyVal = document.getElementById('key-val').innerHTML
var summaryVal = document.getElementById('summary-val')

// Create a string that looks nice when copied (ID-1234: This is a title)
var keySummary = keyVal + ': ' + summaryVal.textContent.replace(/\n|\r/g, '').trim()

// Insert button to the right of project/key breadcrumb
var breadcrumbs = document.getElementsByClassName('aui-nav-breadcrumbs')

function btnClick(ev) {
  GM.setClipboard(keySummary)
}

// Create a nice looking button
var btn = document.createElement ('button');
btn.innerHTML = 'Copy'
btn.setAttribute('id', 'btnContainer');

breadcrumbs[0].appendChild(btn)


document.getElementById("btnContainer").addEventListener (
    "click", btnClick, false
);
addStyle = function (css) {
  var style = document.createElement('style');
  style.textContent = css;
  document.documentElement.appendChild(style);
  return style; //optional, but convenient for changing the styling later.
};

addStyle(`
	#btnContainer {
		top:							0;
		left:      	      0;
    font-size:        10px;
    background:       #091E4221;
		border: 					none;
		border-radius: 		2px;
		color: 						#454545;
    opacity:          0.9;
    padding:          2px 10px;
    cursor:           pointer;
		margin:						auto;
  }
	#btnContainer:hover {
    background:       #45454538;
  }
	#btnContainer:active {
    background:       #45454555;
  }
`);