Add-on SDK: take a screenshot of the active tab's content

gistfile1.js

const { window: { document } } = require('sdk/addon/window');
const { getTabContentWindow, getActiveTab } = require('sdk/tabs/utils');
const { getMostRecentBrowserWindow } = require('sdk/window/utils');

const canvas = document.createElementNS('http://www.w3.org/1999/xhtml', 'canvas');
document.documentElement.appendChild(canvas);

function captureActiveTab() {
  let contentWindow = getTabContentWindow(getActiveTab(getMostRecentBrowserWindow()));

  let w = contentWindow.innerWidth;
  let h = contentWindow.innerHeight;
  let x = contentWindow.scrollX;
  let y = contentWindow.scrollY;

  canvas.width = w;
  canvas.height = h;

  let ctx = canvas.getContext('2d');

  ctx.drawWindow(contentWindow, x, y, w, h, '#000');
  
  return canvas.toDataURL();
}