message: Add descriptive comment for the space behaviour
diff: // space closes the menu, and activates the current link
  // this behavior is identical between both the WAI example, and GitHub's
  if (document.activeElement instanceof HTMLAnchorElement && !document.activeElement.hasAttribute("aria-haspopup")) {
  + // It's supposed to copy the behaviour of the WAI Menu Bar
  + // page, and of GitHub's menus. I've been using these two
  + // sources to judge what is basically "industry standard"
  + // behaviour for menu keyboard activity on the web.
  + //
  + // On GitHub, here's what I notice:
  + //
  + // 1 If you click open a menu, the menu button remains
  + // focused. If, in this stage, I press space, the menu will
  + // close.
  + //
  + // 2 If I use the arrow keys to focus a menu item, and then
  + // press space, the menu item will be activated. For
  + // example, clicking "+", then pressing down, then pressing
  + // space will open the New Repository page.
  + //
  + // Behaviour 1 is why the
  + // `!document.activeElement.hasAttribute("aria-haspopup")`
  + // condition is there. It's to make sure the menu-link on
  + // things like the About dropdown don't get activated.
  + // Behaviour 2 is why this code is required at all; I want to
  + // activate the currently highlighted menu item.
  document.activeElement.click();
  }
  setTimeout(function() {
  