// ... / NixOS / nixpkgs / pull / <number> / ...
const number = new URL(window.location.href).pathname.split('/')[4]

// There are (at the time of writing) two contexts in which the PR
// number is visible:
//
//   1. in the header (following the PR title)
//   2. sufficiently far down that the header is no longer visible (at
//   which point the header shrinks and affixes itself to the top of the
//   page)
//
// These are actually two distinct elements, the visibility of which
// are dependent on your scroll position. They also have completely
// different (and nondescript) CSS selectors, so instead of looking for
// them directly, we locate the immediately preceding PR title elements
// (which do have a common CSS selector), and grab their next sibling.
for (let node of document.querySelectorAll('.js-issue-title')) {
  node = node.nextElementSibling
  const a = document.createElement('a')
  a.setAttribute('href', `https://nixpk.gs/pr-tracker.html?pr=${number}`)
  node.replaceWith(a)
  a.appendChild(node)
}