close-already-viewed-github-pr-files.js

(() => {
  /* Finds the "Viewed" button for each file, selecting only ones marked as already viewed */
  const allViewedFiles = Array.from(document.querySelectorAll('[data-ga-click="File viewed, click, value:true"]'));

  /* Iterates through the viewed files and closes any that are still open. */
  const allOpenViewedFiles = allViewedFiles.forEach(viewedButton => {
    const fileContainer = viewedButton.closest('.js-details-container');
    if (fileContainer.classList.contains('open')) {
      const collapseButton = fileContainer.querySelector('.js-details-target');
      collapseButton.click();
    }
  })
})();

Make the URL for a bookmark be:
javascript: <code>

Simply click the bookmark to collapse all PR files that have already been viewed.