oh dang

One option imho would be to:

• Do strict history management. Every page change has to be handled by JS, and the current page has to be pushed into the history (see JS History API)
• Add custom data, which includes the current scroll position or the last seen post id on the screen
• On browser back, read the custom data and see if there is a last-seen post id or scroll position. Load posts until you find the right id and scroll to it, or try to set the last scroll position after the DOM is fully rendered and styled

// on browser back, load old page and set scroll position
window.addEventListener('popstate', eve => {
  if (eve.state.scrollPos) {
    // load old page
    // change address bar
    document.documentElement.scrollTop = document.body.scrollTop = eve.state.scrollPos;
  }
});

// on anchor click, store scroll position and load new page
for (let a of document.querySelectorAll('a')) {
  a.addEventListener('click', eve => {
    history.pushState(
      {
        scrollPos: window.pageYOffset || document.documentElement.scrollTop,
      },
      null,
      window.location.toString()
    );

    // load new page
    // change address bar
  }
}