Fix SharePoint left sidebar selecting the wrong link with JavaScript

fixLeftNavSelection.js

function fixLeftNavSelection() {
  const leftNavBar = document.getElementById("DeltaPlaceHolderLeftNavBar");
  if (leftNavBar == null) return;
  const href = location.pathname + location.search;
  const correctLink = leftNavBar.querySelector(
    'li > a.ms-core-listMenu-item[href*="' + href + '"]'
  );
  if (correctLink == null) return;

  // Remove selection from everything
  var selectednav = leftNavBar.querySelector(
    "li.selected > a.ms-core-listMenu-selected"
  );
  if (selectednav != null) {
    selectednav.classList.remove("ms-core-listMenu-selected");
    selectednav.parentElement.classList.remove("selected");
  }

  // Add selection to the correct link and li
  correctLink.classList.add("ms-core-listMenu-selected");
  correctLink.parentElement.classList.add("selected");
}

if (typeof _spBodyOnLoadFunctionNames === "object") {
  _spBodyOnLoadFunctionNames.push("fixLeftNavSelection");
}

Keep in mind that this is ES6 code. You may want to convert it to ES5 with Babel before deploying it live