opencv/docs_sphinx/_templates/opencv-version-switcher.html

{# Navbar version switcher: a plain `<select>` rendered right after the
   `navbar-logo` slot, mirroring the legacy docs.opencv.org header where the
   version selector sits inline with the wordmark.

   Single source of truth: the options are built client-side from
   `window.OPENCV_DOC_VERSIONS`, defined in `_static/version.js` — the SAME
   list the legacy Doxygen pages carry. There is no build-time scrape and no
   second list: publishing a release is one edit to that file. We load it via
   `pathto('_static/version.js', 1)` (the theme's own idiom for JS assets) so
   the script resolves relative to each page, which means the dropdown
   populates fully in a local `sphinx-build` preview AND on the deployed site,
   regardless of how deep the page sits or which prefix it lands under.

   The option whose label equals this build's `release` is marked selected;
   every other option carries that version's site-absolute path and the
   `onchange` handler navigates the same tab there. #}
<form class="opencv-version-switcher" role="search"
      aria-label="OpenCV documentation version"
      onsubmit="return false">
  <select id="opencv-version-select"
          aria-label="Select OpenCV documentation version"
          onchange="if(this.value){window.location.href=this.value;}">
    <option value="" selected>{{ release }}</option>
  </select>
</form>
<script src="{{ pathto('_static/version.js', 1) }}"></script>
<script>
  (function () {
    var current = {{ release | tojson }};

    // Each version.js entry is a bare directory path (e.g. "/4.13.0"). Link
    // straight to that directory's index document: the S3 *website* endpoint
    // would resolve "/4.13.0/" to index.html on its own, but the plain REST
    // endpoint (bucket.s3.amazonaws.com) does NOT — there "/4.13.0" is a
    // missing object key. Appending "/index.html" works on both endpoints.
    function toHref(path) {
      if (/\.html?($|[?#])/.test(path)) return path;   // already a file
      return path.replace(/\/+$/, '') + '/index.html';
    }

    function populate() {
      var versions = window.OPENCV_DOC_VERSIONS;
      if (!Array.isArray(versions)) return;   // version.js absent (list stays at current release)
      document.querySelectorAll('#opencv-version-select').forEach(function (sel) {
        // Rebuild from the canonical list so the order matches the legacy
        // docs.opencv.org dropdown exactly.
        while (sel.firstChild) sel.removeChild(sel.firstChild);
        versions.forEach(function (v) {
          var label = v[0], path = v[1];
          var opt = document.createElement('option');
          // Selecting the current version is a no-op (empty value -> the
          // inline `onchange` guard skips navigation); every other option
          // navigates to that version's index document.
          opt.value = (label === current) ? '' : toHref(path);
          opt.textContent = label;
          if (label === current) opt.selected = true;
          sel.appendChild(opt);
        });
      });
    }

    function resetSelection() {
      // After picking an older version and hitting Back, the browser can
      // restore the <select> from bfcache with the OLD choice. Snap it back
      // to the current release (the option whose value is empty).
      document.querySelectorAll('#opencv-version-select').forEach(function (sel) {
        for (var i = 0; i < sel.options.length; i++) {
          if (sel.options[i].value === '') { sel.selectedIndex = i; break; }
        }
      });
    }

    if (document.readyState === 'loading') {
      document.addEventListener('DOMContentLoaded', populate);
    } else {
      populate();
    }
    // `pageshow` fires on initial load AND on bfcache restore (Back button).
    window.addEventListener('pageshow', resetSelection);
  })();
</script>