Fix version dropdown behavior (#12632)

* sets the version dropdown to retain current doc location

* handle rewrites for anchor links

* support master version redirects

docs/_static/js/docversion.js

@@ -0,0 +1,24 @@
+function setVersion(){
+        let doc = window.location.pathname.match(/^\/(api\/.*)$/) || window.location.pathname.match(/^\/versions\/[^*]+\/(api\/.*)$/);
+        if (doc) {
+            if (document.getElementById('dropdown-menu-position-anchor-version')) {
+                    versionNav = $('#dropdown-menu-position-anchor-version a.main-nav-link');
+                    $(versionNav).each( function( index, el ) {
+                            currLink = $( el ).attr('href');
+                            version = currLink.match(/\/versions\/([0-9.master]+)\//);
+                            if (version) {
+                                    versionedDoc = '/versions/' + version[1] + '/' + doc[1] + (window.location.hash || '');
+                                    $( el ).attr('href', versionedDoc);
+                            }
+                    });
+            }        
+        }
+}
+
+$(document).ready(function () {
+    setVersion();
+});
+
+$('a.reference.internal').click(function(){
+    setVersion();
+});

docs/_static/mxnet-theme/layout.html

@@ -191,6 +191,7 @@
     <script type="text/javascript" src="{{ pathto('_static/js/clipboard.min.js', 1) }}"></script>
     <script type="text/javascript" src="{{ pathto('_static/js/copycode.js', 1) }}"></script>
     <script type="text/javascript" src="{{ pathto('_static/js/page.js', 1) }}"></script>
+    <script type="text/javascript" src="{{ pathto('_static/js/docversion.js', 1) }}"></script>
     <script type="text/javascript">
         $('body').ready(function () {
             $('body').css('visibility', 'visible');

docs/build_version_doc/AddVersion.py

@@ -47,7 +47,7 @@
                          'style="position: relative">' \
                          '<a href="#" tabindex="-1">Versions(%s)</a><ul class="dropdown-menu">' % (args.current_version)
     for i, tag in enumerate(tag_list):
-        url = root_url if tag == args.tag_default else root_url + 'versions/%s/index.html' % (tag)
+        url = root_url + 'versions/%s/index.html' % (tag)
         version_str += '<li><a class="main-nav-link" href=%s>%s</a></li>' % (url, tag)
         version_str_mobile += '<li><a tabindex="-1" href=%s>%s</a></li>' % (url, tag)
     version_str += '</ul></span>'