add transparent namespace to all user-generated anchors in GitLab flavored markdown

app/assets/javascripts/lib/utils/common_utils.js

-/* eslint-disable func-names, space-before-function-paren, wrap-iife, no-var, no-unused-expressions, no-param-reassign, no-else-return, quotes, object-shorthand, comma-dangle, camelcase, one-var, vars-on-top, one-var-declaration-per-line, no-return-assign, consistent-return, padded-blocks, max-len */
+/* eslint-disable func-names, space-before-function-paren, wrap-iife, no-var, no-unused-expressions, no-param-reassign, no-else-return, quotes, object-shorthand, comma-dangle, camelcase, one-var, vars-on-top, one-var-declaration-per-line, no-return-assign, consistent-return, padded-blocks, max-len, prefer-template */
 (function() {
   (function(w) {
     var base;
@@ -97,6 +97,9 @@
     // automatically adjust scroll position for hash urls taking the height of the navbar into account
     // https://github.com/twitter/bootstrap/issues/1768
     w.gl.utils.handleLocationHash = function() {
+      var hash = w.gl.utils.getLocationHash();
+      if (!hash) return;
+
       var navbar = document.querySelector('.navbar-gitlab');
       var subnav = document.querySelector('.layout-nav');
       var fixedTabs = document.querySelector('.js-tabs-affix');
@@ -104,9 +107,21 @@
       var adjustment = 0;
       if (navbar) adjustment -= navbar.offsetHeight;
       if (subnav) adjustment -= subnav.offsetHeight;
-      if (fixedTabs) adjustment -= fixedTabs.offsetHeight;
 
-      window.scrollBy(0, adjustment);
+      // scroll to user-generated markdown anchor if we cannot find a match
+      if (document.getElementById(hash) === null) {
+        var target = document.getElementById('user-content_' + hash);
+        if (target && target.scrollIntoView) {
+          target.scrollIntoView(true);
+          window.scrollBy(0, adjustment);
+        }
+      } else {
+        // only adjust for fixedTabs when not targeting user-generated content
+        if (fixedTabs) {
+          adjustment -= fixedTabs.offsetHeight;
+        }
+        window.scrollBy(0, adjustment);
+      }
     };
 
     gl.utils.updateTooltipTitle = function($tooltipEl, newTitle) {

lib/banzai/filter/table_of_contents_filter.rb

@@ -35,9 +35,11 @@ module Banzai
           headers[id] += 1
 
           if header_content = node.children.first
+            # namespace detection will be automatically handled via javascript (see issue #22781)
+            namespace = "user-content_"
             href = "#{id}#{uniq}"
             push_toc(href, text)
-            header_content.add_previous_sibling(anchor_tag(href))
+            header_content.add_previous_sibling(anchor_tag("#{namespace}#{href}", href))
           end
         end
 
@@ -48,8 +50,8 @@ module Banzai
 
       private
 
-      def anchor_tag(href)
-        %Q{<a id="#{href}" class="anchor" href="##{href}" aria-hidden="true"></a>}
+      def anchor_tag(id, href)
+        %Q{<a id="#{id}" class="anchor" href="##{href}" aria-hidden="true"></a>}
       end
 
       def push_toc(href, text)