Merge branch '22781-user-generated-permalinks' into 'master'

Resolve "User-generated permalink IDs collide with GitLab interface"

## What does this MR do?

Prevents ID values automatically generated by headers in [GitLab Flavored Markdown](https://github.com/gitlabhq/gitlabhq/blob/master/doc/user/markdown.md#header-ids-and-links) from colliding with IDs used elsewhere in the GitLab interface.  This can cause confusion when, for instance, a selector looks for a merge request tab with `id="pipelines"` and there is a header with the same ID earlier in the DOM.

How this works:
* All header IDs generated with GitLab Flavored Markdown are namespaced with `id="user-content_foo"`
* All anchor links which point to these IDs continue to use the non-namespaced hash `<a href="#foo">...</a>`
* When a page is loaded or when the `hashchange` event is triggered, javascript will automatically search for `#user-content_foo` if `#foo` cannot be found, and scroll to that position instead.

## Before

![2016-11-21-13.00.28](/uploads/e3be2cd6a9142dfd6e64db5462a6aa76/2016-11-21-13.00.28.gif)

## After:

![2016-11-21-13.12.45](/uploads/f7ae3f3a30c91325eaa3665591b6a850/2016-11-21-13.12.45.gif)  
![2016-11-21-13.03.00](/uploads/3a6a782c081ecaa05b8781548d794909/2016-11-21-13.03.00.gif)  

## Does this MR meet the acceptance criteria?

- [x] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- Tests
  - [x] Added for this feature/bug
  - [ ] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

Closes #22781 

See also prior attempts to address this issue:
#3908, !2023, !2024


See merge request !7631

1 files changed, 5 insertions, 3 deletions

diff --git a/lib/banzai/filter/table_of_contents_filter.rb b/lib/banzai/filter/table_of_contents_filter.rb
index a4eda6fdf76..8e7084f2543 100644
--- a/lib/banzai/filter/table_of_contents_filter.rb
+++ b/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)