Merge branch 'copy-as-md' into 'master'

Copying a rendered issue/comment will paste into GFM textareas as actual GFM

See merge request !8597

12 files changed, 911 insertions, 41 deletions

diff --git a/app/assets/javascripts/copy_as_gfm.js.es6 b/app/assets/javascripts/copy_as_gfm.js.es6
new file mode 100644
index 00000000000..b94125a4210
--- /dev/null
+++ b/app/assets/javascripts/copy_as_gfm.js.es6
@@ -0,0 +1,355 @@
+/* eslint-disable class-methods-use-this, object-shorthand, no-unused-vars, no-use-before-define, no-new, max-len, no-restricted-syntax, guard-for-in, no-continue */
+/* jshint esversion: 6 */
+
+/*= require lib/utils/common_utils */
+
+(() => {
+  const gfmRules = {
+    // The filters referenced in lib/banzai/pipeline/gfm_pipeline.rb convert
+    // GitLab Flavored Markdown (GFM) to HTML.
+    // These handlers consequently convert that same HTML to GFM to be copied to the clipboard.
+    // Every filter in lib/banzai/pipeline/gfm_pipeline.rb that generates HTML
+    // from GFM should have a handler here, in reverse order.
+    // The GFM-to-HTML-to-GFM cycle is tested in spec/features/copy_as_gfm_spec.rb.
+    InlineDiffFilter: {
+      'span.idiff.addition'(el, text) {
+        return `{+${text}+}`;
+      },
+      'span.idiff.deletion'(el, text) {
+        return `{-${text}-}`;
+      },
+    },
+    TaskListFilter: {
+      'input[type=checkbox].task-list-item-checkbox'(el, text) {
+        return `[${el.checked ? 'x' : ' '}]`;
+      },
+    },
+    ReferenceFilter: {
+      'a.gfm:not([data-link=true])'(el, text) {
+        return el.dataset.original || text;
+      },
+    },
+    AutolinkFilter: {
+      'a'(el, text) {
+        // Fallback on the regular MarkdownFilter's `a` handler.
+        if (text !== el.getAttribute('href')) return false;
+
+        return text;
+      },
+    },
+    TableOfContentsFilter: {
+      'ul.section-nav'(el, text) {
+        return '[[_TOC_]]';
+      },
+    },
+    EmojiFilter: {
+      'img.emoji'(el, text) {
+        return el.getAttribute('alt');
+      },
+    },
+    ImageLinkFilter: {
+      'a.no-attachment-icon'(el, text) {
+        return text;
+      },
+    },
+    VideoLinkFilter: {
+      '.video-container'(el, text) {
+        const videoEl = el.querySelector('video');
+        if (!videoEl) return false;
+
+        return CopyAsGFM.nodeToGFM(videoEl);
+      },
+      'video'(el, text) {
+        return `![${el.dataset.title}](${el.getAttribute('src')})`;
+      },
+    },
+    MathFilter: {
+      'pre.code.math[data-math-style=display]'(el, text) {
+        return `\`\`\`math\n${text.trim()}\n\`\`\``;
+      },
+      'code.code.math[data-math-style=inline]'(el, text) {
+        return `$\`${text}\`$`;
+      },
+      'span.katex-display span.katex-mathml'(el, text) {
+        const mathAnnotation = el.querySelector('annotation[encoding="application/x-tex"]');
+        if (!mathAnnotation) return false;
+
+        return `\`\`\`math\n${CopyAsGFM.nodeToGFM(mathAnnotation)}\n\`\`\``;
+      },
+      'span.katex-mathml'(el, text) {
+        const mathAnnotation = el.querySelector('annotation[encoding="application/x-tex"]');
+        if (!mathAnnotation) return false;
+
+        return `$\`${CopyAsGFM.nodeToGFM(mathAnnotation)}\`$`;
+      },
+      'span.katex-html'(el, text) {
+        // We don't want to include the content of this element in the copied text.
+        return '';
+      },
+      'annotation[encoding="application/x-tex"]'(el, text) {
+        return text.trim();
+      },
+    },
+    SanitizationFilter: {
+      'dl'(el, text) {
+        let lines = text.trim().split('\n');
+        // Add two spaces to the front of subsequent list items lines,
+        // or leave the line entirely blank.
+        lines = lines.map((l) => {
+          const line = l.trim();
+          if (line.length === 0) return '';
+
+          return `  ${line}`;
+        });
+
+        return `<dl>\n${lines.join('\n')}\n</dl>`;
+      },
+      'sub, dt, dd, kbd, q, samp, var, ruby, rt, rp, abbr'(el, text) {
+        const tag = el.nodeName.toLowerCase();
+        return `<${tag}>${text}</${tag}>`;
+      },
+    },
+    SyntaxHighlightFilter: {
+      'pre.code.highlight'(el, t) {
+        const text = t.trim();
+
+        let lang = el.getAttribute('lang');
+        if (lang === 'plaintext') {
+          lang = '';
+        }
+
+        // Prefixes lines with 4 spaces if the code contains triple backticks
+        if (lang === '' && text.match(/^```/gm)) {
+          return text.split('\n').map((l) => {
+            const line = l.trim();
+            if (line.length === 0) return '';
+
+            return `    ${line}`;
+          }).join('\n');
+        }
+
+        return `\`\`\`${lang}\n${text}\n\`\`\``;
+      },
+      'pre > code'(el, text) {
+         // Don't wrap code blocks in ``
+        return text;
+      },
+    },
+    MarkdownFilter: {
+      'br'(el, text) {
+        // Two spaces at the end of a line are turned into a BR
+        return '  ';
+      },
+      'code'(el, text) {
+        let backtickCount = 1;
+        const backtickMatch = text.match(/`+/);
+        if (backtickMatch) {
+          backtickCount = backtickMatch[0].length + 1;
+        }
+
+        const backticks = Array(backtickCount + 1).join('`');
+        const spaceOrNoSpace = backtickCount > 1 ? ' ' : '';
+
+        return backticks + spaceOrNoSpace + text + spaceOrNoSpace + backticks;
+      },
+      'blockquote'(el, text) {
+        return text.trim().split('\n').map(s => `> ${s}`.trim()).join('\n');
+      },
+      'img'(el, text) {
+        return `![${el.getAttribute('alt')}](${el.getAttribute('src')})`;
+      },
+      'a.anchor'(el, text) {
+        // Don't render a Markdown link for the anchor link inside a heading
+        return text;
+      },
+      'a'(el, text) {
+        return `[${text}](${el.getAttribute('href')})`;
+      },
+      'li'(el, text) {
+        const lines = text.trim().split('\n');
+        const firstLine = `- ${lines.shift()}`;
+        // Add four spaces to the front of subsequent list items lines,
+        // or leave the line entirely blank.
+        const nextLines = lines.map((s) => {
+          if (s.trim().length === 0) return '';
+
+          return `    ${s}`;
+        });
+
+        return `${firstLine}\n${nextLines.join('\n')}`;
+      },
+      'ul'(el, text) {
+        return text;
+      },
+      'ol'(el, text) {
+        // LIs get a `- ` prefix by default, which we replace by `1. ` for ordered lists.
+        return text.replace(/^- /mg, '1. ');
+      },
+      'h1'(el, text) {
+        return `# ${text.trim()}`;
+      },
+      'h2'(el, text) {
+        return `## ${text.trim()}`;
+      },
+      'h3'(el, text) {
+        return `### ${text.trim()}`;
+      },
+      'h4'(el, text) {
+        return `#### ${text.trim()}`;
+      },
+      'h5'(el, text) {
+        return `##### ${text.trim()}`;
+      },
+      'h6'(el, text) {
+        return `###### ${text.trim()}`;
+      },
+      'strong'(el, text) {
+        return `**${text}**`;
+      },
+      'em'(el, text) {
+        return `_${text}_`;
+      },
+      'del'(el, text) {
+        return `~~${text}~~`;
+      },
+      'sup'(el, text) {
+        return `^${text}`;
+      },
+      'hr'(el, text) {
+        return '-----';
+      },
+      'table'(el, text) {
+        const theadEl = el.querySelector('thead');
+        const tbodyEl = el.querySelector('tbody');
+        if (!theadEl || !tbodyEl) return false;
+
+        const theadText = CopyAsGFM.nodeToGFM(theadEl);
+        const tbodyText = CopyAsGFM.nodeToGFM(tbodyEl);
+
+        return theadText + tbodyText;
+      },
+      'thead'(el, text) {
+        const cells = _.map(el.querySelectorAll('th'), (cell) => {
+          let chars = CopyAsGFM.nodeToGFM(cell).trim().length + 2;
+
+          let before = '';
+          let after = '';
+          switch (cell.style.textAlign) {
+            case 'center':
+              before = ':';
+              after = ':';
+              chars -= 2;
+              break;
+            case 'right':
+              after = ':';
+              chars -= 1;
+              break;
+            default:
+              break;
+          }
+
+          chars = Math.max(chars, 3);
+
+          const middle = Array(chars + 1).join('-');
+
+          return before + middle + after;
+        });
+
+        return `${text}|${cells.join('|')}|`;
+      },
+      'tr'(el, text) {
+        const cells = _.map(el.querySelectorAll('td, th'), cell => CopyAsGFM.nodeToGFM(cell).trim());
+        return `| ${cells.join(' | ')} |`;
+      },
+    },
+  };
+
+  class CopyAsGFM {
+    constructor() {
+      $(document).on('copy', '.md, .wiki', this.handleCopy);
+      $(document).on('paste', '.js-gfm-input', this.handlePaste);
+    }
+
+    handleCopy(e) {
+      const clipboardData = e.originalEvent.clipboardData;
+      if (!clipboardData) return;
+
+      const documentFragment = window.gl.utils.getSelectedFragment();
+      if (!documentFragment) return;
+
+      // If the documentFragment contains more than just Markdown, don't copy as GFM.
+      if (documentFragment.querySelector('.md, .wiki')) return;
+
+      e.preventDefault();
+      clipboardData.setData('text/plain', documentFragment.textContent);
+
+      const gfm = CopyAsGFM.nodeToGFM(documentFragment);
+      clipboardData.setData('text/x-gfm', gfm);
+    }
+
+    handlePaste(e) {
+      const clipboardData = e.originalEvent.clipboardData;
+      if (!clipboardData) return;
+
+      const gfm = clipboardData.getData('text/x-gfm');
+      if (!gfm) return;
+
+      e.preventDefault();
+
+      window.gl.utils.insertText(e.target, gfm);
+    }
+
+    static nodeToGFM(node) {
+      if (node.nodeType === Node.TEXT_NODE) {
+        return node.textContent;
+      }
+
+      const text = this.innerGFM(node);
+
+      if (node.nodeType === Node.DOCUMENT_FRAGMENT_NODE) {
+        return text;
+      }
+
+      for (const filter in gfmRules) {
+        const rules = gfmRules[filter];
+
+        for (const selector in rules) {
+          const func = rules[selector];
+
+          if (!window.gl.utils.nodeMatchesSelector(node, selector)) continue;
+
+          const result = func(node, text);
+          if (result === false) continue;
+
+          return result;
+        }
+      }
+
+      return text;
+    }
+
+    static innerGFM(parentNode) {
+      const nodes = parentNode.childNodes;
+
+      const clonedParentNode = parentNode.cloneNode(true);
+      const clonedNodes = Array.prototype.slice.call(clonedParentNode.childNodes, 0);
+
+      for (let i = 0; i < nodes.length; i += 1) {
+        const node = nodes[i];
+        const clonedNode = clonedNodes[i];
+
+        const text = this.nodeToGFM(node);
+
+        // `clonedNode.replaceWith(text)` is not yet widely supported
+        clonedNode.parentNode.replaceChild(document.createTextNode(text), clonedNode);
+      }
+
+      return clonedParentNode.innerText || clonedParentNode.textContent;
+    }
+  }
+
+  window.gl = window.gl || {};
+  window.gl.CopyAsGFM = CopyAsGFM;
+
+  new CopyAsGFM();
+})();
diff --git a/app/assets/javascripts/lib/utils/common_utils.js.es6 b/app/assets/javascripts/lib/utils/common_utils.js.es6
index 7452879d9a3..51993bb3420 100644
--- a/app/assets/javascripts/lib/utils/common_utils.js.es6
+++ b/app/assets/javascripts/lib/utils/common_utils.js.es6
@@ -160,6 +160,62 @@
       return decodeURIComponent(results[2].replace(/\+/g, ' '));
     };
 
+    w.gl.utils.getSelectedFragment = () => {
+      const selection = window.getSelection();
+      const documentFragment = selection.getRangeAt(0).cloneContents();
+      if (documentFragment.textContent.length === 0) return null;
+
+      return documentFragment;
+    };
+
+    w.gl.utils.insertText = (target, text) => {
+      // Firefox doesn't support `document.execCommand('insertText', false, text)` on textareas
+
+      const selectionStart = target.selectionStart;
+      const selectionEnd = target.selectionEnd;
+      const value = target.value;
+
+      const textBefore = value.substring(0, selectionStart);
+      const textAfter = value.substring(selectionEnd, value.length);
+      const newText = textBefore + text + textAfter;
+
+      target.value = newText;
+      target.selectionStart = target.selectionEnd = selectionStart + text.length;
+
+      // Trigger autosave
+      $(target).trigger('input');
+
+      // Trigger autosize
+      var event = document.createEvent('Event');
+      event.initEvent('autosize:update', true, false);
+      target.dispatchEvent(event);
+    };
+
+    w.gl.utils.nodeMatchesSelector = (node, selector) => {
+      const matches = Element.prototype.matches ||
+        Element.prototype.matchesSelector ||
+        Element.prototype.mozMatchesSelector ||
+        Element.prototype.msMatchesSelector ||
+        Element.prototype.oMatchesSelector ||
+        Element.prototype.webkitMatchesSelector;
+
+      if (matches) {
+        return matches.call(node, selector);
+      }
+
+      // IE11 doesn't support `node.matches(selector)`
+
+      let parentNode = node.parentNode;
+      if (!parentNode) {
+        parentNode = document.createElement('div');
+        node = node.cloneNode(true);
+        parentNode.appendChild(node);
+      }
+
+      const matchingNodes = parentNode.querySelectorAll(selector);
+      return Array.prototype.indexOf.call(matchingNodes, node) !== -1;
+    };
+
     /**
       this will take in the headers from an API response and normalize them
       this way we don't run into production issues when nginx gives us lowercased header keys
diff --git a/app/assets/javascripts/shortcuts_issuable.js b/app/assets/javascripts/shortcuts_issuable.js
index 6f919de3da7..4ef516af8c8 100644
--- a/app/assets/javascripts/shortcuts_issuable.js
+++ b/app/assets/javascripts/shortcuts_issuable.js
@@ -39,29 +39,39 @@
     }
 
     ShortcutsIssuable.prototype.replyWithSelectedText = function() {
-      var quote, replyField, selected, separator;
-      if (window.getSelection) {
-        selected = window.getSelection().toString();
-        replyField = $('.js-main-target-form #note_note');
-        if (selected.trim() === "") {
-          return;
-        }
-        // Put a '>' character before each non-empty line in the selection
-        quote = _.map(selected.split("\n"), function(val) {
-          if (val.trim() !== '') {
-            return "> " + val + "\n";
-          }
-        });
-        // If replyField already has some content, add a newline before our quote
-        separator = replyField.val().trim() !== "" && "\n" || '';
-        replyField.val(function(_, current) {
-          return current + separator + quote.join('') + "\n";
-        });
-        // Trigger autosave for the added text
-        replyField.trigger('input');
-        // Focus the input field
-        return replyField.focus();
+      var quote, replyField, documentFragment, selected, separator;
+
+      documentFragment = window.gl.utils.getSelectedFragment();
+      if (!documentFragment) return;
+
+      // If the documentFragment contains more than just Markdown, don't copy as GFM.
+      if (documentFragment.querySelector('.md, .wiki')) return;
+
+      selected = window.gl.CopyAsGFM.nodeToGFM(documentFragment);
+
+      replyField = $('.js-main-target-form #note_note');
+      if (selected.trim() === "") {
+        return;
       }
+      quote = _.map(selected.split("\n"), function(val) {
+        return ("> " + val).trim() + "\n";
+      });
+      // If replyField already has some content, add a newline before our quote
+      separator = replyField.val().trim() !== "" && "\n\n" || '';
+      replyField.val(function(_, current) {
+        return current + separator + quote.join('') + "\n";
+      });
+
+      // Trigger autosave
+      replyField.trigger('input');
+
+      // Trigger autosize
+      var event = document.createEvent('Event');
+      event.initEvent('autosize:update', true, false);
+      replyField.get(0).dispatchEvent(event);
+
+      // Focus the input field
+      return replyField.focus();
     };
 
     ShortcutsIssuable.prototype.editIssue = function() {
diff --git a/changelogs/unreleased/copy-as-md.yml b/changelogs/unreleased/copy-as-md.yml
new file mode 100644
index 00000000000..637e9dc36e2
--- /dev/null
+++ b/changelogs/unreleased/copy-as-md.yml
@@ -0,0 +1,4 @@
+---
+title: Copying a rendered issue/comment will paste into GFM textareas as actual GFM
+merge_request:
+author:
diff --git a/lib/banzai/filter/abstract_reference_filter.rb b/lib/banzai/filter/abstract_reference_filter.rb
index 6d04f68c8f9..a3d495a5da0 100644
--- a/lib/banzai/filter/abstract_reference_filter.rb
+++ b/lib/banzai/filter/abstract_reference_filter.rb
@@ -153,7 +153,7 @@ module Banzai
             title = object_link_title(object)
             klass = reference_class(object_sym)
 
-            data = data_attributes_for(link_content || match, project, object)
+            data = data_attributes_for(link_content || match, project, object, link: !!link_content)
 
             if matches.names.include?("url") && matches[:url]
               url = matches[:url]
@@ -172,9 +172,10 @@ module Banzai
         end
       end
 
-      def data_attributes_for(text, project, object)
+      def data_attributes_for(text, project, object, link: false)
         data_attribute(
           original:     text,
+          link:         link,
           project:      project.id,
           object_sym => object.id
         )
diff --git a/lib/banzai/filter/issue_reference_filter.rb b/lib/banzai/filter/issue_reference_filter.rb
index 4d1bc687696..fd6b9704132 100644
--- a/lib/banzai/filter/issue_reference_filter.rb
+++ b/lib/banzai/filter/issue_reference_filter.rb
@@ -62,7 +62,7 @@ module Banzai
         end
       end
 
-      def data_attributes_for(text, project, object)
+      def data_attributes_for(text, project, object, link: false)
         if object.is_a?(ExternalIssue)
           data_attribute(
             project: project.id,
diff --git a/lib/banzai/filter/syntax_highlight_filter.rb b/lib/banzai/filter/syntax_highlight_filter.rb
index 026b81ac175..a447e2b8bff 100644
--- a/lib/banzai/filter/syntax_highlight_filter.rb
+++ b/lib/banzai/filter/syntax_highlight_filter.rb
@@ -20,17 +20,19 @@ module Banzai
         code = node.text
         css_classes = "code highlight"
         lexer = lexer_for(language)
+        lang = lexer.tag
 
         begin
           code = format(lex(lexer, code))
 
-          css_classes << " js-syntax-highlight #{lexer.tag}"
+          css_classes << " js-syntax-highlight #{lang}"
         rescue
+          lang = nil
           # Gracefully handle syntax highlighter bugs/errors to ensure
           # users can still access an issue/comment/etc.
         end
 
-        highlighted = %(<pre class="#{css_classes}" v-pre="true"><code>#{code}</code></pre>)
+        highlighted = %(<pre class="#{css_classes}" lang="#{lang}" v-pre="true"><code>#{code}</code></pre>)
 
         # Extracted to a method to measure it
         replace_parent_pre_element(node, highlighted)
diff --git a/lib/banzai/filter/video_link_filter.rb b/lib/banzai/filter/video_link_filter.rb
index ac7bbcb0d10..b64a1287d4d 100644
--- a/lib/banzai/filter/video_link_filter.rb
+++ b/lib/banzai/filter/video_link_filter.rb
@@ -35,7 +35,8 @@ module Banzai
           src: element['src'],
           width: '400',
           controls: true,
-          'data-setup' => '{}')
+          'data-setup' => '{}',
+          'data-title' => element['title'] || element['alt'])
 
         link = doc.document.create_element(
           'a',
diff --git a/lib/banzai/pipeline/gfm_pipeline.rb b/lib/banzai/pipeline/gfm_pipeline.rb
index 5a1f873496c..ac95a79009b 100644
--- a/lib/banzai/pipeline/gfm_pipeline.rb
+++ b/lib/banzai/pipeline/gfm_pipeline.rb
@@ -1,6 +1,12 @@
 module Banzai
   module Pipeline
     class GfmPipeline < BasePipeline
+      # These filters convert GitLab Flavored Markdown (GFM) to HTML.
+      # The handlers defined in app/assets/javascripts/copy_as_gfm.js.es6
+      # consequently convert that same HTML to GFM to be copied to the clipboard.
+      # Every filter that generates HTML from GFM should have a handler in
+      # app/assets/javascripts/copy_as_gfm.js.es6, in reverse order.
+      # The GFM-to-HTML-to-GFM cycle is tested in spec/features/copy_as_gfm_spec.rb.
       def self.filters
         @filters ||= FilterArray[
           Filter::SyntaxHighlightFilter,
diff --git a/spec/features/copy_as_gfm_spec.rb b/spec/features/copy_as_gfm_spec.rb
new file mode 100644
index 00000000000..f3a5b565122
--- /dev/null
+++ b/spec/features/copy_as_gfm_spec.rb
@@ -0,0 +1,432 @@
+require 'spec_helper'
+
+describe 'Copy as GFM', feature: true, js: true do
+  include GitlabMarkdownHelper
+  include ActionView::Helpers::JavaScriptHelper
+
+  before do
+    @feat = MarkdownFeature.new
+
+    # `markdown` helper expects a `@project` variable
+    @project = @feat.project
+
+    visit namespace_project_issue_path(@project.namespace, @project, @feat.issue)
+  end
+
+  # The filters referenced in lib/banzai/pipeline/gfm_pipeline.rb convert GitLab Flavored Markdown (GFM) to HTML.
+  # The handlers defined in app/assets/javascripts/copy_as_gfm.js.es6 consequently convert that same HTML to GFM.
+  # To make sure these filters and handlers are properly aligned, this spec tests the GFM-to-HTML-to-GFM cycle
+  # by verifying (`html_to_gfm(gfm_to_html(gfm)) == gfm`) for a number of examples of GFM for every filter, using the `verify` helper.
+
+  # These are all in a single `it` for performance reasons.
+  it 'works', :aggregate_failures do
+    verify(
+      'nesting',
+
+      '> 1. [x] **[$`2 + 2`$ {-=-}{+=+} 2^2 ~~:thumbsup:~~](http://google.com)**'
+    )
+
+    verify(
+      'a real world example from the gitlab-ce README',
+
+      <<-GFM.strip_heredoc
+        # GitLab
+
+        [![Build status](https://gitlab.com/gitlab-org/gitlab-ce/badges/master/build.svg)](https://gitlab.com/gitlab-org/gitlab-ce/commits/master)
+        [![CE coverage report](https://gitlab.com/gitlab-org/gitlab-ce/badges/master/coverage.svg?job=coverage)](https://gitlab-org.gitlab.io/gitlab-ce/coverage-ruby)
+        [![Code Climate](https://codeclimate.com/github/gitlabhq/gitlabhq.svg)](https://codeclimate.com/github/gitlabhq/gitlabhq)
+        [![Core Infrastructure Initiative Best Practices](https://bestpractices.coreinfrastructure.org/projects/42/badge)](https://bestpractices.coreinfrastructure.org/projects/42)
+
+        ## Canonical source
+
+        The canonical source of GitLab Community Edition is [hosted on GitLab.com](https://gitlab.com/gitlab-org/gitlab-ce/).
+
+        ## Open source software to collaborate on code
+
+        To see how GitLab looks please see the [features page on our website](https://about.gitlab.com/features/).
+
+
+        - Manage Git repositories with fine grained access controls that keep your code secure
+
+        - Perform code reviews and enhance collaboration with merge requests
+
+        - Complete continuous integration (CI) and CD pipelines to builds, test, and deploy your applications
+
+        - Each project can also have an issue tracker, issue board, and a wiki
+
+        - Used by more than 100,000 organizations, GitLab is the most popular solution to manage Git repositories on-premises
+
+        - Completely free and open source (MIT Expat license)
+      GFM
+    )
+
+    verify(
+      'InlineDiffFilter',
+
+      '{-Deleted text-}',
+      '{+Added text+}'
+    )
+
+    verify(
+      'TaskListFilter',
+
+      '- [ ] Unchecked task',
+      '- [x] Checked task',
+      '1. [ ] Unchecked numbered task',
+      '1. [x] Checked numbered task'
+    )
+
+    verify(
+      'ReferenceFilter',
+
+      # issue reference
+      @feat.issue.to_reference,
+      # full issue reference
+      @feat.issue.to_reference(full: true),
+      # issue URL
+      namespace_project_issue_url(@project.namespace, @project, @feat.issue),
+      # issue URL with note anchor
+      namespace_project_issue_url(@project.namespace, @project, @feat.issue, anchor: 'note_123'),
+      # issue link
+      "[Issue](#{namespace_project_issue_url(@project.namespace, @project, @feat.issue)})",
+      # issue link with note anchor
+      "[Issue](#{namespace_project_issue_url(@project.namespace, @project, @feat.issue, anchor: 'note_123')})",
+    )
+
+    verify(
+      'AutolinkFilter',
+
+      'https://example.com'
+    )
+
+    verify(
+      'TableOfContentsFilter',
+
+      '[[_TOC_]]'
+    )
+
+    verify(
+      'EmojiFilter',
+
+      ':thumbsup:'
+    )
+
+    verify(
+      'ImageLinkFilter',
+
+      '![Image](https://example.com/image.png)'
+    )
+
+    verify(
+      'VideoLinkFilter',
+
+      '![Video](https://example.com/video.mp4)'
+    )
+
+    verify(
+      'MathFilter: math as converted from GFM to HTML',
+
+      '$`c = \pm\sqrt{a^2 + b^2}`$',
+
+      # math block
+      <<-GFM.strip_heredoc
+        ```math
+        c = \pm\sqrt{a^2 + b^2}
+        ```
+      GFM
+    )
+
+    aggregate_failures('MathFilter: math as transformed from HTML to KaTeX') do
+      gfm = '$`c = \pm\sqrt{a^2 + b^2}`$'
+
+      html = <<-HTML.strip_heredoc
+        <span class="katex">
+          <span class="katex-mathml">
+            <math>
+              <semantics>
+                <mrow>
+                  <mi>c</mi>
+                  <mo>=</mo>
+                  <mo>±</mo>
+                  <msqrt>
+                    <mrow>
+                      <msup>
+                        <mi>a</mi>
+                        <mn>2</mn>
+                      </msup>
+                      <mo>+</mo>
+                      <msup>
+                        <mi>b</mi>
+                        <mn>2</mn>
+                      </msup>
+                    </mrow>
+                  </msqrt>
+                </mrow>
+                <annotation encoding="application/x-tex">c = \\pm\\sqrt{a^2 + b^2}</annotation>
+              </semantics>
+            </math>
+          </span>
+          <span class="katex-html" aria-hidden="true">
+              <span class="strut" style="height: 0.913389em;"></span>
+              <span class="strut bottom" style="height: 1.04em; vertical-align: -0.126611em;"></span>
+              <span class="base textstyle uncramped">
+                <span class="mord mathit">c</span>
+                <span class="mrel">=</span>
+                <span class="mord">±</span>
+                <span class="sqrt mord"><span class="sqrt-sign" style="top: -0.073389em;">
+                  <span class="style-wrap reset-textstyle textstyle uncramped">√</span>
+                </span>
+                <span class="vlist">
+                  <span class="" style="top: 0em;">
+                    <span class="fontsize-ensurer reset-size5 size5">
+                      <span class="" style="font-size: 1em;">​</span>
+                    </span>
+                    <span class="mord textstyle cramped">
+                      <span class="mord">
+                        <span class="mord mathit">a</span>
+                        <span class="msupsub">
+                          <span class="vlist">
+                            <span class="" style="top: -0.289em; margin-right: 0.05em;">
+                              <span class="fontsize-ensurer reset-size5 size5">
+                                <span class="" style="font-size: 0em;">​</span>
+                              </span>
+                              <span class="reset-textstyle scriptstyle cramped">
+                                <span class="mord mathrm">2</span>
+                              </span>
+                            </span>
+                            <span class="baseline-fix">
+                              <span class="fontsize-ensurer reset-size5 size5">
+                                <span class="" style="font-size: 0em;">​</span>
+                              </span>
+                            ​</span>
+                          </span>
+                        </span>
+                      </span>
+                      <span class="mbin">+</span>
+                      <span class="mord">
+                        <span class="mord mathit">b</span>
+                        <span class="msupsub">
+                          <span class="vlist">
+                            <span class="" style="top: -0.289em; margin-right: 0.05em;">
+                              <span class="fontsize-ensurer reset-size5 size5">
+                                <span class="" style="font-size: 0em;">​</span>
+                              </span>
+                              <span class="reset-textstyle scriptstyle cramped">
+                                <span class="mord mathrm">2</span>
+                              </span>
+                            </span>
+                            <span class="baseline-fix">
+                              <span class="fontsize-ensurer reset-size5 size5">
+                                <span class="" style="font-size: 0em;">​</span>
+                              </span>
+                            ​</span>
+                          </span>
+                        </span>
+                      </span>
+                    </span>
+                  </span>
+                  <span class="" style="top: -0.833389em;">
+                    <span class="fontsize-ensurer reset-size5 size5">
+                      <span class="" style="font-size: 1em;">​</span>
+                    </span>
+                    <span class="reset-textstyle textstyle uncramped sqrt-line"></span>
+                  </span>
+                  <span class="baseline-fix">
+                    <span class="fontsize-ensurer reset-size5 size5">
+                      <span class="" style="font-size: 1em;">​</span>
+                    </span>
+                  ​</span>
+                </span>
+              </span>
+            </span>
+          </span>
+        </span>
+      HTML
+
+      output_gfm = html_to_gfm(html)
+      expect(output_gfm.strip).to eq(gfm.strip)
+    end
+
+    verify(
+      'SanitizationFilter',
+
+      <<-GFM.strip_heredoc
+      <sub>sub</sub>
+
+      <dl>
+        <dt>dt</dt>
+        <dd>dd</dd>
+      </dl>
+
+      <kbd>kbd</kbd>
+
+      <q>q</q>
+
+      <samp>samp</samp>
+
+      <var>var</var>
+
+      <ruby>ruby</ruby>
+
+      <rt>rt</rt>
+
+      <rp>rp</rp>
+
+      <abbr>abbr</abbr>
+      GFM
+    )
+
+    verify(
+      'SanitizationFilter',
+
+      <<-GFM.strip_heredoc,
+        ```
+        Plain text
+        ```
+      GFM
+
+      <<-GFM.strip_heredoc,
+        ```ruby
+        def foo
+          bar
+        end
+        ```
+      GFM
+
+      <<-GFM.strip_heredoc
+        Foo
+
+            This is an example of GFM
+
+            ```js
+            Code goes here
+            ```
+      GFM
+    )
+
+    verify(
+      'MarkdownFilter',
+
+      "Line with two spaces at the end  \nto insert a linebreak",
+
+      '`code`',
+      '`` code with ` ticks ``',
+
+      '> Quote',
+
+      # multiline quote
+      <<-GFM.strip_heredoc,
+        > Multiline
+        > Quote
+        >
+        > With multiple paragraphs
+      GFM
+
+      '![Image](https://example.com/image.png)',
+
+      '# Heading with no anchor link',
+
+      '[Link](https://example.com)',
+
+      '- List item',
+
+      # multiline list item
+      <<-GFM.strip_heredoc,
+        - Multiline
+            List item
+      GFM
+
+      # nested lists
+      <<-GFM.strip_heredoc,
+        - Nested
+
+
+            - Lists
+      GFM
+
+      # list with blockquote
+      <<-GFM.strip_heredoc,
+        - List
+
+            > Blockquote
+      GFM
+
+      '1. Numbered list item',
+
+      # multiline numbered list item
+      <<-GFM.strip_heredoc,
+        1. Multiline
+            Numbered list item
+      GFM
+
+      # nested numbered list
+      <<-GFM.strip_heredoc,
+        1. Nested
+
+
+            1. Numbered lists
+      GFM
+
+      '# Heading',
+      '## Heading',
+      '### Heading',
+      '#### Heading',
+      '##### Heading',
+      '###### Heading',
+
+      '**Bold**',
+
+      '_Italics_',
+
+      '~~Strikethrough~~',
+
+      '2^2',
+
+      '-----',
+
+      # table
+      <<-GFM.strip_heredoc,
+        | Centered | Right | Left |
+        |:--------:|------:|------|
+        | Foo | Bar | **Baz** |
+        | Foo | Bar | **Baz** |
+      GFM
+
+      # table with empty heading
+      <<-GFM.strip_heredoc,
+        |  | x | y |
+        |---|---|---|
+        | a | 1 | 0 |
+        | b | 0 | 1 |
+      GFM
+    )
+  end
+
+  alias_method :gfm_to_html, :markdown
+
+  def html_to_gfm(html)
+    js = <<-JS.strip_heredoc
+      (function(html) {
+        var node = document.createElement('div');
+        node.innerHTML = html;
+        return window.gl.CopyAsGFM.nodeToGFM(node);
+      })("#{escape_javascript(html)}")
+    JS
+    page.evaluate_script(js)
+  end
+
+  def verify(label, *gfms)
+    aggregate_failures(label) do
+      gfms.each do |gfm|
+        html = gfm_to_html(gfm)
+        output_gfm = html_to_gfm(html)
+        expect(output_gfm.strip).to eq(gfm.strip)
+      end
+    end
+  end
+
+  # Fake a `current_user` helper
+  def current_user
+    @feat.user
+  end
+end
diff --git a/spec/javascripts/shortcuts_issuable_spec.js b/spec/javascripts/shortcuts_issuable_spec.js
index 755dba19744..386fc8f514e 100644
--- a/spec/javascripts/shortcuts_issuable_spec.js
+++ b/spec/javascripts/shortcuts_issuable_spec.js
@@ -1,6 +1,7 @@
 /* eslint-disable space-before-function-paren, no-return-assign, no-var, quotes */
 /* global ShortcutsIssuable */
 
+/*= require copy_as_gfm */
 /*= require shortcuts_issuable */
 
 (function() {
@@ -14,10 +15,12 @@
     });
     return describe('#replyWithSelectedText', function() {
       var stubSelection;
-      // Stub window.getSelection to return the provided String.
-      stubSelection = function(text) {
-        return window.getSelection = function() {
-          return text;
+      // Stub window.gl.utils.getSelectedFragment to return a node with the provided HTML.
+      stubSelection = function(html) {
+        window.gl.utils.getSelectedFragment = function() {
+          var node = document.createElement('div');
+          node.innerHTML = html;
+          return node;
         };
       };
       beforeEach(function() {
@@ -32,13 +35,13 @@
       });
       describe('with any selection', function() {
         beforeEach(function() {
-          return stubSelection('Selected text.');
+          return stubSelection('<p>Selected text.</p>');
         });
         it('leaves existing input intact', function() {
           $(this.selector).val('This text was already here.');
           expect($(this.selector).val()).toBe('This text was already here.');
           this.shortcut.replyWithSelectedText();
-          return expect($(this.selector).val()).toBe("This text was already here.\n> Selected text.\n\n");
+          return expect($(this.selector).val()).toBe("This text was already here.\n\n> Selected text.\n\n");
         });
         it('triggers `input`', function() {
           var triggered;
@@ -61,16 +64,16 @@
       });
       describe('with a one-line selection', function() {
         return it('quotes the selection', function() {
-          stubSelection('This text has been selected.');
+          stubSelection('<p>This text has been selected.</p>');
           this.shortcut.replyWithSelectedText();
           return expect($(this.selector).val()).toBe("> This text has been selected.\n\n");
         });
       });
       return describe('with a multi-line selection', function() {
         return it('quotes the selected lines as a group', function() {
-          stubSelection("Selected line one.\n\nSelected line two.\nSelected line three.\n");
+          stubSelection("<p>Selected line one.</p>\n\n<p>Selected line two.</p>\n\n<p>Selected line three.</p>");
           this.shortcut.replyWithSelectedText();
-          return expect($(this.selector).val()).toBe("> Selected line one.\n> Selected line two.\n> Selected line three.\n\n");
+          return expect($(this.selector).val()).toBe("> Selected line one.\n>\n> Selected line two.\n>\n> Selected line three.\n\n");
         });
       });
     });
diff --git a/spec/lib/banzai/filter/syntax_highlight_filter_spec.rb b/spec/lib/banzai/filter/syntax_highlight_filter_spec.rb
index d265d29ee86..69e3c52b35a 100644
--- a/spec/lib/banzai/filter/syntax_highlight_filter_spec.rb
+++ b/spec/lib/banzai/filter/syntax_highlight_filter_spec.rb
@@ -6,21 +6,21 @@ describe Banzai::Filter::SyntaxHighlightFilter, lib: true do
   context "when no language is specified" do
     it "highlights as plaintext" do
       result = filter('<pre><code>def fun end</code></pre>')
-      expect(result.to_html).to eq('<pre class="code highlight js-syntax-highlight plaintext" v-pre="true"><code>def fun end</code></pre>')
+      expect(result.to_html).to eq('<pre class="code highlight js-syntax-highlight plaintext" lang="plaintext" v-pre="true"><code>def fun end</code></pre>')
     end
   end
 
   context "when a valid language is specified" do
     it "highlights as that language" do
       result = filter('<pre><code class="ruby">def fun end</code></pre>')
-      expect(result.to_html).to eq('<pre class="code highlight js-syntax-highlight ruby" v-pre="true"><code><span class="k">def</span> <span class="nf">fun</span> <span class="k">end</span></code></pre>')
+      expect(result.to_html).to eq('<pre class="code highlight js-syntax-highlight ruby" lang="ruby" v-pre="true"><code><span class="k">def</span> <span class="nf">fun</span> <span class="k">end</span></code></pre>')
     end
   end
 
   context "when an invalid language is specified" do
     it "highlights as plaintext" do
       result = filter('<pre><code class="gnuplot">This is a test</code></pre>')
-      expect(result.to_html).to eq('<pre class="code highlight js-syntax-highlight plaintext" v-pre="true"><code>This is a test</code></pre>')
+      expect(result.to_html).to eq('<pre class="code highlight js-syntax-highlight plaintext" lang="plaintext" v-pre="true"><code>This is a test</code></pre>')
     end
   end
 
@@ -31,7 +31,7 @@ describe Banzai::Filter::SyntaxHighlightFilter, lib: true do
 
     it "highlights as plaintext" do
       result = filter('<pre><code class="ruby">This is a test</code></pre>')
-      expect(result.to_html).to eq('<pre class="code highlight" v-pre="true"><code>This is a test</code></pre>')
+      expect(result.to_html).to eq('<pre class="code highlight" lang="" v-pre="true"><code>This is a test</code></pre>')
     end
   end
 end