Fix serious performance bug with rendering Markdown with InlineDiffFilter

Nokogiri's `node.replace` was being unnecessarily called for every text node in
the document due to a comparison bug. The code previously was comparing the
HTML representation of the full document against the text node, which would
always fail. Fix the comparison to just compare the modified text.

Closes #18011

CHANGELOG

@@ -30,6 +30,7 @@ v 8.9.0 (unreleased)
 v 8.8.3
   - Fix incorrect links on pipeline page when merge request created from fork
   - Fix gitlab importer failing to import new projects due to missing credentials
+  - Fix serious performance bug with rendering Markdown with InlineDiffFilter
   - Fix import URL migration not rescuing with the correct Error
   - In search results, only show notes on confidential issues that the user has access to
   - Fix health check access token changing due to old application settings being used

lib/banzai/filter/inline_diff_filter.rb

@@ -8,15 +8,19 @@ module Banzai
           next if has_ancestor?(node, IGNORED_ANCESTOR_TAGS)
 
           content = node.to_html
-          content = content.gsub(/(?:\[\-(.*?)\-\]|\{\-(.*?)\-\})/, '<span class="idiff left right deletion">\1\2</span>')
-          content = content.gsub(/(?:\[\+(.*?)\+\]|\{\+(.*?)\+\})/, '<span class="idiff left right addition">\1\2</span>')
+          html_content = inline_diff_filter(content)
 
-          next if html == content
+          next if content == html_content
 
-          node.replace(content)
+          node.replace(html_content)
         end
         doc
       end
+
+      def inline_diff_filter(text)
+        html_content = text.gsub(/(?:\[\-(.*?)\-\]|\{\-(.*?)\-\})/, '<span class="idiff left right deletion">\1\2</span>')
+        html_content.gsub(/(?:\[\+(.*?)\+\]|\{\+(.*?)\+\})/, '<span class="idiff left right addition">\1\2</span>')
+      end
     end
   end
 end