Ensure exact match is the first result in hashtag searches (#21315)

Fixes #17494

app/services/tag_search_service.rb

@@ -76,11 +76,27 @@ class TagSearchService < BaseService
     definition = TagsIndex.query(query)
     definition = definition.filter(filter) if @options[:exclude_unreviewed]
 
-    definition.limit(@limit).offset(@offset).objects.compact
+    ensure_exact_match(definition.limit(@limit).offset(@offset).objects.compact)
   rescue Faraday::ConnectionFailed, Parslet::ParseFailed
     nil
   end
 
+  # Since the ElasticSearch Query doesn't guarantee the exact match will be the
+  # first result or that it will even be returned, patch the results accordingly
+  def ensure_exact_match(results)
+    return results unless @offset.nil? || @offset.zero?
+
+    normalized_query = Tag.normalize(@query)
+    exact_match = results.find { |tag| tag.name.downcase == normalized_query }
+    exact_match ||= Tag.find_normalized(normalized_query)
+    unless exact_match.nil?
+      results.delete(exact_match)
+      results = [exact_match] + results
+    end
+
+    results
+  end
+
   def from_database
     Tag.search_for(@query, @limit, @offset, @options)
   end