Don't display comment on unchanged line on both sides in parallel diff

app/helpers/diff_helper.rb

@@ -66,12 +66,12 @@ module DiffHelper
 
     discussions_left = discussions_right = nil
 
-    if left && (left.unchanged? || left.discussable?)
+    if left && left.discussable? && (left.unchanged? || left.removed?)
       line_code = diff_file.line_code(left)
       discussions_left = @grouped_diff_discussions[line_code]
     end
 
-    if right&.discussable?
+    if right && right.discussable? && right.added?
       line_code = diff_file.line_code(right)
       discussions_right = @grouped_diff_discussions[line_code]
     end

app/models/legacy_diff_note.rb

@@ -42,7 +42,7 @@ class LegacyDiffNote < Note
   end
 
   def for_line?(line)
-    !line.meta? && diff_file.line_code(line) == self.line_code
+    line.discussable? && diff_file.line_code(line) == self.line_code
   end
 
   def original_line_code

lib/gitlab/diff/line.rb

@@ -42,25 +42,25 @@ module Gitlab
       end
 
       def added?
-        type == 'new' || type == 'new-nonewline'
+        %w[new new-nonewline].include?(type)
       end
 
       def removed?
-        type == 'old' || type == 'old-nonewline'
-      end
-
-      def rich_text
-        @parent_file.highlight_lines! if @parent_file && !@rich_text
-
-        @rich_text
+        %w[old old-nonewline].include?(type)
       end
 
       def meta?
-        type == 'match'
+        %w[match new-nonewline old-nonewline].include?(type)
       end
 
       def discussable?
-        !['match', 'new-nonewline', 'old-nonewline'].include?(type)
+        !meta?
+      end
+
+      def rich_text
+        @parent_file.highlight_lines! if @parent_file && !@rich_text
+
+        @rich_text
       end
 
       def as_json(opts = nil)

lib/gitlab/diff/parallel_diff.rb

@@ -14,16 +14,7 @@ module Gitlab
         lines = []
         highlighted_diff_lines = diff_file.highlighted_diff_lines
         highlighted_diff_lines.each do |line|
-          if line.meta? || line.unchanged?
-            # line in the right panel is the same as in the left one
-            lines << {
-              left: line,
-              right: line
-            }
-
-            free_right_index = nil
-            i += 1
-          elsif line.removed?
+          if line.removed?
             lines << {
               left: line,
               right: nil
@@ -51,6 +42,15 @@ module Gitlab
               free_right_index = nil
               i += 1
             end
+          elsif line.meta? || line.unchanged?
+            # line in the right panel is the same as in the left one
+            lines << {
+              left: line,
+              right: line
+            }
+
+            free_right_index = nil
+            i += 1
           end
         end
 

spec/helpers/diff_helper_spec.rb

@@ -148,12 +148,21 @@ describe DiffHelper do
 
     it 'puts comments on added lines' do
       left = Gitlab::Diff::Line.new('\\nonewline', 'old-nonewline', 3, 3, 3)
-      right = Gitlab::Diff::Line.new('new line', 'add', 3, 3, 3)
+      right = Gitlab::Diff::Line.new('new line', 'new', 3, 3, 3)
 
       result = helper.parallel_diff_discussions(left, right, diff_file)
 
       expect(result).to eq([nil, 'comment'])
     end
+
+    it 'puts comments on unchanged lines' do
+      left = Gitlab::Diff::Line.new('unchanged line', nil, 3, 3, 3)
+      right = Gitlab::Diff::Line.new('unchanged line', nil, 3, 3, 3)
+
+      result = helper.parallel_diff_discussions(left, right, diff_file)
+
+      expect(result).to eq(['comment', nil])
+    end
   end
 
   describe "#diff_match_line" do