Wrap should always be enabled for parallel diff.

app/views/projects/diffs/_file.html.haml

@@ -15,6 +15,7 @@
         %span.file-mode= "#{diff.a_mode} → #{diff.b_mode}"
 
       .diff-btn-group
+        - unless params[:view] == 'parallel'
           %label
             = check_box_tag nil, 1, false, class: "js-toggle-diff-line-wrap"
             Wrap text

app/views/projects/diffs/_parallel_view.html.haml

 / Side-by-side diff view
-%div.text-file
+%div.text-file.diff-wrap-lines
   %table
     - parallel_diff(diff_file, index).each do |line|
       - type_left = line[0]