added whitespace toggle to diffs page and set it to return the project compare path with the selected whitespace params

Updated CHANGELOG

Moved CHANGELOG entry

CHANGELOG

@@ -38,6 +38,7 @@ v 8.9.0 (unreleased)
   - Add support for using Yubikeys (U2F) for two-factor authentication
   - Link to blank group icon doesn't throw a 404 anymore
   - Remove 'main language' feature
+  - Toggle whitespace button now available for compare branches diffs #17881
   - Pipelines can be canceled only when there are running builds
   - Use downcased path to container repository as this is expected path by Docker
   - Projects pending deletion will render a 404 page

app/helpers/diff_helper.rb

@@ -135,6 +135,11 @@ module DiffHelper
     toggle_whitespace_link(url, options)
   end
 
+  def diff_compare_whitespace_link(project, from, to, options)
+    url = namespace_project_compare_path(project.namespace, project, from, to, params_with_whitespace)
+    toggle_whitespace_link(url, options)
+  end
+
   private
 
   def hide_whitespace?

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

@@ -11,6 +11,8 @@
         = commit_diff_whitespace_link(@project, @commit, class: 'hidden-xs')
       - elsif current_controller?(:merge_requests)
         = diff_merge_request_whitespace_link(@project, @merge_request, class: 'hidden-xs')
+      - elsif current_controller?(:compare)
+        = diff_compare_whitespace_link(@project, params[:from], params[:to], class: 'hidden-xs')
     .btn-group
       = inline_diff_btn
       = parallel_diff_btn