Default the `single_mr_diff_view` FF to true

app/controllers/projects/merge_requests_controller.rb

@@ -21,7 +21,7 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
   before_action only: [:show] do
     push_frontend_feature_flag(:diffs_batch_load, @project, default_enabled: true)
     push_frontend_feature_flag(:deploy_from_footer, @project, default_enabled: true)
-    push_frontend_feature_flag(:single_mr_diff_view, @project)
+    push_frontend_feature_flag(:single_mr_diff_view, @project, default_enabled: true)
     push_frontend_feature_flag(:suggest_pipeline) if experiment_enabled?(:suggest_pipeline)
   end
 

app/serializers/diff_file_entity.rb

@@ -67,14 +67,14 @@ class DiffFileEntity < DiffFileBaseEntity
   private
 
   def parallel_diff_view?(options, diff_file)
-    return true unless Feature.enabled?(:single_mr_diff_view, diff_file.repository.project)
+    return true unless Feature.enabled?(:single_mr_diff_view, diff_file.repository.project, default_enabled: true)
 
     # If we're not rendering inline, we must be rendering parallel
     !inline_diff_view?(options, diff_file)
   end
 
   def inline_diff_view?(options, diff_file)
-    return true unless Feature.enabled?(:single_mr_diff_view, diff_file.repository.project)
+    return true unless Feature.enabled?(:single_mr_diff_view, diff_file.repository.project, default_enabled: true)
 
     # If nothing is present, inline will be the default.
     options.fetch(:diff_view, :inline).to_sym == :inline

changelogs/unreleased/feature-enable-split-diffs-by-default.yml

+---
+title: Diffs load each view style separately, on demand
+merge_request: 24821
+author:
+type: performance