Default diff version to HEAD

app/assets/javascripts/diffs/store/getters_versions_dropdowns.js

@@ -11,7 +11,9 @@ export const diffCompareDropdownTargetVersions = (state, getters) => {
   // startVersion only exists if the user has selected a version other
   // than "base" so if startVersion is null then base must be selected
 
-  const diffHead = parseBoolean(getParameterByName('diff_head'));
+  const defaultMergeRefForDiffs = window.gon?.features?.defaultMergeRefForDiffs;
+  const diffHeadParam = getParameterByName('diff_head');
+  const diffHead = parseBoolean(diffHeadParam) || (!diffHeadParam && defaultMergeRefForDiffs);
   const isBaseSelected = !state.startVersion && !diffHead;
   const isHeadSelected = !state.startVersion && diffHead;
 
@@ -40,7 +42,11 @@ export const diffCompareDropdownTargetVersions = (state, getters) => {
     };
   };
 
-  return [...state.mergeRequestDiffs.slice(1).map(formatVersion), baseVersion, headVersion];
+  return [
+    ...state.mergeRequestDiffs.slice(1).map(formatVersion),
+    defaultMergeRefForDiffs && baseVersion,
+    headVersion,
+  ].filter(a => a);
 };
 
 export const diffCompareDropdownSourceVersions = (state, getters) => {

app/controllers/projects/merge_requests_controller.rb

@@ -41,6 +41,7 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
     push_frontend_feature_flag(:merge_request_widget_graphql, @project)
     push_frontend_feature_flag(:unified_diff_lines, @project)
     push_frontend_feature_flag(:highlight_current_diff_row, @project)
+    push_frontend_feature_flag(:default_merge_ref_for_diffs, @project)
   end
 
   before_action do
@@ -451,6 +452,10 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
     params = request.query_parameters
     params[:view] = cookies[:diff_view] if params[:view].blank? && cookies[:diff_view].present?
 
+    if Feature.enabled?(:default_merge_ref_for_diffs, @project)
+      params = params.merge(diff_head: true)
+    end
+
     diffs_metadata_project_json_merge_request_path(project, merge_request, 'json', params)
   end
 end

app/serializers/merge_request_diff_entity.rb

@@ -31,11 +31,7 @@ class MergeRequestDiffEntity < Grape::Entity
 
     next unless project
 
-    if Feature.enabled?(:default_merge_ref_for_diffs)
-      diffs_project_merge_request_path(project, merge_request, diff_head: true)
-    else
-      merge_request_version_path(project, merge_request, merge_request_diff)
-    end
+    merge_request_version_path(project, merge_request, merge_request_diff)
   end
 
   expose :head_version_path do |merge_request_diff|

app/views/projects/merge_requests/show.html.haml

@@ -70,10 +70,13 @@
           = render 'projects/commit/pipelines_list', disable_initialization: true, endpoint: pipelines_project_merge_request_path(@project, @merge_request)
       - if mr_action === "diffs"
         - add_page_startup_api_call @endpoint_metadata_url
+      - params = request.query_parameters
+      - if Feature.enabled?(:default_merge_ref_for_diffs, @project)
+        - params = params.merge(diff_head: true)
       = render "projects/merge_requests/tabs/pane", name: "diffs", id: "js-diffs-app", class: "diffs", data: { "is-locked": @merge_request.discussion_locked?,
-        endpoint: diffs_project_merge_request_path(@project, @merge_request, 'json', request.query_parameters),
+        endpoint: diffs_project_merge_request_path(@project, @merge_request, 'json', params,
         endpoint_metadata: @endpoint_metadata_url,
-        endpoint_batch: diffs_batch_project_json_merge_request_path(@project, @merge_request, 'json', request.query_parameters),
+        endpoint_batch: diffs_batch_project_json_merge_request_path(@project, @merge_request, 'json', params,
         endpoint_coverage: @coverage_path,
         help_page_path: suggest_changes_help_path,
         current_user_data: @current_user_data,