Merge branch '198458-make-merge-ref-diffs-the-default-comparison-mode' into 'master'

Make merge-ref diffs the default comparison mode

Closes #198458

See merge request gitlab-org/gitlab!34472

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

@@ -11,17 +11,26 @@ 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 || false;
+  const diffHeadParam = getParameterByName('diff_head');
+  const diffHead = parseBoolean(diffHeadParam) || (!diffHeadParam && defaultMergeRefForDiffs);
   const isBaseSelected = !state.startVersion && !diffHead;
   const isHeadSelected = !state.startVersion && diffHead;
+  let baseVersion = null;
 
-  const baseVersion = {
+  if (
+    !defaultMergeRefForDiffs ||
+    (defaultMergeRefForDiffs && !state.mergeRequestDiff.head_version_path)
+  ) {
+    baseVersion = {
       versionName: state.targetBranchName,
       version_index: DIFF_COMPARE_BASE_VERSION_INDEX,
       href: state.mergeRequestDiff.base_version_path,
       isBase: true,
-    selected: isBaseSelected,
+      selected:
+        isBaseSelected || (defaultMergeRefForDiffs && !state.mergeRequestDiff.head_version_path),
     };
+  }
 
   const headVersion = {
     versionName: state.targetBranchName,
@@ -40,7 +49,11 @@ export const diffCompareDropdownTargetVersions = (state, getters) => {
     };
   };
 
-  return [...state.mergeRequestDiffs.slice(1).map(formatVersion), baseVersion, headVersion];
+  return [
+    ...state.mergeRequestDiffs.slice(1).map(formatVersion),
+    baseVersion,
+    state.mergeRequestDiff.head_version_path && 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
@@ -457,6 +458,10 @@ 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/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,

spec/controllers/projects/merge_requests_controller_spec.rb

@@ -87,6 +87,22 @@ RSpec.describe Projects::MergeRequestsController do
         end
       end
 
+      context 'with `default_merge_ref_for_diffs` feature flag enabled' do
+        before do
+          stub_feature_flags(default_merge_ref_for_diffs: true)
+          go
+        end
+
+        it 'adds the diff_head parameter' do
+          expect(assigns["endpoint_metadata_url"]).to eq(
+            diffs_metadata_project_json_merge_request_path(
+              project,
+              merge_request,
+              'json',
+              diff_head: true))
+        end
+      end
+
       context 'when diff is missing' do
         render_views