Fixes diff metadata StartupJS URL not matching

With unified diffs we always send the view param as inline
this wasn't taken into account when setting up the startup JS
url.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/280579

app/controllers/projects/merge_requests/diffs_controller.rb

@@ -69,7 +69,7 @@ class Projects::MergeRequests::DiffsController < Projects::MergeRequests::Applic
     }
 
     options = additional_attributes.merge(
-      diff_view: Feature.enabled?(:unified_diff_lines, @merge_request.project, default_enabled: true) ? "inline" : diff_view,
+      diff_view: unified_diff_lines_view_type(@merge_request.project),
       merge_ref_head_diff: render_merge_ref_head_diff?
     )
 

app/controllers/projects/merge_requests_controller.rb

@@ -481,7 +481,7 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
 
   def endpoint_metadata_url(project, merge_request)
     params = request.query_parameters
-    params[:view] = cookies[:diff_view] if params[:view].blank? && cookies[:diff_view].present?
+    params[:view] = unified_diff_lines_view_type(project)
 
     if Feature.enabled?(:default_merge_ref_for_diffs, project)
       params = params.merge(diff_head: true)

app/helpers/diff_helper.rb

@@ -203,6 +203,14 @@ module DiffHelper
     set_secure_cookie(:diff_view, params.delete(:view), type: CookiesHelper::COOKIE_TYPE_PERMANENT) if params[:view].present?
   end
 
+  def unified_diff_lines_view_type(project)
+    if Feature.enabled?(:unified_diff_lines, project, default_enabled: true)
+      'inline'
+    else
+      diff_view
+    end
+  end
+
   private
 
   def diff_btn(title, name, selected)

changelogs/unreleased/ph-280579-fixDiffMetadataStartupJS.yml

+---
+title: Fixed diff metadata endpoint being called twice
+merge_request: 47265
+author:
+type: fixed

spec/controllers/projects/merge_requests_controller_spec.rb

@@ -95,7 +95,8 @@ RSpec.describe Projects::MergeRequestsController do
               project,
               merge_request,
               'json',
-              diff_head: true))
+              diff_head: true,
+              view: 'inline'))
         end
       end
 

spec/helpers/diff_helper_spec.rb

@@ -358,4 +358,30 @@ RSpec.describe DiffHelper do
       expect(diff_file_path_text(diff_file, max: 10)).to eq("...open.rb")
     end
   end
+
+  describe 'unified_diff_lines_view_type' do
+    before do
+      controller.params[:view] = 'parallel'
+    end
+
+    describe 'unified diffs enabled' do
+      before do
+        stub_feature_flags(unified_diff_lines: true)
+      end
+
+      it 'returns inline view' do
+        expect(helper.unified_diff_lines_view_type(project)).to eq 'inline'
+      end
+    end
+
+    describe 'unified diffs disabled' do
+      before do
+        stub_feature_flags(unified_diff_lines: false)
+      end
+
+      it 'returns parallel view' do
+        expect(helper.unified_diff_lines_view_type(project)).to eq :parallel
+      end
+    end
+  end
 end