Add feature flag for merge_ref_head

Add check for diff_compare_with_head before appending head version
Push diff_compare_with_head to frontend flags
Add test to confirm head is not added unless diff_compare_with_head is
enabled

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

@@ -39,7 +39,11 @@ export const diffCompareDropdownTargetVersions = (state, getters) => {
       ...v,
     };
   };
-  return [...state.mergeRequestDiffs.slice(1).map(formatVersion), baseVersion, headVersion];
+
+  if (gon.features?.diffCompareWithHead) {
+    return [...state.mergeRequestDiffs.slice(1).map(formatVersion), baseVersion, headVersion];
+  }
+  return [...state.mergeRequestDiffs.slice(1).map(formatVersion), baseVersion];
 };
 
 export const diffCompareDropdownSourceVersions = (state, getters) => {

app/controllers/projects/merge_requests_controller.rb

@@ -26,6 +26,7 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
     push_frontend_feature_flag(:code_navigation, @project)
     push_frontend_feature_flag(:widget_visibility_polling, @project, default_enabled: true)
     push_frontend_feature_flag(:merge_ref_head_comments, @project)
+    push_frontend_feature_flag(:diff_compare_with_head, @project)
   end
 
   before_action do

doc/user/project/merge_requests/versions.md

@@ -67,6 +67,26 @@ current default comparison.
 
 ![Merge request versions compare HEAD](img/versions_compare_head_v12_10.png)
 
+### Enable or disable `HEAD` comparison mode **(CORE ONLY)**
+
+`HEAD` comparison mode is under development and not ready for production use. It is
+deployed behind a feature flag that is **disabled by default**.
+[GitLab administrators with access to the GitLab Rails console](../../../administration/troubleshooting/navigating_gitlab_via_rails_console.md#starting-a-rails-console-session)
+can enable it for your instance. You're welcome to test it, but use it at your
+own risk.
+
+To enable it:
+
+```ruby
+Feature.enable(:diff_compare_with_head)
+```
+
+To disable it:
+
+```ruby
+Feature.disable(:diff_compare_with_head)
+```
+
 <!-- ## Troubleshooting
 
 Include any troubleshooting steps that you can foresee. If you know beforehand what issues

spec/frontend/diffs/store/getters_versions_dropdowns_spec.js

@@ -18,6 +18,7 @@ describe('Compare diff version dropdowns', () => {
     };
     localState.targetBranchName = 'baseVersion';
     localState.mergeRequestDiffs = diffsMockData;
+    gon.features = { diffCompareWithHead: true };
   });
 
   describe('selectedTargetIndex', () => {
@@ -128,6 +129,14 @@ describe('Compare diff version dropdowns', () => {
       });
       assertVersions(targetVersions);
     });
+
+    it('does not list head version if feature flag is not enabled', () => {
+      gon.features = { diffCompareWithHead: false };
+      setupTest();
+      const targetVersions = getters.diffCompareDropdownTargetVersions(localState, getters);
+
+      expect(targetVersions.find(version => version.isHead)).toBeUndefined();
+    });
   });
 
   it('diffCompareDropdownSourceVersions', () => {