1 files changed, 22 insertions, 9 deletions
diff --git a/app/assets/javascripts/diffs/store/getters_versions_dropdowns.js b/app/assets/javascripts/diffs/store/getters_versions_dropdowns.js
index 1e8e736c028..135b1c61ef5 100644
--- a/app/assets/javascripts/diffs/store/getters_versions_dropdowns.js
+++ b/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 = {
-    versionName: state.targetBranchName,
-    version_index: DIFF_COMPARE_BASE_VERSION_INDEX,
-    href: state.mergeRequestDiff.base_version_path,
-    isBase: true,
-    selected: isBaseSelected,
-  };
+  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 || (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) => {