diff --git a/polygerrit-ui/app/elements/diff/gr-diff-builder/gr-diff-builder.html b/polygerrit-ui/app/elements/diff/gr-diff-builder/gr-diff-builder.html
index cc66e3bdff..0997fee43f 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-builder/gr-diff-builder.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff-builder/gr-diff-builder.html
@@ -114,7 +114,6 @@ limitations under the License.
           baseImage: Object,
           revisionImage: Object,
           projectName: String,
-          parentIndex: Number,
           /**
            * @type {Defs.LineOfInterest|null}
            */
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.html b/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.html
index e3bf86690a..0e0f5422c4 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.html
@@ -16,9 +16,9 @@ limitations under the License.
 -->
 
 <link rel="import" href="../../../bower_components/polymer/polymer.html">
+<link rel="import" href="../../../behaviors/gr-patch-set-behavior/gr-patch-set-behavior.html">
 <link rel="import" href="../../core/gr-reporting/gr-reporting.html">
 <link rel="import" href="../../shared/gr-rest-api-interface/gr-rest-api-interface.html">
-
 <link rel="import" href="../gr-diff/gr-diff.html">
 
 <dom-module id="gr-diff-host">
@@ -47,7 +47,8 @@ limitations under the License.
         base-image="[[_baseImage]]"
         revision-image=[[_revisionImage]]
         blame="[[_blame]]"
-        diff="[[_diff]]"></gr-diff>
+        diff="[[_diff]]"
+        parent-index="[[_parentIndex]]"></gr-diff>
     <gr-rest-api-interface id="restAPI"></gr-rest-api-interface>
     <gr-reporting id="reporting" category="diff"></gr-reporting>
   </template>
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.js b/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.js
index 19fb308ed8..5ab37e1d31 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.js
+++ b/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.js
@@ -174,8 +174,17 @@
       },
 
       _loadedWhitespaceLevel: String,
+
+      _parentIndex: {
+        type: Number,
+        computed: '_computeParentIndex(patchRange.*)',
+      },
     },
 
+    behaviors: [
+      Gerrit.PatchSetBehavior,
+    ],
+
     listeners: {
       'create-comment': '_handleCreateComment',
     },
@@ -530,5 +539,15 @@
         this.reload();
       }
     },
+
+    /**
+     * @param {Object} patchRangeRecord
+     * @return {number|null}
+     */
+    _computeParentIndex(patchRangeRecord) {
+      return this.isMergeParent(patchRangeRecord.base.basePatchNum) ?
+          this.getParentIndex(patchRangeRecord.base.basePatchNum) : null;
+    },
+
   });
 })();
diff --git a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
index bddfc6d636..adb4dd6055 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
@@ -290,7 +290,6 @@ limitations under the License.
               is-image-diff="[[isImageDiff]]"
               base-image="[[baseImage]]"
               revision-image="[[revisionImage]]"
-              parent-index="[[_parentIndex]]"
               create-comment-fn="[[_createThreadGroupFn]]"
               line-of-interest="[[lineOfInterest]]">
             <table
diff --git a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.js b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.js
index 69ac283a2c..e8fb5ea465 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.js
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.js
@@ -176,10 +176,7 @@
         observer: '_blameChanged',
       },
 
-      _parentIndex: {
-        type: Number,
-        computed: '_computeParentIndex(patchRange.*)',
-      },
+      parentIndex: Number,
 
       _newlineWarning: {
         type: String,
@@ -693,16 +690,6 @@
       return errorMessage ? 'showError' : '';
     },
 
-    /**
-     * @return {number|null}
-     */
-    _computeParentIndex(patchRangeRecord) {
-      if (!this.isMergeParent(patchRangeRecord.base.basePatchNum)) {
-        return null;
-      }
-      return this.getParentIndex(patchRangeRecord.base.basePatchNum);
-    },
-
     expandAllContext() {
       this._handleFullBypass();
     },