diff --git a/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.ts b/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.ts
index 7610e5e..799baec 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.ts
+++ b/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.ts
@@ -709,7 +709,8 @@
   }
 
   _handleCreateComment(e: CustomEvent) {
-    const {lineNum, side, patchNum, range, path, commentSide} = e.detail;
+    const {lineNum, side, patchNum, range, path} = e.detail;
+    const commentSide = this._sideToCommentSide(side);
     const threadEl = this._getOrCreateThread(
       patchNum,
       lineNum,
@@ -723,6 +724,15 @@
     this.reporting.recordDraftInteraction();
   }
 
+  _sideToCommentSide(side: Side): CommentSide {
+    if (!this.patchRange) throw Error('patch range not set');
+    return side === Side.LEFT &&
+      (this.patchRange.basePatchNum === 'PARENT' ||
+        isMergeParent(this.patchRange.basePatchNum))
+      ? CommentSide.PARENT
+      : CommentSide.REVISION;
+  }
+
   /**
    * Gets or creates a comment thread at a given location.
    * May provide a range, to get/create a range comment.
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host_test.js b/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host_test.js
index 20063f4..25a4eff 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host_test.js
+++ b/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host_test.js
@@ -21,7 +21,7 @@
 import {GerritNav} from '../../core/gr-navigation/gr-navigation.js';
 import {dom} from '@polymer/polymer/lib/legacy/polymer.dom.js';
 import {createCommentThreads} from '../../../utils/comment-util.js';
-import {Side, CommentSide} from '../../../constants/constants.js';
+import {Side} from '../../../constants/constants.js';
 import {createChange} from '../../../test/test-data-generators.js';
 import {CoverageType} from '../../../types/types.js';
 import {addListenerForTest, stubRestApi} from '../../../test/test-utils.js';
@@ -958,14 +958,16 @@
   suite('create-comment', () => {
     test('creates comments if they do not exist yet', () => {
       const diffSide = Side.LEFT;
-      const commentSide = CommentSide.PARENT;
+      element.patchRange = {
+        basePatchNum: 'PARENT',
+        patchNum: 2,
+      };
 
       element.dispatchEvent(new CustomEvent('create-comment', {
         detail: {
-          patchNum: '2',
+          patchNum: 2,
           lineNum: 3,
           side: diffSide,
-          commentSide,
           path: '/p',
         },
       }));
@@ -975,6 +977,7 @@
 
       assert.equal(threads.length, 1);
       assert.equal(threads[0].diffSide, diffSide);
+      assert.isTrue(threads[0].isOnParent);
       assert.equal(threads[0].range, undefined);
       assert.equal(threads[0].patchNum, 2);
 
@@ -988,10 +991,9 @@
 
       element.dispatchEvent(new CustomEvent('create-comment', {
         detail: {
-          patchNum: '3',
+          patchNum: 3,
           lineNum: 1,
           side: diffSide,
-          commentSide,
           path: '/p',
           range,
         },
@@ -1002,21 +1004,101 @@
 
       assert.equal(threads.length, 2);
       assert.equal(threads[1].diffSide, diffSide);
+      assert.isTrue(threads[0].isOnParent);
       assert.equal(threads[1].range, range);
       assert.equal(threads[1].patchNum, 3);
     });
 
+    test('should not be on parent if on the right', () => {
+      element.patchRange = {
+        basePatchNum: 2,
+        patchNum: 3,
+      };
+
+      element.dispatchEvent(new CustomEvent('create-comment', {
+        detail: {
+          patchNum: 2,
+          side: Side.RIGHT,
+        },
+      }));
+
+      const thread = dom(element.$.diff)
+          .queryDistributedElements('gr-comment-thread')[0];
+
+      assert.isFalse(thread.isOnParent);
+    });
+
+    test('should be on parent if right and base is PARENT', () => {
+      element.patchRange = {
+        basePatchNum: 'PARENT',
+        patchNum: 3,
+      };
+
+      element.dispatchEvent(new CustomEvent('create-comment', {
+        detail: {
+          patchNum: 3,
+          side: Side.LEFT,
+        },
+      }));
+
+      const thread = dom(element.$.diff)
+          .queryDistributedElements('gr-comment-thread')[0];
+
+      assert.isTrue(thread.isOnParent);
+    });
+
+    test('should be on parent if right and base negative', () => {
+      element.patchRange = {
+        basePatchNum: -2, // merge parents have negative numbers
+        patchNum: 3,
+      };
+
+      element.dispatchEvent(new CustomEvent('create-comment', {
+        detail: {
+          patchNum: 3,
+          side: Side.LEFT,
+        },
+      }));
+
+      const thread = dom(element.$.diff)
+          .queryDistributedElements('gr-comment-thread')[0];
+
+      assert.isTrue(thread.isOnParent);
+    });
+
+    test('should not be on parent otherwise', () => {
+      element.patchRange = {
+        basePatchNum: 2, // merge parents have negative numbers
+        patchNum: 3,
+      };
+
+      element.dispatchEvent(new CustomEvent('create-comment', {
+        detail: {
+          patchNum: 3,
+          side: Side.LEFT,
+        },
+      }));
+
+      const thread = dom(element.$.diff)
+          .queryDistributedElements('gr-comment-thread')[0];
+
+      assert.isFalse(thread.isOnParent);
+    });
+
     test('thread should use old file path if first created ' +
     'on patch set (left) before renaming', () => {
       const diffSide = Side.LEFT;
+      element.patchRange = {
+        basePatchNum: 2,
+        patchNum: 3,
+      };
       element.file = {basePath: 'file_renamed.txt', path: element.path};
 
       element.dispatchEvent(new CustomEvent('create-comment', {
         detail: {
-          patchNum: '2',
+          patchNum: 2,
           lineNum: 3,
           side: diffSide,
-          commentSide: CommentSide.REVISION,
           path: '/p',
         },
       }));
@@ -1032,14 +1114,17 @@
     test('thread should use new file path if first created' +
     'on patch set (right) after renaming', () => {
       const diffSide = Side.RIGHT;
+      element.patchRange = {
+        basePatchNum: 2,
+        patchNum: 3,
+      };
       element.file = {basePath: 'file_renamed.txt', path: element.path};
 
       element.dispatchEvent(new CustomEvent('create-comment', {
         detail: {
-          patchNum: '2',
+          patchNum: 2,
           lineNum: 3,
           side: diffSide,
-          commentSide: CommentSide.REVISION,
           path: '/p',
         },
       }));
@@ -1055,14 +1140,17 @@
     test('thread should use new file path if first created' +
     'on patch set (left) but is base', () => {
       const diffSide = Side.LEFT;
+      element.patchRange = {
+        basePatchNum: 'PARENT',
+        patchNum: 3,
+      };
       element.file = {basePath: 'file_renamed.txt', path: element.path};
 
       element.dispatchEvent(new CustomEvent('create-comment', {
         detail: {
-          patchNum: '2',
+          patchNum: 2,
           lineNum: 3,
           side: diffSide,
-          commentSide: CommentSide.PARENT,
           path: '/p',
           range: undefined,
         },
diff --git a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.ts b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.ts
index 359db1f..79d529d 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.ts
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.ts
@@ -63,7 +63,7 @@
   PolymerDomWrapper,
 } from '../../../types/types';
 import {CommentRangeLayer} from '../gr-ranged-comment-layer/gr-ranged-comment-layer';
-import {CommentSide, DiffViewMode, Side} from '../../../constants/constants';
+import {DiffViewMode, Side} from '../../../constants/constants';
 import {KeyLocations} from '../gr-diff-processor/gr-diff-processor';
 import {FlattenedNodesObserver} from '@polymer/polymer/lib/utils/flattened-nodes-observer';
 import {PolymerDeepPropertyChange} from '@polymer/polymer/interfaces';
@@ -647,11 +647,7 @@
       lineEl,
       contentEl
     );
-    const isOnParent = this._getIsParentCommentByLineAndContent(
-      lineEl,
-      contentEl
-    );
-    const commentSide = isOnParent ? CommentSide.PARENT : CommentSide.REVISION;
+
     this.dispatchEvent(
       new CustomEvent('create-comment', {
         bubbles: true,
@@ -659,7 +655,6 @@
         detail: {
           lineNum,
           side,
-          commentSide,
           patchNum: patchForNewThreads,
           range,
           path: this.path,
@@ -715,25 +710,11 @@
     return patchNum;
   }
 
-  _getIsParentCommentByLineAndContent(lineEl: Element, contentEl: Element) {
-    if (!this.patchRange) throw Error('patch range not set');
-    return (
-      (lineEl.classList.contains(Side.LEFT) ||
-        contentEl.classList.contains('remove')) &&
-      (this.patchRange.basePatchNum === 'PARENT' ||
-        isMergeParent(this.patchRange.basePatchNum))
-    );
-  }
-
   _getCommentSideByLineAndContent(lineEl: Element, contentEl: Element): Side {
-    let side = Side.RIGHT;
-    if (
-      lineEl.classList.contains(Side.LEFT) ||
+    return lineEl.classList.contains(Side.LEFT) ||
       contentEl.classList.contains('remove')
-    ) {
-      side = Side.LEFT;
-    }
-    return side;
+      ? Side.LEFT
+      : Side.RIGHT;
   }
 
   _prefsObserver(newPrefs: DiffPreferencesInfo, oldPrefs: DiffPreferencesInfo) {
diff --git a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff_test.js b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff_test.js
index 2cc9c10..baee7aa 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff_test.js
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff_test.js
@@ -87,7 +87,7 @@
     assert.isNotOk(getComputedStyleValue('--line-limit', element));
   });
 
-  suite('_get{PatchNum|IsParentComment}ByLineAndContent', () => {
+  suite('_getPatchNumByLineAndContent', () => {
     let lineEl;
     let contentEl;
 
@@ -97,78 +97,39 @@
       contentEl = document.createElement('span');
     });
 
-    suite('_getPatchNumByLineAndContent', () => {
-      test('right side', () => {
-        element.patchRange = {patchNum: 4, basePatchNum: 'PARENT'};
-        lineEl.classList.add('right');
-        assert.equal(element._getPatchNumByLineAndContent(lineEl, contentEl),
-            4);
-      });
-
-      test('left side parent by linenum', () => {
-        element.patchRange = {patchNum: 4, basePatchNum: 'PARENT'};
-        lineEl.classList.add('left');
-        assert.equal(element._getPatchNumByLineAndContent(lineEl, contentEl),
-            4);
-      });
-
-      test('left side parent by content', () => {
-        element.patchRange = {patchNum: 4, basePatchNum: 'PARENT'};
-        contentEl.classList.add('remove');
-        assert.equal(element._getPatchNumByLineAndContent(lineEl, contentEl),
-            4);
-      });
-
-      test('left side merge parent', () => {
-        element.patchRange = {patchNum: 4, basePatchNum: -2};
-        contentEl.classList.add('remove');
-        assert.equal(element._getPatchNumByLineAndContent(lineEl, contentEl),
-            4);
-      });
-
-      test('left side non parent', () => {
-        element.patchRange = {patchNum: 4, basePatchNum: 3};
-        contentEl.classList.add('remove');
-        assert.equal(element._getPatchNumByLineAndContent(lineEl, contentEl),
-            3);
-      });
+    test('right side', () => {
+      element.patchRange = {patchNum: 4, basePatchNum: 'PARENT'};
+      lineEl.classList.add('right');
+      assert.equal(element._getPatchNumByLineAndContent(lineEl, contentEl),
+          4);
     });
 
-    suite('_getIsParentCommentByLineAndContent', () => {
-      test('right side', () => {
-        element.patchRange = {patchNum: 4, basePatchNum: 'PARENT'};
-        lineEl.classList.add('right');
-        assert.isFalse(
-            element._getIsParentCommentByLineAndContent(lineEl, contentEl));
-      });
+    test('left side parent by linenum', () => {
+      element.patchRange = {patchNum: 4, basePatchNum: 'PARENT'};
+      lineEl.classList.add('left');
+      assert.equal(element._getPatchNumByLineAndContent(lineEl, contentEl),
+          4);
+    });
 
-      test('left side parent by linenum', () => {
-        element.patchRange = {patchNum: 4, basePatchNum: 'PARENT'};
-        lineEl.classList.add('left');
-        assert.isTrue(
-            element._getIsParentCommentByLineAndContent(lineEl, contentEl));
-      });
+    test('left side parent by content', () => {
+      element.patchRange = {patchNum: 4, basePatchNum: 'PARENT'};
+      contentEl.classList.add('remove');
+      assert.equal(element._getPatchNumByLineAndContent(lineEl, contentEl),
+          4);
+    });
 
-      test('left side parent by content', () => {
-        element.patchRange = {patchNum: 4, basePatchNum: 'PARENT'};
-        contentEl.classList.add('remove');
-        assert.isTrue(
-            element._getIsParentCommentByLineAndContent(lineEl, contentEl));
-      });
+    test('left side merge parent', () => {
+      element.patchRange = {patchNum: 4, basePatchNum: -2};
+      contentEl.classList.add('remove');
+      assert.equal(element._getPatchNumByLineAndContent(lineEl, contentEl),
+          4);
+    });
 
-      test('left side merge parent', () => {
-        element.patchRange = {patchNum: 4, basePatchNum: -2};
-        contentEl.classList.add('remove');
-        assert.isTrue(
-            element._getIsParentCommentByLineAndContent(lineEl, contentEl));
-      });
-
-      test('left side non parent', () => {
-        element.patchRange = {patchNum: 4, basePatchNum: 3};
-        contentEl.classList.add('remove');
-        assert.isFalse(
-            element._getIsParentCommentByLineAndContent(lineEl, contentEl));
-      });
+    test('left side non parent', () => {
+      element.patchRange = {patchNum: 4, basePatchNum: 3};
+      contentEl.classList.add('remove');
+      assert.equal(element._getPatchNumByLineAndContent(lineEl, contentEl),
+          3);
     });
   });