diff --git a/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.js b/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.js
index 3049e1e98e..d770dddc80 100644
--- a/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.js
+++ b/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.js
@@ -43,6 +43,7 @@ import '../gr-file-list-header/gr-file-list-header.js';
 import '../gr-file-list/gr-file-list.js';
 import '../gr-included-in-dialog/gr-included-in-dialog.js';
 import '../gr-messages-list/gr-messages-list.js';
+import '../gr-messages-list/gr-messages-list-experimental.js';
 import '../gr-related-changes-list/gr-related-changes-list.js';
 import '../../diff/gr-apply-fix-dialog/gr-apply-fix-dialog.js';
 import '../gr-reply-dialog/gr-reply-dialog.js';
@@ -67,6 +68,7 @@ import {RevisionInfo} from '../../shared/revision-info/revision-info.js';
 
 import {PrimaryTabs, SecondaryTabs} from '../../../constants/constants.js';
 import {NO_ROBOT_COMMENTS_THREADS_MSG} from '../../../constants/messages.js';
+import {appContext} from '../../../services/app-context.js';
 
 const CHANGE_ID_ERROR = {
   MISMATCH: 'mismatch',
@@ -444,6 +446,11 @@ class GrChangeView extends mixinBehaviors( [
     };
   }
 
+  constructor() {
+    super();
+    this.flagsService = appContext.flagsService;
+  }
+
   /** @override */
   created() {
     super.created();
@@ -531,8 +538,14 @@ class GrChangeView extends mixinBehaviors( [
     }
   }
 
+  _isChangeLogExperimentEnabled() {
+    return this.flagsService.isEnabled('UiFeature__cleaner_changelog');
+  }
+
   get messagesList() {
-    return this.shadowRoot.querySelector('gr-messages-list');
+    const tagName = this._isChangeLogExperimentEnabled()
+      ? 'gr-messages-list-experimental' : 'gr-messages-list';
+    return this.shadowRoot.querySelector(tagName);
   }
 
   get threadList() {
diff --git a/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view_html.js b/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view_html.js
index 219ee893f7..13ffc02f89 100644
--- a/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view_html.js
+++ b/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view_html.js
@@ -216,7 +216,8 @@ export const htmlTemplate = html`
         --paper-tab-ink: var(--link-color);
       }
       gr-thread-list,
-      gr-messages-list {
+      gr-messages-list,
+      gr-messages-list-experimental {
         display: block;
       }
       gr-thread-list {
@@ -500,7 +501,12 @@ export const htmlTemplate = html`
       </paper-tabs>
       <section class="changeLog">
         <template is="dom-if" if="[[_isTabActive(_constants.SecondaryTabs.CHANGE_LOG, _activeTabs)]]">
-          <gr-messages-list class="hideOnMobileOverlay" change-num="[[_changeNum]]" labels="[[_change.labels]]" messages="[[_change.messages]]" reviewer-updates="[[_change.reviewer_updates]]" change-comments="[[_changeComments]]" project-name="[[_change.project]]" show-reply-buttons="[[_loggedIn]]" on-message-anchor-tap="_handleMessageAnchorTap" on-reply="_handleMessageReply"></gr-messages-list>
+          <template is="dom-if" if="[[!_isChangeLogExperimentEnabled()]]">
+            <gr-messages-list class="hideOnMobileOverlay" change-num="[[_changeNum]]" labels="[[_change.labels]]" messages="[[_change.messages]]" reviewer-updates="[[_change.reviewer_updates]]" change-comments="[[_changeComments]]" project-name="[[_change.project]]" show-reply-buttons="[[_loggedIn]]" on-message-anchor-tap="_handleMessageAnchorTap" on-reply="_handleMessageReply"></gr-messages-list>
+          </template>
+          <template is="dom-if" if="[[_isChangeLogExperimentEnabled()]]">
+            <gr-messages-list-experimental class="hideOnMobileOverlay" change-num="[[_changeNum]]" labels="[[_change.labels]]" messages="[[_change.messages]]" reviewer-updates="[[_change.reviewer_updates]]" change-comments="[[_changeComments]]" project-name="[[_change.project]]" show-reply-buttons="[[_loggedIn]]" on-message-anchor-tap="_handleMessageAnchorTap" on-reply="_handleMessageReply"></gr-messages-list-experimental>
+          </template>
         </template>
         <template is="dom-if" if="[[_isTabActive(_constants.SecondaryTabs.COMMENT_THREADS, _activeTabs)]]">
           <gr-thread-list threads="[[_commentThreads]]" change="[[_change]]" change-num="[[_changeNum]]" logged-in="[[_loggedIn]]" only-show-robot-comments-with-human-reply="" on-thread-list-modified="_handleReloadDiffComments"></gr-thread-list>
diff --git a/polygerrit-ui/app/elements/change/gr-message/gr-message.js b/polygerrit-ui/app/elements/change/gr-message/gr-message.js
index 9028782850..101f304528 100644
--- a/polygerrit-ui/app/elements/change/gr-message/gr-message.js
+++ b/polygerrit-ui/app/elements/change/gr-message/gr-message.js
@@ -287,8 +287,8 @@ class GrMessage extends GestureEventListeners(
     return hideAutomated && isAutomated;
   }
 
-  _computeIsReviewerUpdate(event) {
-    return event.type === 'REVIEWER_UPDATE';
+  _computeIsReviewerUpdate(message) {
+    return message.type === 'REVIEWER_UPDATE';
   }
 
   _getScores(message, labelExtremes) {
diff --git a/polygerrit-ui/app/elements/change/gr-messages-list/gr-messages-list-experimental.js b/polygerrit-ui/app/elements/change/gr-messages-list/gr-messages-list-experimental.js
new file mode 100644
index 0000000000..f939f34c6b
--- /dev/null
+++ b/polygerrit-ui/app/elements/change/gr-messages-list/gr-messages-list-experimental.js
@@ -0,0 +1,365 @@
+/**
+ * @license
+ * Copyright (C) 2020 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+import '../../../scripts/bundled-polymer.js';
+
+import '@polymer/paper-toggle-button/paper-toggle-button.js';
+import '../../core/gr-reporting/gr-reporting.js';
+import '../../shared/gr-button/gr-button.js';
+import '../gr-message/gr-message.js';
+import '../../../styles/shared-styles.js';
+import {dom} from '@polymer/polymer/lib/legacy/polymer.dom.js';
+import {mixinBehaviors} from '@polymer/polymer/lib/legacy/class.js';
+import {GestureEventListeners} from '@polymer/polymer/lib/mixins/gesture-event-listeners.js';
+import {LegacyElementMixin} from '@polymer/polymer/lib/legacy/legacy-element-mixin.js';
+import {PolymerElement} from '@polymer/polymer/polymer-element.js';
+import {htmlTemplate} from './gr-messages-list-experimental_html.js';
+import {KeyboardShortcutBehavior} from '../../../behaviors/keyboard-shortcut-behavior/keyboard-shortcut-behavior.js';
+import {util} from '../../../scripts/util.js';
+
+/**
+ * The content of the enum is also used in the UI for the button text.
+ *
+ * @enum {string}
+ */
+const ExpandAllState = {
+  EXPAND_ALL: 'Expand All',
+  COLLAPSE_ALL: 'Collapse All',
+};
+
+/**
+ * @extends Polymer.Element
+ */
+class GrMessagesListExperimental extends mixinBehaviors( [
+  KeyboardShortcutBehavior,
+], GestureEventListeners(
+    LegacyElementMixin(
+        PolymerElement))) {
+  static get template() { return htmlTemplate; }
+
+  static get is() { return 'gr-messages-list-experimental'; }
+
+  static get properties() {
+    return {
+      changeNum: Number,
+      /**
+       * These are just the change messages. They are combined with reviewer
+       * updates below. So _combinedMessages is the more important property.
+       */
+      messages: {
+        type: Array,
+        value() { return []; },
+      },
+      /**
+       * These are just the reviewer updates. They are combined with change
+       * messages above. So _combinedMessages is the more important property.
+       */
+      reviewerUpdates: {
+        type: Array,
+        value() { return []; },
+      },
+      changeComments: Object,
+      projectName: String,
+      showReplyButtons: {
+        type: Boolean,
+        value: false,
+      },
+      labels: Object,
+
+      /**
+       * Keeps track of the state of the "Expand All" toggle button. Note that
+       * you can individually expand/collapse some messages without affecting
+       * the toggle button's state.
+       *
+       * @type {ExpandAllState}
+       */
+      _expandAllState: {
+        type: String,
+        value: ExpandAllState.EXPAND_ALL,
+      },
+      _expandAllTitle: {
+        type: String,
+        computed: '_computeExpandAllTitle(_expandAllState)',
+      },
+
+      _hideAutomated: {
+        type: Boolean,
+        value: false,
+        observer: '_hideAutomatedChanged',
+      },
+      /**
+       * The merged array of change messages and reviewer updates.
+       */
+      _combinedMessages: {
+        type: Array,
+        computed: '_computeCombinedMessages(messages, reviewerUpdates)',
+        observer: '_combinedMessagesChanged',
+      },
+
+      _labelExtremes: {
+        type: Object,
+        computed: '_computeLabelExtremes(labels.*)',
+      },
+    };
+  }
+
+  scrollToMessage(messageID) {
+    const selector = `[data-message-id="${messageID}"]`;
+    const el = this.shadowRoot.querySelector(selector);
+
+    if (!el && !this._hideAutomated) {
+      console.warn(`Failed to scroll to message: ${messageID}`);
+      return;
+    }
+    if (!el) {
+      this._hideAutomated = false;
+      setTimeout(() => this.scrollToMessage(messageID));
+      return;
+    }
+
+    el.set('message.expanded', true);
+    let top = el.offsetTop;
+    for (let offsetParent = el.offsetParent;
+      offsetParent;
+      offsetParent = offsetParent.offsetParent) {
+      top += offsetParent.offsetTop;
+    }
+    window.scrollTo(0, top);
+    this._highlightEl(el);
+  }
+
+  _isAutomated(message) {
+    const isReviewerUpdate =
+        !!(message.reviewer || message.type === 'REVIEWER_UPDATE');
+    const isAutoGenerated =
+        !!(message.tag && message.tag.startsWith('autogenerated'));
+    return isReviewerUpdate || isAutoGenerated;
+  }
+
+  _hideAutomatedChanged(hideAutomated) {
+    // We have to call render() such that the dom-repeat filter picks up the
+    // change.
+    this.$.messageRepeat.render();
+  }
+
+  /**
+   * Filter for the dom-repeat of combinedMessages.
+   */
+  _isMessageVisible(message) {
+    return !(this._hideAutomated && this._isAutomated(message));
+  }
+
+  /**
+   * Merges change messages and reviewer updates into one array.
+   */
+  _computeCombinedMessages(messages, reviewerUpdates) {
+    messages = messages || [];
+    reviewerUpdates = reviewerUpdates || [];
+    let mi = 0;
+    let ri = 0;
+    let combinedMessages = [];
+    let mDate;
+    let rDate;
+    for (let i = 0; i < messages.length; i++) {
+      messages[i]._index = i;
+    }
+
+    while (mi < messages.length || ri < reviewerUpdates.length) {
+      if (mi >= messages.length) {
+        combinedMessages = combinedMessages.concat(reviewerUpdates.slice(ri));
+        break;
+      }
+      if (ri >= reviewerUpdates.length) {
+        combinedMessages = combinedMessages.concat(messages.slice(mi));
+        break;
+      }
+      mDate = mDate || util.parseDate(messages[mi].date);
+      rDate = rDate || util.parseDate(reviewerUpdates[ri].date);
+      if (rDate < mDate) {
+        combinedMessages.push(reviewerUpdates[ri++]);
+        rDate = null;
+      } else {
+        combinedMessages.push(messages[mi++]);
+        mDate = null;
+      }
+    }
+    combinedMessages.forEach(m => {
+      if (m.expanded === undefined) {
+        m.expanded = false;
+      }
+    });
+    return combinedMessages;
+  }
+
+  _updateExpandedStateOfAllMessages(exp) {
+    if (this._combinedMessages) {
+      for (let i = 0; i < this._combinedMessages.length; i++) {
+        this._combinedMessages[i].expanded = exp;
+        this.notifyPath(`_combinedMessages.${i}.expanded`);
+      }
+    }
+  }
+
+  _computeExpandAllTitle(_expandAllState) {
+    if (_expandAllState === ExpandAllState.COLLAPSED_ALL) {
+      return this.createTitle(
+          this.Shortcut.COLLAPSE_ALL_MESSAGES, this.ShortcutSection.ACTIONS);
+    }
+    if (_expandAllState === ExpandAllState.EXPAND_ALL) {
+      return this.createTitle(
+          this.Shortcut.EXPAND_ALL_MESSAGES, this.ShortcutSection.ACTIONS);
+    }
+    return '';
+  }
+
+  _highlightEl(el) {
+    const highlightedEls =
+        dom(this.root).querySelectorAll('.highlighted');
+    for (const highlighedEl of highlightedEls) {
+      highlighedEl.classList.remove('highlighted');
+    }
+    function handleAnimationEnd() {
+      el.removeEventListener('animationend', handleAnimationEnd);
+      el.classList.remove('highlighted');
+    }
+    el.addEventListener('animationend', handleAnimationEnd);
+    el.classList.add('highlighted');
+  }
+
+  /**
+   * @param {boolean} expand
+   */
+  handleExpandCollapse(expand) {
+    this._expandAllState = expand ? ExpandAllState.COLLAPSE_ALL
+      : ExpandAllState.EXPAND_ALL;
+    this._updateExpandedStateOfAllMessages(expand);
+  }
+
+  _handleExpandCollapseTap(e) {
+    e.preventDefault();
+    this.handleExpandCollapse(
+        this._expandAllState === ExpandAllState.EXPAND_ALL);
+  }
+
+  _handleAnchorClick(e) {
+    this.scrollToMessage(e.detail.id);
+  }
+
+  _hasAutomatedMessages(messages) {
+    if (!messages) { return false; }
+    for (const message of messages) {
+      if (this._isAutomated(message)) {
+        return true;
+      }
+    }
+    return false;
+  }
+
+  /**
+   * Computes message author's file comments for change's message.
+   * Method uses this.messages to find next message and relies on messages
+   * to be sorted by date field descending.
+   *
+   * @param {!Object} changeComments changeComment object, which includes
+   *     a method to get all published comments (including robot comments),
+   *     which returns a Hash of arrays of comments, filename as key.
+   * @param {!Object} message
+   * @return {!Object} Hash of arrays of comments, filename as key.
+   */
+  _computeCommentsForMessage(changeComments, message) {
+    if ([changeComments, message].some(arg => arg === undefined)) {
+      return [];
+    }
+    const comments = changeComments.getAllPublishedComments();
+    if (message._index === undefined || !comments || !this.messages) {
+      return [];
+    }
+    const messages = this.messages || [];
+    const index = message._index;
+    const authorId = message.author && message.author._account_id;
+    const mDate = util.parseDate(message.date).getTime();
+    // NB: Messages array has oldest messages first.
+    let nextMDate;
+    if (index > 0) {
+      for (let i = index - 1; i >= 0; i--) {
+        if (messages[i] && messages[i].author &&
+            messages[i].author._account_id === authorId) {
+          nextMDate = util.parseDate(messages[i].date).getTime();
+          break;
+        }
+      }
+    }
+    const msgComments = {};
+    for (const file in comments) {
+      if (!comments.hasOwnProperty(file)) { continue; }
+      const fileComments = comments[file];
+      for (let i = 0; i < fileComments.length; i++) {
+        if (fileComments[i].author &&
+            fileComments[i].author._account_id !== authorId) {
+          continue;
+        }
+        const cDate = util.parseDate(fileComments[i].updated).getTime();
+        if (cDate <= mDate) {
+          if (nextMDate && cDate <= nextMDate) {
+            continue;
+          }
+          msgComments[file] = msgComments[file] || [];
+          msgComments[file].push(fileComments[i]);
+        }
+      }
+    }
+    return msgComments;
+  }
+
+  /**
+   * This method is for reporting stats only.
+   */
+  _combinedMessagesChanged(combinedMessages) {
+    if (combinedMessages) {
+      if (combinedMessages.length === 0) return;
+      const tags = combinedMessages.map(
+          message => message.tag || message.type ||
+              (message.comments ? 'comments' : 'none'));
+      const tagsCounted = tags.reduce((acc, val) => {
+        acc[val] = (acc[val] || 0) + 1;
+        return acc;
+      }, {all: combinedMessages.length});
+      this.$.reporting.reportInteraction('messages-count', tagsCounted);
+    }
+  }
+
+  /**
+   * Compute a mapping from label name to objects representing the minimum and
+   * maximum possible values for that label.
+   */
+  _computeLabelExtremes(labelRecord) {
+    const extremes = {};
+    const labels = labelRecord.base;
+    if (!labels) { return extremes; }
+    for (const key of Object.keys(labels)) {
+      if (!labels[key] || !labels[key].values) { continue; }
+      const values = Object.keys(labels[key].values)
+          .map(v => parseInt(v, 10));
+      values.sort((a, b) => a - b);
+      if (!values.length) { continue; }
+      extremes[key] = {min: values[0], max: values[values.length - 1]};
+    }
+    return extremes;
+  }
+}
+
+customElements.define(GrMessagesListExperimental.is,
+    GrMessagesListExperimental);
diff --git a/polygerrit-ui/app/elements/change/gr-messages-list/gr-messages-list-experimental_html.js b/polygerrit-ui/app/elements/change/gr-messages-list/gr-messages-list-experimental_html.js
new file mode 100644
index 0000000000..83e2815964
--- /dev/null
+++ b/polygerrit-ui/app/elements/change/gr-messages-list/gr-messages-list-experimental_html.js
@@ -0,0 +1,74 @@
+/**
+ * @license
+ * Copyright (C) 2020 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+import {html} from '@polymer/polymer/lib/utils/html-tag.js';
+
+export const htmlTemplate = html`
+    <style include="shared-styles">
+      :host {
+        display: flex;
+        justify-content: space-between;
+      }
+      .header {
+        align-items: center;
+        border-top: 1px solid var(--border-color);
+        border-bottom: 1px solid var(--border-color);
+        display: flex;
+        justify-content: space-between;
+        padding: var(--spacing-s) var(--spacing-l);
+      }
+      .highlighted {
+        animation: 3s fadeOut;
+      }
+      @keyframes fadeOut {
+        0% { background-color: var(--emphasis-color); }
+        100% { background-color: var(--view-background-color); }
+      }
+      .container {
+        align-items: center;
+        display: flex;
+      }
+      gr-message:not(:last-of-type) {
+        border-bottom: 1px solid var(--border-color);
+      }
+      gr-message:nth-child(2n) {
+        background-color: var(--background-color-secondary);
+      }
+      gr-message:nth-child(2n+1) {
+        background-color: var(--background-color-tertiary);
+      }
+    </style>
+    <div class="header">
+      <span id="automatedMessageToggleContainer" class="container" hidden\$="[[!_hasAutomatedMessages(messages)]]">
+        <paper-toggle-button id="automatedMessageToggle" checked="{{_hideAutomated}}"></paper-toggle-button>Only comments
+        <span class="transparent separator"></span>
+      </span>
+      <gr-button id="collapse-messages" link="" title="[[_expandAllTitle]]" on-click="_handleExpandCollapseTap">
+        [[_expandAllState]]
+      </gr-button>
+    </div>
+    <template id="messageRepeat" is="dom-repeat" items="[[_combinedMessages]]" as="message" filter="_isMessageVisible">
+      <gr-message change-num="[[changeNum]]"
+                  message="[[message]]"
+                  comments="[[_computeCommentsForMessage(changeComments, message)]]"
+                  project-name="[[projectName]]"
+                  show-reply-button="[[showReplyButtons]]"
+                  on-message-anchor-tap="_handleAnchorClick"
+                  label-extremes="[[_labelExtremes]]"
+                  data-message-id\$="[[message.id]]"></gr-message>
+    </template>
+    <gr-reporting id="reporting" category="message-list"></gr-reporting>
+`;
diff --git a/polygerrit-ui/app/elements/change/gr-messages-list/gr-messages-list-experimental_test.html b/polygerrit-ui/app/elements/change/gr-messages-list/gr-messages-list-experimental_test.html
new file mode 100644
index 0000000000..68a4c0b816
--- /dev/null
+++ b/polygerrit-ui/app/elements/change/gr-messages-list/gr-messages-list-experimental_test.html
@@ -0,0 +1,430 @@
+<!DOCTYPE html>
+<!--
+@license
+Copyright (C) 2020 The Android Open Source Project
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+-->
+
+<meta name="viewport" content="width=device-width, minimum-scale=1.0, initial-scale=1.0, user-scalable=yes">
+<title>gr-messages-list-experimental</title>
+
+<script src="/node_modules/@webcomponents/webcomponentsjs/custom-elements-es5-adapter.js"></script>
+
+<script src="/node_modules/@webcomponents/webcomponentsjs/webcomponents-lite.js"></script>
+<script src="/components/wct-browser-legacy/browser.js"></script>
+
+<dom-module id="comment-api-mock">
+  <template>
+    <gr-messages-list-experimental
+        id="messagesList"
+        change-comments="[[_changeComments]]"></gr-messages-list-experimental>
+    <gr-comment-api id="commentAPI"></gr-comment-api>
+  </template>
+  </dom-module>
+
+<test-fixture id="basic">
+  <template>
+    <comment-api-mock>
+      <gr-messages-list-experimental></gr-messages-list-experimental>
+    </comment-api-mock>
+  </template>
+</test-fixture>
+
+<script type="module">
+import '../../../test/common-test-setup.js';
+import '../../diff/gr-comment-api/gr-comment-api.js';
+import './gr-messages-list-experimental.js';
+import '../../diff/gr-comment-api/gr-comment-api-mock_test.js';
+import {dom} from '@polymer/polymer/lib/legacy/polymer.dom.js';
+const randomMessage = function(opt_params) {
+  const params = opt_params || {};
+  const author1 = {
+    _account_id: 1115495,
+    name: 'Andrew Bonventre',
+    email: 'andybons@chromium.org',
+  };
+  return {
+    id: params.id || Math.random().toString(),
+    date: params.date || '2016-01-12 20:28:33.038000',
+    message: params.message || Math.random().toString(),
+    _revision_number: params._revision_number || 1,
+    author: params.author || author1,
+  };
+};
+
+const randomAutomated = function(opt_params) {
+  return Object.assign({tag: 'autogenerated:gerrit:replace'},
+      randomMessage(opt_params));
+};
+
+suite('gr-messages-list-experimental tests', () => {
+  let element;
+  let messages;
+  let sandbox;
+  let commentApiWrapper;
+
+  const getMessages = function() {
+    return dom(element.root).querySelectorAll('gr-message');
+  };
+
+  const author = {
+    _account_id: 42,
+    name: 'Marvin the Paranoid Android',
+    email: 'marvin@sirius.org',
+  };
+
+  const comments = {
+    file1: [
+      {
+        message: 'message text',
+        updated: '2016-09-27 00:18:03.000000000',
+        in_reply_to: '6505d749_f0bec0aa',
+        line: 62,
+        id: '6505d749_10ed44b2',
+        patch_set: 2,
+        author: {
+          email: 'some@email.com',
+          _account_id: 123,
+        },
+      },
+      {
+        message: 'message text',
+        updated: '2016-09-27 00:18:03.000000000',
+        in_reply_to: 'c5912363_6b820105',
+        line: 42,
+        id: '450a935e_0f1c05db',
+        patch_set: 2,
+        author,
+      },
+      {
+        message: 'message text',
+        updated: '2016-09-27 00:18:03.000000000',
+        in_reply_to: '6505d749_f0bec0aa',
+        line: 62,
+        id: '6505d749_10ed44b2',
+        patch_set: 2,
+        author,
+      },
+    ],
+    file2: [
+      {
+        message: 'message text',
+        updated: '2016-09-27 00:18:03.000000000',
+        in_reply_to: 'c5912363_4b7d450a',
+        line: 132,
+        id: '450a935e_4f260d25',
+        patch_set: 2,
+        author,
+      },
+    ],
+  };
+
+  suite('basic tests', () => {
+    setup(() => {
+      stub('gr-rest-api-interface', {
+        getConfig() { return Promise.resolve({}); },
+        getLoggedIn() { return Promise.resolve(false); },
+        getDiffComments() { return Promise.resolve(comments); },
+        getDiffRobotComments() { return Promise.resolve({}); },
+        getDiffDrafts() { return Promise.resolve({}); },
+      });
+      sandbox = sinon.sandbox.create();
+      messages = _.times(3, randomMessage);
+      // Element must be wrapped in an element with direct access to the
+      // comment API.
+      commentApiWrapper = fixture('basic');
+      element = commentApiWrapper.$.messagesList;
+      element.messages = messages;
+
+      // Stub methods on the changeComments object after changeComments has
+      // been initialized.
+      return commentApiWrapper.loadComments();
+    });
+
+    teardown(() => {
+      sandbox.restore();
+    });
+
+    test('expand/collapse all', () => {
+      let allMessageEls = getMessages();
+      for (const message of allMessageEls) {
+        message._expanded = false;
+      }
+      MockInteractions.tap(allMessageEls[1]);
+      assert.isTrue(allMessageEls[1]._expanded);
+
+      MockInteractions.tap(element.shadowRoot
+          .querySelector('#collapse-messages'));
+      allMessageEls = getMessages();
+      for (const message of allMessageEls) {
+        assert.isTrue(message._expanded);
+      }
+
+      MockInteractions.tap(element.shadowRoot
+          .querySelector('#collapse-messages'));
+      allMessageEls = getMessages();
+      for (const message of allMessageEls) {
+        assert.isFalse(message._expanded);
+      }
+    });
+
+    test('expand/collapse from external keypress', () => {
+      // Start with one expanded message. -> not all collapsed
+      element.scrollToMessage(messages[1].id);
+      assert.isFalse([...getMessages()].filter(m => m._expanded).length == 0);
+
+      // Press 'z' -> all collapsed
+      element.handleExpandCollapse(false);
+      assert.isTrue([...getMessages()].filter(m => m._expanded).length == 0);
+
+      // Press 'x' -> all expanded
+      element.handleExpandCollapse(true);
+      assert.isTrue([...getMessages()].filter(m => !m._expanded).length == 0);
+
+      // Press 'z' -> all collapsed
+      element.handleExpandCollapse(false);
+      assert.isTrue([...getMessages()].filter(m => m._expanded).length == 0);
+    });
+
+    test('hide messages does not appear when no automated messages', () => {
+      assert.isOk(element.shadowRoot
+          .querySelector('#automatedMessageToggleContainer[hidden]'));
+    });
+
+    test('scroll to message', () => {
+      const allMessageEls = getMessages();
+      for (const message of allMessageEls) {
+        message.set('message.expanded', false);
+      }
+
+      const scrollToStub = sandbox.stub(window, 'scrollTo');
+      const highlightStub = sandbox.stub(element, '_highlightEl');
+
+      element.scrollToMessage('invalid');
+
+      for (const message of allMessageEls) {
+        assert.isFalse(message._expanded,
+            'expected gr-message to not be expanded');
+      }
+
+      const messageID = messages[1].id;
+      element.scrollToMessage(messageID);
+      assert.isTrue(
+          element.shadowRoot
+              .querySelector('[data-message-id="' + messageID + '"]')
+              ._expanded);
+
+      assert.isTrue(scrollToStub.calledOnce);
+      assert.isTrue(highlightStub.calledOnce);
+    });
+
+    test('scroll to message offscreen', () => {
+      const scrollToStub = sandbox.stub(window, 'scrollTo');
+      const highlightStub = sandbox.stub(element, '_highlightEl');
+      element.messages = _.times(25, randomMessage);
+      flushAsynchronousOperations();
+      assert.isFalse(scrollToStub.called);
+      assert.isFalse(highlightStub.called);
+
+      const messageID = element.messages[1].id;
+      element.scrollToMessage(messageID);
+      assert.isTrue(scrollToStub.calledOnce);
+      assert.isTrue(highlightStub.calledOnce);
+      assert.isTrue(
+          element.shadowRoot
+              .querySelector('[data-message-id="' + messageID + '"]')
+              ._expanded);
+    });
+
+    test('messages', () => {
+      const messages = [].concat(
+          randomMessage(),
+          {
+            _index: 5,
+            _revision_number: 4,
+            message: 'Uploaded patch set 4.',
+            date: '2016-09-28 13:36:33.000000000',
+            author,
+            id: '8c19ccc949c6d482b061be6a28e10782abf0e7af',
+          },
+          {
+            _index: 6,
+            _revision_number: 4,
+            message: 'Patch Set 4:\n\n(6 comments)',
+            date: '2016-09-28 13:36:33.000000000',
+            author,
+            id: 'e7bfdbc842f6b6d8064bc68e0f52b673f40c0ca5',
+          }
+      );
+      element.messages = messages;
+      const isAuthor = function(author, message) {
+        return message.author._account_id === author._account_id;
+      };
+      const isMarvin = isAuthor.bind(null, author);
+      flushAsynchronousOperations();
+      const messageElements = getMessages();
+      assert.equal(messageElements.length, messages.length);
+      assert.deepEqual(messageElements[1].message, messages[1]);
+      assert.deepEqual(messageElements[2].message, messages[2]);
+      assert.deepEqual(messageElements[1].comments.file1,
+          comments.file1.filter(isMarvin));
+      assert.deepEqual(messageElements[1].comments.file2,
+          comments.file2.filter(isMarvin));
+      assert.deepEqual(messageElements[2].comments, {});
+    });
+
+    test('messages without author do not throw', () => {
+      const messages = [{
+        _index: 5,
+        _revision_number: 4,
+        message: 'Uploaded patch set 4.',
+        date: '2016-09-28 13:36:33.000000000',
+        id: '8c19ccc949c6d482b061be6a28e10782abf0e7af',
+      }];
+      element.messages = messages;
+      flushAsynchronousOperations();
+      const messageEls = getMessages();
+      assert.equal(messageEls.length, 1);
+      assert.equal(messageEls[0].message.message, messages[0].message);
+    });
+  });
+
+  suite('gr-messages-list-experimental automate tests', () => {
+    let element;
+    let messages;
+    let sandbox;
+    let commentApiWrapper;
+
+    const getMessages = function() {
+      return dom(element.root).querySelectorAll('gr-message');
+    };
+    const getHiddenMessages = function() {
+      return dom(element.root).querySelectorAll('gr-message[hidden]');
+    };
+
+    const randomMessageReviewer = {
+      reviewer: {},
+      date: '2016-01-13 20:30:33.038000',
+    };
+
+    setup(() => {
+      stub('gr-rest-api-interface', {
+        getConfig() { return Promise.resolve({}); },
+        getLoggedIn() { return Promise.resolve(false); },
+        getDiffComments() { return Promise.resolve({}); },
+        getDiffRobotComments() { return Promise.resolve({}); },
+        getDiffDrafts() { return Promise.resolve({}); },
+      });
+
+      sandbox = sinon.sandbox.create();
+      messages = _.times(2, randomAutomated);
+      messages.push(randomMessageReviewer);
+
+      // Element must be wrapped in an element with direct access to the
+      // comment API.
+      commentApiWrapper = fixture('basic');
+      element = commentApiWrapper.$.messagesList;
+      sandbox.spy(commentApiWrapper.$.commentAPI, 'loadAll');
+      element.messages = messages;
+
+      // Stub methods on the changeComments object after changeComments has
+      // been initialized.
+      return commentApiWrapper.loadComments();
+    });
+
+    teardown(() => {
+      sandbox.restore();
+    });
+
+    test('hide autogenerated button is not hidden', () => {
+      assert.isNotOk(element.shadowRoot
+          .querySelector('#automatedMessageToggle[hidden]'));
+    });
+
+    test('autogenerated messages are not hidden initially', () => {
+      const allHiddenMessageEls = getHiddenMessages();
+
+      // There are no hidden messages.
+      assert.isFalse(!!allHiddenMessageEls.length);
+    });
+
+    test('autogenerated messages hidden after comments only toggle', () => {
+      let allHiddenMessageEls = getHiddenMessages();
+
+      element._hideAutomated = false;
+      MockInteractions.tap(element.$.automatedMessageToggle);
+      flushAsynchronousOperations();
+      const allMessageEls = getMessages();
+      allHiddenMessageEls = getHiddenMessages();
+
+      // Autogenerated messages are now hidden.
+      assert.equal(allHiddenMessageEls.length, allMessageEls.length);
+    });
+
+    test('autogenerated messages not hidden after comments only toggle',
+        () => {
+          let allHiddenMessageEls = getHiddenMessages();
+
+          element._hideAutomated = true;
+          MockInteractions.tap(element.$.automatedMessageToggle);
+          allHiddenMessageEls = getHiddenMessages();
+
+          // Autogenerated messages are now hidden.
+          assert.isFalse(!!allHiddenMessageEls.length);
+        });
+
+    test('_computeLabelExtremes', () => {
+      const computeSpy = sandbox.spy(element, '_computeLabelExtremes');
+
+      element.labels = null;
+      assert.isTrue(computeSpy.calledOnce);
+      assert.deepEqual(computeSpy.lastCall.returnValue, {});
+
+      element.labels = {};
+      assert.isTrue(computeSpy.calledTwice);
+      assert.deepEqual(computeSpy.lastCall.returnValue, {});
+
+      element.labels = {'my-label': {}};
+      assert.isTrue(computeSpy.calledThrice);
+      assert.deepEqual(computeSpy.lastCall.returnValue, {});
+
+      element.labels = {'my-label': {values: {}}};
+      assert.equal(computeSpy.callCount, 4);
+      assert.deepEqual(computeSpy.lastCall.returnValue, {});
+
+      element.labels = {'my-label': {values: {'-12': {}}}};
+      assert.equal(computeSpy.callCount, 5);
+      assert.deepEqual(computeSpy.lastCall.returnValue,
+          {'my-label': {min: -12, max: -12}});
+
+      element.labels = {
+        'my-label': {values: {'-2': {}, '-1': {}, '0': {}, '+1': {}, '+2': {}}},
+      };
+      assert.equal(computeSpy.callCount, 6);
+      assert.deepEqual(computeSpy.lastCall.returnValue,
+          {'my-label': {min: -2, max: 2}});
+
+      element.labels = {
+        'my-label': {values: {'-12': {}}},
+        'other-label': {values: {'-1': {}, ' 0': {}, '+1': {}}},
+      };
+      assert.equal(computeSpy.callCount, 7);
+      assert.deepEqual(computeSpy.lastCall.returnValue, {
+        'my-label': {min: -12, max: -12},
+        'other-label': {min: -1, max: 1},
+      });
+    });
+  });
+});
+</script>
diff --git a/polygerrit-ui/app/test/tests.js b/polygerrit-ui/app/test/tests.js
index 26a3069bc9..504a1766d3 100644
--- a/polygerrit-ui/app/test/tests.js
+++ b/polygerrit-ui/app/test/tests.js
@@ -80,6 +80,7 @@ const elements = [
   'change/gr-label-scores/gr-label-scores_test.html',
   'change/gr-message/gr-message_test.html',
   'change/gr-messages-list/gr-messages-list_test.html',
+  'change/gr-messages-list/gr-messages-list-experimental_test.html',
   'change/gr-related-changes-list/gr-related-changes-list_test.html',
   'change/gr-reply-dialog/gr-reply-dialog-it_test.html',
   'change/gr-reply-dialog/gr-reply-dialog_test.html',