Fix issue where amp-img are not rendered inside amp-list in story pag…

…e attachment (#39806)

examples/amp-story/attachment.html

@@ -6,6 +6,8 @@
     <script async custom-element="amp-form" src="https://cdn.ampproject.org/v0/amp-form-0.1.js"></script>
     <script async custom-element="amp-story" src="https://cdn.ampproject.org/v0/amp-story-1.0.js"></script>
     <script async custom-element="amp-story-page-attachment" src="https://cdn.ampproject.org/v0/amp-story-page-attachment-0.1.js"></script>
+    <script async custom-element="amp-list" src="https://cdn.ampproject.org/v0/amp-list-0.1.js"></script>
+    <script async custom-template="amp-mustache" src="https://cdn.ampproject.org/v0/amp-mustache-latest.js"></script>
     <link href='https://fonts.googleapis.com/css?family=Georgia|Open+Sans|Roboto' rel='stylesheet' type='text/css'>
     <link href="https://fonts.googleapis.com/css?family=Roboto+Condensed" rel="stylesheet">
     <title>Attachments</title>
@@ -232,6 +234,27 @@
         </amp-story-page-attachment>
       </amp-story-page>
 
+      <amp-story-page id="amp-list-of-amp-img">
+        <amp-story-grid-layer template="fill">
+          <amp-img layout="fill" src="/examples/visual-tests/amp-story/img/cat1.jpg"></amp-img>
+        </amp-story-grid-layer>
+        <amp-story-grid-layer template="vertical">
+          <p>
+            <span data-text-background-color="#00A3DB">amp-story-page-attachment: amp-list of amp-img.</span></br></br>
+          </p>
+        </amp-story-grid-layer>
+        <amp-story-page-attachment
+          title="Lorem ipsum"
+          layout="nodisplay"
+        >
+          <amp-list src="https://afdsi.com/164189302300000061/data.json" width="auto" height="100" layout="fixed-height">
+            <template type="amp-mustache">
+              <amp-img src="{{imageUrl}}" width="160" height="160"></amp-img>
+            </template>
+          </amp-list>
+        </amp-story-page-attachment>
+      </amp-story-page>
+
       <amp-story-page id="outlink-default">
         <amp-story-grid-layer template="fill">
           <amp-img layout="fill" src="/examples/visual-tests/amp-story/img/cat2.jpg"></amp-img>

extensions/amp-list/0.1/amp-list.js

@@ -1147,6 +1147,16 @@ export class AmpList extends AMP.BaseElement {
         this.addElementsToContainer_(elements, container);
       }
 
+      // For amp-list embedded in the amp-story-page-attachment, because the attachment is initially in
+      // nodisplay mode, the layoutCallback of all amp-img elements are not called at all, even after
+      // the attachment is opened and amp-list fetches results and inserted built amp-img(from templates)
+      // into DOM. Force call layoutCallback to render images.
+      if (this.element.closest('amp-story-page-attachment')) {
+        this.container_.querySelectorAll('amp-img').forEach((element) => {
+          element.getImpl().then((impl) => impl.layoutCallback());
+        });
+      }
+
       const event = createCustomEvent(
         this.win,
         AmpEvents_Enum.DOM_UPDATE,