MD Settings: in cr_dialog, prevent intersectionObserver from firing early.

chrome/test/data/webui/cr_elements/cr_dialog_test.js

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 suite('cr-dialog', function() {
   function pressEnter(element) {
     MockInteractions.keyEventOn(element, 'keypress', 13, undefined, 'Enter');
   }
 
   setup(function() {
@@ skipping 127 matching lines @@
 
     assertNotEquals(dialog, document.activeElement);
     assertNotEquals(button, document.activeElement);
 
     dialog.showModal();
 
     expectNotEquals(dialog, document.activeElement);
     expectEquals(button, document.activeElement);
   });
 
+  // Ensuring that intersectionObserver does not fire any callbacks before the
+  // dialog has been opened.
+  test('body scrollable border not added before modal shown', function(done) {
+    document.body.innerHTML = `
+      <dialog is="cr-dialog" show-scroll-borders>
+        <div class="title">title</div>
+        <div class="body">body</div>
+      </dialog>`;
+
+    var dialog = document.body.querySelector('dialog');
+    assertFalse(dialog.open);
+    var bodyContainer = dialog.$$('.body-container');
+    assertTrue(!!bodyContainer);
+
+    // Waiting for 1ms because IntersectionObserver fires one message loop after
+    // dialog.attached.
+    setTimeout(function() {
+      assertFalse(bodyContainer.classList.contains('top-scrollable'));
+      assertFalse(bodyContainer.classList.contains('bottom-scrollable'));
+      done();
+    }, 1);
+  });
+
   test('dialog body scrollable border when appropriate', function(done) {
     document.body.innerHTML = `
       <dialog is="cr-dialog" show-scroll-borders>
         <div class="title">title</div>
         <div class="body">body</div>
       </dialog>`;
 
     var dialog = document.body.querySelector('dialog');
     var bodyContainer = dialog.$$('.body-container');
     assertTrue(!!bodyContainer);
@@ skipping 29 matching lines @@
           break;
       }
     });
     observer.observe(bodyContainer, {attributes: true});
 
     // Height is normally set via CSS, but mixin doesn't work with innerHTML.
     bodyContainer.style.height = '1px';
     bodyContainer.scrollTop = 100;
   });
 });