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

ui/webui/resources/cr_elements/cr_dialog/cr_dialog.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.
 
 /**
  * @fileoverview 'cr-dialog' is a component for showing a modal dialog. If the
  * dialog is closed via close(), a 'close' event is fired. If the dialog is
  * canceled via cancel(), a 'cancel' event is fired followed by a 'close' event.
  * Additionally clients can inspect the dialog's |returnValue| property inside
  * the 'close' event listener to determine whether it was canceled or just
@@ skipping 30 matching lines @@
     showScrollBorders: {
       type: Boolean,
       value: false,
       reflectToAttribute: true,
     },
   },
 
   /** @private {?IntersectionObserver} */
   intersectionObserver_: null,
 
+  /** @private {?MutationObserver} */
+  mutationObserver_: null,
+
   /** @override */
   ready: function() {
     // If the active history entry changes (i.e. user clicks back button),
     // all open dialogs should be cancelled.
     window.addEventListener('popstate', function() {
       if (!this.ignorePopstate && this.open)
         this.cancel();
     }.bind(this));
 
     if (!this.ignoreEnterKey)
       this.addEventListener('keypress', this.onKeypress_.bind(this));
   },
 
   /** @override */
   attached: function() {
     if (this.showScrollBorders) {
+      this.mutationObserver_ = new MutationObserver(function() {
+        if (this.open) {
+          this.addIntersectionObserver_();
+        } else {
+          this.removeIntersectionObserver_();
+        }
+      }.bind(this));
+
+      this.mutationObserver_.observe(this, {
+        attributes: true,
+        attributeFilter: ['open'],
+      });
+    }
+  },
+
+  /** @override */
+  detached: function() {
+    this.removeIntersectionObserver_();
+    if (this.mutationObserver_) {
+      this.mutationObserver_.disconnect();
+      this.mutationObserver_ = null;
+    }
+  },
+
+  /** @private */
+  addIntersectionObserver_: function() {

[dpapad, 2017/04/13 22:02:03]

Nit: You can reduce indentation throughout this method as follows

if (this.intersectionObserver)
  return;

// do stuff here....

[scottchen, 2017/04/13 22:33:29]

Done.

+    if (!this.intersectionObserver_) {
       var bodyContainer = this.$$('.body-container');
 
       var bottomMarker = this.$.bodyBottomMarker;
       var topMarker = this.$.bodyTopMarker;
 
       var callback = function(entries) {
         assert(entries.length <= 2);
         for (var i = 0; i < entries.length; i++) {
           var target = entries[i].target;
           assert(target == bottomMarker || target == topMarker);
@@ skipping 10 matching lines @@
           callback,
           /** @type {IntersectionObserverInit} */ ({
             root: bodyContainer,
             threshold: 0,
           }));
       this.intersectionObserver_.observe(bottomMarker);
       this.intersectionObserver_.observe(topMarker);
     }
   },
 
-  /** @override */
-  detached: function() {
+  /** @private */
+  removeIntersectionObserver_: function() {
     if (this.intersectionObserver_) {
       this.intersectionObserver_.disconnect();
       this.intersectionObserver_ = null;
     }
   },
 
   cancel: function() {
     this.fire('cancel');
     HTMLDialogElement.prototype.close.call(this, '');
   },
@@ skipping 24 matching lines @@
       return;
 
     var actionButton =
         this.querySelector('.action-button:not([disabled]):not([hidden])');
     if (actionButton) {
       actionButton.click();
       e.preventDefault();
     }
   },
 });