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

ui/webui/resources/cr_elements/cr_dialog/cr_dialog.js

Index: ui/webui/resources/cr_elements/cr_dialog/cr_dialog.js
diff --git a/ui/webui/resources/cr_elements/cr_dialog/cr_dialog.js b/ui/webui/resources/cr_elements/cr_dialog/cr_dialog.js
index c98c75ac4053eda397cdc7758103f9edb91fd87d..3fa57b54f22259340cbfa7f52f7c942e355872b1 100644
--- a/ui/webui/resources/cr_elements/cr_dialog/cr_dialog.js
+++ b/ui/webui/resources/cr_elements/cr_dialog/cr_dialog.js
@@ -45,6 +45,8 @@ Polymer({
     },
   },
 
+  observers: ['onOpenChanged_(open)'],

[dpapad, 2017/04/13 01:12:12]

You probably checked this already, but does this work without adding an "open"
as a Polymer property within the "properties" section?

[scottchen, 2017/04/13 20:17:51]

It does work, but unfortunately found out that observing |open| messes with its
native behavior. We started getting failing tests because dialog.open became
''/null instead of true/false after being observed.

Submitted another patch to use MutationObserver instead.

+
   /** @private {?IntersectionObserver} */
   intersectionObserver_: null,
 
@@ -61,9 +63,13 @@ Polymer({
       this.addEventListener('keypress', this.onKeypress_.bind(this));
   },
 
-  /** @override */
-  attached: function() {
-    if (this.showScrollBorders) {
+  /** @private */
+  onOpenChanged_: function() {
+    if (!this.showScrollBorders) {
+      return;
+    }
+
+    if (this.open !== null && !this.intersectionObserver_) {
       var bodyContainer = this.$$('.body-container');
 
       var bottomMarker = this.$.bodyBottomMarker;
@@ -89,14 +95,10 @@ Polymer({
             root: bodyContainer,
             threshold: 0,
           }));
+
       this.intersectionObserver_.observe(bottomMarker);
       this.intersectionObserver_.observe(topMarker);
-    }
-  },
-
-  /** @override */
-  detached: function() {
-    if (this.intersectionObserver_) {
+    } else if (this.open === null && this.intersectionObserver_) {
       this.intersectionObserver_.disconnect();
       this.intersectionObserver_ = null;
     }