MD Settings: Save and restore scroll position in iron-list

ui/webui/resources/cr_elements/cr_scrollable_behavior.js

Index: ui/webui/resources/cr_elements/cr_scrollable_behavior.js
diff --git a/ui/webui/resources/cr_elements/cr_scrollable_behavior.js b/ui/webui/resources/cr_elements/cr_scrollable_behavior.js
index d0f4b0265ea93def612096690337cea623eef010..df9e5c5bc218109abad7601915a23446668369a9 100644
--- a/ui/webui/resources/cr_elements/cr_scrollable_behavior.js
+++ b/ui/webui/resources/cr_elements/cr_scrollable_behavior.js
@@ -36,10 +36,17 @@
 
 /** @polymerBehavior */
 var CrScrollableBehavior = {
-  properties: {
-    /** @private {number|null} */
-    intervalId_: {type: Number, value: null}
-  },
+
+  /** @private {number|null} */
+  intervalId_: null,
+
+  /**
+   * A dictionary of integer arrays, one entry per scrollable contianer id.
+   * Each array keeps a queue of scroll positions. This ensures that multiple
+   * calls to save/restore scroll do not interfere with each other.
+   * @private {!Object<!Array<number>>}
+   */
+  saveScrollTop_: {},
 
   ready: function() {
     var scrollableElements = this.root.querySelectorAll('[scrollable]');
@@ -90,6 +97,26 @@ var CrScrollableBehavior = {
     }.bind(this), 10);
   },
 
+  /** @param {!IronListElement} list */
+  saveScroll(list) {
+    let target = list.scrollTarget;
+    // Ignore scrollTop of 0 in case it is intermittent (we do not need to
+    // explicity scroll to 0).
+    let scroll = target.scrollTop || -1;
+    this.saveScrollTop_[target.id] = this.saveScrollTop_[target.id] || [];
+    this.saveScrollTop_[target.id].push(scroll);
+  },
+
+  /** @param {!IronListElement} list */
+  restoreScroll(list) {
+    let target = list.scrollTarget;
+    this.async(function() {
+      let scroll = this.saveScrollTop_[target.id].shift();
+      if (scroll != -1)
+        list.scroll(0, scroll);
+    });
+  },
+
   /**
    * Event wrapper for updateScroll_.
    * @param {!Event} event