Improve recycler APIs used by dropdown

src/mm-item-recycler/mm-item-recycler.js

@@ -68,6 +68,11 @@ found here: https://github.com/Polymer/core-list
 				value: "2d",
 			},
 			_gpuAssignedOnce: String,
+			xScroll: {
+				type: String,
+				value: "all",
+				reflectToAttribute: true,
+			},
 			scope: {
 				type: Object,
 				value: null,
@@ -677,10 +682,13 @@ found here: https://github.com/Polymer/core-list
 
 
 		getHeightAtIndex: function () {
+			var recycled = null;
 			if (this.itemHeight) {
 				return this._recycler.getHeightAtIndex.apply(this._recycler, arguments);
 			} else {
-				return null;
+				// especially relevant when measurements are available before mutation observer fires
+				recycled = Polymer.dom(this.$.middle).querySelector(".recycler-panel");
+				return recycled && recycled.offsetHeight || null;
 			}
 		},
 	});

src/mm-item-recycler/mm-item-recycler.scss

@@ -9,12 +9,18 @@
 
 :host {
 	display: block;
+	overflow: auto;
+}
+
+:host([x-scroll="none"]) #constrainer {
+	max-width: 100%;
 }
 
 #pane {
 	width: 100%;
 	height: 100%;
-	overflow: auto;
+	overflow-x: inherit;
+	overflow-y: inherit;
 }
 
 #constrainer {

src/shared/behaviors/positionable-dropdown.html

@@ -80,9 +80,7 @@
 
 			// if it's an item-recycler, we need to have NO horizontal scrolling
 			if (this.$.itemRecycler) {
-				this.$.itemRecycler.$.constrainer.style.width = panelWidth;
-				this.$.itemRecycler.$.pane.style.overflowY = "auto";
-				this.$.itemRecycler.$.pane.style.overflowX = "hidden";
+				this.$.itemRecycler.xScroll = "none";
 			}
 
 			panelRect.toCSS(this.panel,true);