Delay item-recycler initialization by a tick

	- improved scrolling performance for nested recyclers
	- publish recycler "measuring" status to property
	- reset recycler transform debounce timing

src/strand-grid/strand-grid.html

@@ -19,7 +19,7 @@
 <dom-module id="strand-grid">
 	<link rel="import" type="css" href="strand-grid.css"/>
 	<template>
-		<strand-item-recycler id="viewport" gpu="{{gpu}}" data="{{data}}" scope="{{scope}}" index="{{index}}" template-finder="{{templateFinder}}" template-findable="{{templateFindable}}" mixin-findable="{{mixinFindable}}">
+		<strand-item-recycler id="viewport" gpu="{{gpu}}" data="{{data}}" scope="{{scope}}" index="{{index}}" template-finder="{{templateFinder}}" template-findable="{{templateFindable}}" mixin-findable="{{mixinFindable}}" measuring="{{_measuring}}">
 			<div id="header">
 				<div class="_mm_column checkbox" hidden$="{{!selectable}}">
 					<strand-checkbox id="selectAll" on-tap="_toggleAllSelections" state="{{_selectAllState}}"></strand-checkbox>
@@ -36,7 +36,7 @@
 				<strand-grid-item model="{{model}}" scope="{{scope}}"></strand-grid-item>
 			</template>
 		</strand-item-recycler>
- 		<template is="dom-if" if="{{isLoading}}">
+		<template is="dom-if" if="{{ _showLoader(isLoading, _measuring) }}">
 			<strand-loader id="loader"><label>Loading...</label></strand-loader>
 		</template>
 		<div id="separator"></div>

src/strand-grid/strand-grid.js

@@ -44,6 +44,9 @@
 				type: Boolean,
 				value: false
 			},
+			_measuring: {
+				type: Boolean,
+			},
 			expanded: {
 				type: Boolean,
 				value: false,
@@ -269,6 +272,10 @@
 			}
 		},
 
+		_showLoader: function (isLoading, _measuring) {
+			return isLoading;
+		},
+
 		requestInitialization: function () {
 			return this.$.viewport.initialize();
 		},

src/strand-item-recycler/strand-item-recycler.js

@@ -52,7 +52,9 @@ found here: https://github.com/Polymer/core-list
 						//Initialize scrollTop to supplied index
 						itemRecycler.async(itemRecycler.scrollToIndex);
 					}
-					itemRecycler.toggleAttribute("measuring", false, itemRecycler.$.middle);
+					if (itemRecycler._initialized) {
+						itemRecycler._setMeasuring(false);
+					}
 				} else if (itemRecycler._itemHeight < 0) {
 					change = itemRecycler._accommodateGlobalHeightAdjustment(0|!initialization, bound, delta);
 					itemRecycler.async(itemRecycler._modifyPadding, 1);
@@ -236,6 +238,13 @@ found here: https://github.com/Polymer/core-list
 					return new Continuum(this._getItemHeight.bind(this));
 				},
 			},
+			measuring: {
+				type: Boolean,
+				value: false,
+				notify: true,
+				readOnly: true,
+				reflectToAttribute: true,
+			},
 			data: {
 				type: Array,
 				value: null,
@@ -552,7 +561,7 @@ found here: https://github.com/Polymer/core-list
 		},
 
 		_needsInitialization: function (data, _templateFound) {
-			this._initializable = false;
+			this._initializable = true;
 			this._initialized = false;
 			this.initialize();
 		},
@@ -563,13 +572,14 @@ found here: https://github.com/Polymer/core-list
 			} else if (!this.hasTemplate()) {
 				return 0|false;
 			} else {
-				this.debounce("initializeRecycler", this.initializeRecycler);
+				this._setMeasuring(true);
+				this.debounce("initializeRecycler", this.initializeRecycler, 1);
 				return 0|true;
 			}
 		},
 
 		initializeRecycler: function() {
-			if (!this._initializable) {
+			if (this._initializable) {
 				if (this._recycler.truncate(0)) {
 					this._scrollTop = 0;
 					this.$.pane.scrollTop = 0;
@@ -580,12 +590,16 @@ found here: https://github.com/Polymer/core-list
 
 				this._measurements.terminate(0);
 
-				this.toggleAttribute("measuring", true, this.$.middle);
-
 				this._initializeViewport();
 
-				this._initializable = true;
+				this._initializable = false;
 				this._initialized = true;
+
+				if (!this.data ||
+					!this.data.length) {
+					this._setMeasuring(false);
+				}
+
 				return 0|true;
 			} else {
 				return 0|false;
@@ -812,7 +826,7 @@ found here: https://github.com/Polymer/core-list
 
 				if (place < this._maxPixels) {
 					this._repositionBound(bound);
-					this.debounce("rebase-transform", this._rebaseTransform, 250000);
+					this.debounce("rebase-transform", this._rebaseTransform, 250);
 				} else {
 					this._rebaseTransform();
 				}

src/strand-item-recycler/strand-item-recycler.scss

@@ -43,12 +43,12 @@
 	position: relative;
 }
 
-#middle[measuring] {
-	visibility: hidden;
-}
-
 .recycler-panel {
 	position: relative;
 }
 
+:host([measuring]) #middle {
+	visibility: hidden;
+}
+