Added showing slow scroll rectangles in Layers panel.

Source/devtools/front_end/Layers3DView.js

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 32 matching lines @@
     this._model.addEventListener(WebInspector.LayerTreeModel.Events.LayerPainted, this._onLayerPainted, this);
     this._rotatingContainerElement = this.element.createChild("div", "fill rotating-container");
     this.element.addEventListener("mousemove", this._onMouseMove.bind(this), false);
     this.element.addEventListener("mouseout", this._onMouseMove.bind(this), false);
     this.element.addEventListener("mousedown", this._onMouseDown.bind(this), false);
     this.element.addEventListener("mouseup", this._onMouseUp.bind(this), false);
     this.element.addEventListener("contextmenu", this._onContextMenu.bind(this), false);
     this.element.addEventListener("dblclick", this._onDoubleClick.bind(this), false);
     this.element.addEventListener("click", this._onClick.bind(this), false);
     this._elementsByLayerId = {};
+    this._scrollRectElementsByLayerId = {}
     this._rotateX = 0;
     this._rotateY = 0;
     this._scaleAdjustmentStylesheet = this.element.ownerDocument.head.createChild("style");
     this._scaleAdjustmentStylesheet.disabled = true;
     this._lastOutlinedElement = {};
     this._layerImage = document.createElement("img");
     WebInspector.settings.showPaintRects.addChangeListener(this._update, this);
 }
 
 /**
@@ skipping 12 matching lines @@
     LayerSelected: "LayerSelected",
     LayerSnapshotRequested: "LayerSnapshotRequested"
 }
 
 WebInspector.Layers3DView.PaintRectColors = [
     WebInspector.Color.fromRGBA([0, 0x5F, 0, 0x3F]),
     WebInspector.Color.fromRGBA([0, 0xAF, 0, 0x3F]),
     WebInspector.Color.fromRGBA([0, 0xFF, 0, 0x3F])
 ]
 
+/**
+ * @enum {string}
+ */
+WebInspector.Layers3DView.ScrollRectTitles = {
+    NonFastScrollable: "repaints on scroll",

[caseq, 2014/02/20 09:34:06]

Use WebInspector.UIString()

+    TouchEventHandler: "touch event listener",
+    WheelEventHandler: "mousewheel event listener"
+}
+
 WebInspector.Layers3DView.prototype = {
     onResize: function()
     {
         this._update();
     },
 
     willHide: function()
     {
         this._scaleAdjustmentStylesheet.disabled = true;
     },
@@ skipping 109 matching lines @@
         this._clientWidth = this.element.clientWidth;
         this._clientHeight = this.element.clientHeight;
         for (var layerId in this._elementsByLayerId) {
             if (this._model.layerById(layerId))
                 continue;
             this._elementsByLayerId[layerId].remove();
             delete this._elementsByLayerId[layerId];
         }
         this._scaleToFit();
         this._model.forEachLayer(updateLayer.bind(this), this._model.contentRoot());
+        this._paintScrollRects();
         this._needsUpdate = false;
     },
 
     /**
      * @param {!WebInspector.Event} event
      */
     _onLayerPainted: function(event)
     {
         var layer = /** @type {!WebInspector.Layer} */ (event.data);
         this._updatePaintRect(this._elementForLayer(layer));
@@ skipping 53 matching lines @@
             style.webkitTransform = "";
             style.webkitTransformOrigin = "";
         }
 
         function toFixed5(x)
         {
             return x.toFixed(5);
         }
     },
 
+    _createScrollRectElement: function(scrollRect)
+    {
+        var element = document.createElement("div");
+        var style = element.style;
+        var parentLayerElement = this._model._layersById[scrollRect.layerId].nodeId() ? this._elementsByLayerId[scrollRect.layerId] :
+            this._elementsByLayerId[this._model.contentRoot().id()];
+        element.className = "scroll-rect";
+        element.title = WebInspector.Layers3DView.ScrollRectTitles[scrollRect.type];
+        style.width = scrollRect.width + "px";
+        style.height = scrollRect.height + "px";
+        style.left = scrollRect.x + "px";
+        style.top = scrollRect.y + "px";
+        var color = WebInspector.Color.fromRGBA([0xB2, 0, 0, 0.2]);

[caseq, 2014/02/20 09:34:06]

Do this as part of scroll-rect CSS class instead.

+        style.backgroundColor = color.toString(WebInspector.Color.Format.RGBA) || "";
+        color = WebInspector.Color.fromRGBA([0xB2, 0, 0, 1]);
+        style.borderColor = color.toString(WebInspector.Color.Format.RGBA) || "";
+        style.borderWidth = "1px";
+        parentLayerElement.appendChild(element);
+        element._scrollRect = scrollRect;
+        return element;
+    },
+
+    _paintScrollRects: function()
+    {
+        var i, layerId;
+        for (layerId in this._scrollRectElementsByLayerId) {
+            if (!this._model._scrollRectsByLayerId[layerId]) {
+                for (i = 0; i < this._scrollRectElementsByLayerId[layerId].length; ++i) {
+                    this._scrollRectElementsByLayerId[layerId][i].remove();
+                }
+                delete this._scrollRectElementsByLayerId[layerId];
+            }
+        }
+        for (layerId in this._model._scrollRectsByLayerId) {
+            if (!this._scrollRectElementsByLayerId[layerId])
+                this._scrollRectElementsByLayerId[layerId] = [];
+            for (i = 0; i < this._model._scrollRectsByLayerId[layerId].length; ++i) {
+                if (i >= this._scrollRectElementsByLayerId[layerId].length) {
+                    this._scrollRectElementsByLayerId[layerId].push(
+                        this._createScrollRectElement(this._model._scrollRectsByLayerId[layerId][i]));
+                } else if (this._model._scrollRectsByLayerId[layerId][i] != this._scrollRectElementsByLayerId[layerId][i]) {
+                    this._scrollRectElementsByLayerId[layerId][i].remove();
+                    this._scrollRectElementsByLayerId[layerId][i] =
+                        this._createScrollRectElement(this._model._scrollRectsByLayerId[layerId][i]);
+                }
+            }
+            if (this._scrollRectElementsByLayerId[layerId].length > this._model._scrollRectsByLayerId[layerId].length) {

[caseq, 2014/02/20 09:34:06]

No need for this if.

+                for (i = this._model._scrollRectsByLayerId[layerId].length; i < this._scrollRectElementsByLayerId[layerId].length; ++i)
+                    this._scrollRectElementsByLayerId[layerId][i].remove();
+                this._scrollRectElementsByLayerId[layerId].splice(this._model._scrollRectsByLayerId[layerId].length,

[caseq, 2014/02/20 09:34:06]

How about 
this._scrollRectElementsByLayerId[layerId].splice(this._model._scrollRectsByLayerId[layerId].length).forEach(removeElement)

function removeElement(element)
{
    element.remove()
}

+                    this._scrollRectElementsByLayerId[layerId].length - this._model._scrollRectsByLayerId[layerId].length);
+            }
+        }
+    },
+
     _updatePaintRect: function(element)
     {
         var details = element.__layerDetails;
         var paintRect = details.layer.lastPaintRect();
         var paintRectElement = details.paintRectElement;
         if (!paintRect || !WebInspector.settings.showPaintRects.get()) {
             paintRectElement.classList.add("hidden");
             return;
         }
         paintRectElement.classList.remove("hidden");
@@ skipping 143 matching lines @@
  * @param {!WebInspector.Layer} layer
  * @param {!Element} paintRectElement
  */
 WebInspector.LayerDetails = function(layer, paintRectElement)
 {
     this.layer = layer;
     this.depth = 0;
     this.paintRectElement = paintRectElement;
     this.paintCount = 0;
 }