Added showing slow scroll rectangles in Layers panel.

Source/devtools/front_end/Layers3DView.js

Index: Source/devtools/front_end/Layers3DView.js
diff --git a/Source/devtools/front_end/Layers3DView.js b/Source/devtools/front_end/Layers3DView.js
index 0ba03ccf3652e9198f5530eba4362e9a849770e6..3028c747ad91c12c26f3149e6874b7f0dc1b1002 100644
--- a/Source/devtools/front_end/Layers3DView.js
+++ b/Source/devtools/front_end/Layers3DView.js
@@ -50,6 +50,7 @@ WebInspector.Layers3DView = function(model)
     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");
@@ -82,6 +83,15 @@ WebInspector.Layers3DView.PaintRectColors = [
     WebInspector.Color.fromRGBA([0, 0xFF, 0, 0x3F])
 ]
 
+/**
+ * @enum {string}
+ */
+WebInspector.Layers3DView.ScrollRectTitles = {
+    RepaintsOnScroll: WebInspector.UIString("repaints on scroll"),
+    TouchEventHandler: WebInspector.UIString("touch event listener"),
+    WheelEventHandler: WebInspector.UIString("mousewheel event listener")
+}
+
 WebInspector.Layers3DView.prototype = {
     onResize: function()
     {
@@ -181,6 +191,65 @@ WebInspector.Layers3DView.prototype = {
         element.style.top = ((this._clientHeight - root.height() * this._scale) >> 1) + "px";
     },
 
+    /**
+     * @param {!Element} element
+     */
+    _removeElement: function(element)
+    {
+        element.remove()
+    },
+
+    /**
+     * @param {!string} layerId
+     */
+    _removeScrollRects: function(layerId)
+    {
+        this._scrollRectElementsByLayerId[layerId].forEach(this._removeElement);
+        delete this._scrollRectElementsByLayerId[layerId];
+    },
+
+    /**
+     * @param {!LayerTreeAgent.ScrollRect} scrollRect
+     * @param {!WebInspector.Layer} layer
+     * @return {!Element}
+     */
+    _createScrollRectElement: function(scrollRect, layer)
+    {
+        var element = document.createElement("div");
+        var style = element.style;
+        var parentLayerId = layer.nodeId() ? layer.id() : this._model.contentRoot().id();
+        var parentLayerElement = this._elementsByLayerId[parentLayerId];
+        element.className = "scroll-rect";
+        element.title = WebInspector.Layers3DView.ScrollRectTitles[scrollRect.type];
+        style.width = scrollRect.rect.width + "px";
+        style.height = scrollRect.rect.height + "px";
+        style.left = scrollRect.rect.x + "px";
+        style.top = scrollRect.rect.y + "px";
+        parentLayerElement.appendChild(element);
+        element.__scrollRect = scrollRect;
+        return element;
+    },
+
+    /**
+     * @param {!WebInspector.Layer} layer
+     */
+     _updateScrollRectsForLayer: function(layer)
+     {
+        if (!this._scrollRectElementsByLayerId[layer.id()])
+            this._scrollRectElementsByLayerId[layer.id()] = [];
+        var newScrollRects = layer.scrollRects(),
+            scrollRectElements = this._scrollRectElementsByLayerId[layer.id()];
+        for (var i = 0; i < newScrollRects.length; ++i) {
+            if (i >= scrollRectElements.length) {
+                scrollRectElements.push(this._createScrollRectElement(newScrollRects[i], layer));
+            } else if (newScrollRects[i] != scrollRectElements[i].__scrollRect) {
+                scrollRectElements[i].remove();
+                scrollRectElements[i] = this._createScrollRectElement(newScrollRects[i], layer);
+            }
+        }
+        scrollRectElements.splice(newScrollRects.length).forEach(this._removeElement);
+    },
+
     _update: function()
     {
         if (!this.isShowing()) {
@@ -208,9 +277,11 @@ WebInspector.Layers3DView.prototype = {
                 continue;
             this._elementsByLayerId[layerId].remove();
             delete this._elementsByLayerId[layerId];
+            this._removeScrollRects(layerId);
         }
         this._scaleToFit();
         this._model.forEachLayer(updateLayer.bind(this), this._model.contentRoot());
+        this._model.forEachLayer(this._updateScrollRectsForLayer.bind(this));
         this._needsUpdate = false;
     },
 
@@ -284,6 +355,9 @@ WebInspector.Layers3DView.prototype = {
         }
     },
 
+    /**
+     * @param {!Element} element
+     */
     _updatePaintRect: function(element)
     {
         var details = element.__layerDetails;