Bug 790505, part 2: Add a GetLayerTransformForFrame() helper for computing a frame's layer transform outside a proper transaction, and mark frames that have been fully prerendered. r=roc

layout/base/nsLayoutUtils.cpp

@@ -1312,6 +1312,41 @@ nsLayoutUtils::GetTransformToAncestor(nsIFrame *aFrame, const nsIFrame *aAncesto
   return ctm;
 }
 
+bool
+nsLayoutUtils::GetLayerTransformForFrame(nsIFrame* aFrame,
+                                         gfx3DMatrix* aTransform)
+{
+  // FIXME/bug 796690: we can sometimes compute a transform in these
+  // cases, it just increases complexity considerably.  Punt for now.
+  if (aFrame->Preserves3DChildren() || aFrame->HasTransformGetter()) {
+    return false;
+  }
+
+  nsIFrame* root = nsLayoutUtils::GetDisplayRootFrame(aFrame);
+  if (root->HasAnyStateBits(NS_FRAME_UPDATE_LAYER_TREE)) {
+    // Content may have been invalidated, so we can't reliably compute
+    // the "layer transform" in general.
+    return false;
+  }
+  // If the caller doesn't care about the value, early-return to skip
+  // overhead below.
+  if (!aTransform) {
+    return true;
+  }
+
+  nsDisplayListBuilder builder(root, nsDisplayListBuilder::OTHER,
+                               false/*don't build caret*/);
+  nsDisplayList list;  
+  nsDisplayTransform* item =
+    new (&builder) nsDisplayTransform(&builder, aFrame, &list);
+
+  *aTransform =
+    item->GetTransform(aFrame->PresContext()->AppUnitsPerDevPixel());
+  list.DeleteAll();
+
+  return true;
+}
+
 static gfxPoint
 TransformGfxPointFromAncestor(nsIFrame *aFrame,
                               const gfxPoint &aPoint,

layout/base/nsLayoutUtils.h

@@ -536,6 +536,15 @@ public:
    */
   static gfx3DMatrix GetTransformToAncestor(nsIFrame *aFrame, const nsIFrame *aAncestor);
 
+  /**
+   * Return true if a "layer transform" could be computed for aFrame,
+   * and optionally return the computed transform.  The returned
+   * transform is what would be set on the layer currently if a layers
+   * transaction were opened at the time this helper is called.
+   */
+  static bool GetLayerTransformForFrame(nsIFrame* aFrame,
+                                        gfx3DMatrix* aTransform);
+
   /**
    * Given a point in the global coordinate space, returns that point expressed
    * in the coordinate system of aFrame.  This effectively inverts all transforms

layout/generic/nsIFrame.h

@@ -1271,6 +1271,11 @@ public:
   
   bool HasOpacity() const;
 
+   /**
+   * Return true if this frame might be using a transform getter.
+   */
+  virtual bool HasTransformGetter() const { return false; }
+
   /**
    * Returns true if this frame is an SVG frame that has SVG transforms applied
    * to it, or if its parent frame is an SVG frame that has children-only
@@ -2225,7 +2230,7 @@ public:
    * 
    */
   virtual void InvalidateFrameForRemoval() {}
-  
+
   /**
    * Checks if a frame has had InvalidateFrame() called on it since the
    * last paint.

layout/generic/nsPageContentFrame.h

@@ -42,6 +42,8 @@ public:
                              nsSize aMargin, nsSize aBorder, nsSize aPadding,
                              uint32_t aFlags) MOZ_OVERRIDE;
 
+  virtual bool HasTransformGetter() const MOZ_OVERRIDE { return true; }
+
   /**
    * Get the "type" of the frame
    *

layout/generic/nsSimplePageSequence.h

@@ -91,6 +91,8 @@ public:
   // user's settings
   virtual bool HonorPrintBackgroundSettings() { return false; }
 
+  virtual bool HasTransformGetter() const MOZ_OVERRIDE { return true; }
+
   /**
    * Get the "type" of the frame
    *