Bug 1306107 - Stop calling ProjectTo2D() for leaf basic layers. r=mattwoodrow

gfx/layers/Layers.cpp

@@ -604,14 +604,13 @@ Layer::SnapTransformTranslation(const Matrix4x4& aTransform,
   }
 
   Matrix matrix2D;
-  Matrix4x4 result;
   if (aTransform.CanDraw2D(&matrix2D) &&
       !matrix2D.HasNonTranslation() &&
       matrix2D.HasNonIntegerTranslation()) {
     auto snappedTranslation = IntPoint::Round(matrix2D.GetTranslation());
     Matrix snappedMatrix = Matrix::Translation(snappedTranslation.x,
                                                snappedTranslation.y);
-    result = Matrix4x4::From2D(snappedMatrix);
+    Matrix4x4 result = Matrix4x4::From2D(snappedMatrix);
     if (aResidualTransform) {
       // set aResidualTransform so that aResidual * snappedMatrix == matrix2D.
       // (I.e., appying snappedMatrix after aResidualTransform gives the
@@ -623,6 +622,13 @@ Layer::SnapTransformTranslation(const Matrix4x4& aTransform,
     return result;
   }
 
+  return SnapTransformTranslation3D(aTransform, aResidualTransform);
+}
+
+Matrix4x4
+Layer::SnapTransformTranslation3D(const Matrix4x4& aTransform,
+                                  Matrix* aResidualTransform)
+{
   if(aTransform.IsSingular() ||
      aTransform.HasPerspectiveComponent() ||
      aTransform.HasNonTranslation() ||
@@ -672,7 +678,7 @@ Layer::SnapTransformTranslation(const Matrix4x4& aTransform,
   // Translate transformed origin to transformed snap since the
   // residual transform would trnslate the snap to the origin.
   Point3D transformedShift = transformedSnap - transformedOrigin;
-  result = aTransform;
+  Matrix4x4 result = aTransform;
   result.PostTranslate(transformedShift.x,
                        transformedShift.y,
                        transformedShift.z);

gfx/layers/Layers.h

@@ -1821,6 +1821,8 @@ protected:
    */
   gfx::Matrix4x4 SnapTransformTranslation(const gfx::Matrix4x4& aTransform,
                                           gfx::Matrix* aResidualTransform);
+  gfx::Matrix4x4 SnapTransformTranslation3D(const gfx::Matrix4x4& aTransform,
+                                            gfx::Matrix* aResidualTransform);
   /**
    * See comment for SnapTransformTranslation.
    * This function implements type 2 snapping. If aTransform is a translation

gfx/layers/basic/BasicContainerLayer.cpp

@@ -37,18 +37,23 @@ BasicContainerLayer::ComputeEffectiveTransforms(const Matrix4x4& aTransformToSur
   // are aligned in device space, so it doesn't really matter how we snap
   // containers.
   Matrix residual;
-  Matrix4x4 idealTransform = GetLocalTransform() * aTransformToSurface;
-  if (!Extend3DContext() && !Is3DContextLeaf()) {
-    // For 3D transform leaked from extended parent layer.
+  Matrix4x4 transformToSurface = aTransformToSurface;
+  bool participate3DCtx = Extend3DContext() || Is3DContextLeaf();
+  if (!participate3DCtx &&
+      GetContentFlags() & CONTENT_BACKFACE_HIDDEN) {
+    // For backface-hidden layers
+    transformToSurface.ProjectTo2D();
+  }
+  Matrix4x4 idealTransform = GetLocalTransform() * transformToSurface;
+  if (!participate3DCtx &&
+      !(GetContentFlags() & CONTENT_BACKFACE_HIDDEN)) {
+    // For non-backface-hidden layers,
+    // 3D components are required to handle CONTENT_BACKFACE_HIDDEN.
     idealTransform.ProjectTo2D();
   }
 
   if (!idealTransform.CanDraw2D()) {
-    if (!Extend3DContext() ||
-        (!idealTransform.Is2D() && Creates3DContextWithExtendingChildren())) {
-      if (!Creates3DContextWithExtendingChildren()) {
-        idealTransform.ProjectTo2D();
-      }
+    if (!Extend3DContext()) {
       mEffectiveTransform = idealTransform;
       ComputeEffectiveTransformsForChildren(Matrix4x4());
       ComputeEffectiveTransformForMaskLayers(Matrix4x4());
@@ -84,12 +89,12 @@ BasicContainerLayer::ComputeEffectiveTransforms(const Matrix4x4& aTransformToSur
     (GetMixBlendMode() != CompositionOp::OP_OVER && HasMultipleChildren()) ||
     (GetEffectiveOpacity() != 1.0 && ((HasMultipleChildren() && !Extend3DContext()) || hasSingleBlendingChild));
 
-  if (!Extend3DContext()) {
-    idealTransform.ProjectTo2D();
-  }
   mEffectiveTransform =
     !mUseIntermediateSurface ?
-    idealTransform : SnapTransformTranslation(idealTransform, &residual);
+    idealTransform :
+    (!(GetContentFlags() & CONTENT_BACKFACE_HIDDEN) ?
+     SnapTransformTranslation(idealTransform, &residual) :
+     SnapTransformTranslation3D(idealTransform, &residual));
   Matrix4x4 childTransformToSurface =
     (!mUseIntermediateSurface ||
      (mUseIntermediateSurface && !Extend3DContext() /* 2D */)) ?