Make sure we set "should flatten transform" correctly.

Source/core/rendering/compositing/CompositedLayerMapping.cpp

Index: Source/core/rendering/compositing/CompositedLayerMapping.cpp
diff --git a/Source/core/rendering/compositing/CompositedLayerMapping.cpp b/Source/core/rendering/compositing/CompositedLayerMapping.cpp
index 6507b1b25cb3aa974c3e452bb08cd770b7864838..ff29959bc8d2e5a4e6ab3c105f4c37a27c0d3836 100644
--- a/Source/core/rendering/compositing/CompositedLayerMapping.cpp
+++ b/Source/core/rendering/compositing/CompositedLayerMapping.cpp
@@ -1096,20 +1096,9 @@ void CompositedLayerMapping::updateChildrenTransform()
     if (GraphicsLayer* childTransformLayer = layerForChildrenTransform()) {
         childTransformLayer->setTransform(owningLayer().perspectiveTransform());
         childTransformLayer->setTransformOrigin(FloatPoint3D(childTransformLayer->size().width() * 0.5f, childTransformLayer->size().height() * 0.5f, 0.f));
-        bool hasPerspective = false;
-        if (RenderStyle* style = m_owningLayer.renderer()->style())
-            hasPerspective = style->hasPerspective();
-        if (hasPerspective)
-            childTransformLayer->setShouldFlattenTransform(false);
-
-        // Note, if the target is the scrolling layer, we need to ensure that the
-        // scrolling content layer doesn't flatten the transform. (It would be nice
-        // if we could apply transform to the scrolling content layer, but that's
-        // too late, we need the children transform to be applied _before_ the
-        // scrolling offset.)
-        if (childTransformLayer == m_scrollingLayer.get())
-            m_scrollingContentsLayer->setShouldFlattenTransform(false);
     }
+
+    updateShouldFlattenTransform();
 }
 
 // Return true if the layers changed.
@@ -1154,7 +1143,6 @@ bool CompositedLayerMapping::updateChildTransformLayer(bool needsChildTransformL
         if (!m_childTransformLayer) {
             m_childTransformLayer = createGraphicsLayer(CompositingReasonLayerForPerspective);
             m_childTransformLayer->setDrawsContent(false);
-            m_childTransformLayer->setShouldFlattenTransform(false);
             layersChanged = true;
         }
     } else if (m_childTransformLayer) {
@@ -1163,6 +1151,7 @@ bool CompositedLayerMapping::updateChildTransformLayer(bool needsChildTransformL
         layersChanged = true;
     }
 
+    updateShouldFlattenTransform();

[chrishtr, 2014/07/17 20:07:32]

Is this call needed? Why isn't the other one sufficient?

[Ian Vollick, 2014/07/17 20:30:15]

Good point! I think you're right that this is redundant. I've removed this and
the tests seem to pass (at least the ones that this patch affected).

     return layersChanged;
 }
 
@@ -1280,6 +1269,8 @@ static void ApplyToGraphicsLayers(const CompositedLayerMapping* mapping, const F
         f(mapping->clippingLayer());
     if ((mode & ApplyToCoreLayers) && mapping->scrollingLayer())
         f(mapping->scrollingLayer());
+    if ((mode & ApplyToCoreLayers) && mapping->scrollingBlockSelectionLayer())
+        f(mapping->scrollingBlockSelectionLayer());

[chrishtr, 2014/07/17 20:07:32]

Is this an unrelated-ish change?

[Ian Vollick, 2014/07/17 20:30:15]

It isn't, actually. In updateShouldFlattenTransform there are two phases. The
first updates should-flatten-transform as required by preserves-3d, and the
second for perspective. Neither of these phases was operating on the scrolling
block selection layer, but they needed to -- this is the bug this CL is trying
to address. The first phase applies the functor using ApplyToCoreLayers and the
block selection layer needed to be included in that pass. This is what
ApplyToCoreLayers is for, so it's not going to cause the block selection layer
to be included in other, unexpected operations.

[chrishtr, 2014/07/17 20:38:40]

Great. Thanks for the explanation.

     if (((mode & ApplyToCoreLayers) || (mode & ApplyToContentLayers)) && mapping->scrollingContentsLayer())
         f(mapping->scrollingContentsLayer());
     if (((mode & ApplyToCoreLayers) || (mode & ApplyToContentLayers)) && mapping->foregroundLayer())
@@ -1345,6 +1336,26 @@ void CompositedLayerMapping::updateShouldFlattenTransform()
     UpdateShouldFlattenTransformFunctor functor = { !m_owningLayer.shouldPreserve3D() };
     ApplyToGraphicsLayersMode mode = ApplyToCoreLayers;
     ApplyToGraphicsLayers(this, functor, mode);
+
+    // Note, if we apply perspective, we have to set should flatten differently
+    // so that the transform propagates to child layers correctly.
+    if (GraphicsLayer* childTransformLayer = layerForChildrenTransform()) {
+        bool hasPerspective = false;
+        if (RenderStyle* style = m_owningLayer.renderer()->style())
+            hasPerspective = style->hasPerspective();
+        if (hasPerspective)
+            childTransformLayer->setShouldFlattenTransform(false);
+
+        // Note, if the target is the scrolling layer, we need to ensure that the
+        // scrolling content layer doesn't flatten the transform. (It would be nice
+        // if we could apply transform to the scrolling content layer, but that's
+        // too late, we need the children transform to be applied _before_ the
+        // scrolling offset.)
+        if (childTransformLayer == m_scrollingLayer.get()) {
+            m_scrollingContentsLayer->setShouldFlattenTransform(false);
+            m_scrollingBlockSelectionLayer->setShouldFlattenTransform(false);
+        }
+    }
 }
 
 bool CompositedLayerMapping::updateForegroundLayer(bool needsForegroundLayer)