Make squashing work with subpixel layout

Source/core/rendering/compositing/RenderLayerCompositor.cpp

Index: Source/core/rendering/compositing/RenderLayerCompositor.cpp
diff --git a/Source/core/rendering/compositing/RenderLayerCompositor.cpp b/Source/core/rendering/compositing/RenderLayerCompositor.cpp
index 1ec3750969ebc2b8613ca5c6ebce60d59bc70495..ea4001085d4de854c20d724e8b11ec251c2f53f2 100644
--- a/Source/core/rendering/compositing/RenderLayerCompositor.cpp
+++ b/Source/core/rendering/compositing/RenderLayerCompositor.cpp
@@ -648,12 +648,12 @@ bool RenderLayerCompositor::allocateOrClearCompositedLayerMapping(RenderLayer* l
     return compositedLayerMappingChanged || nonCompositedReasonChanged;
 }
 
-static IntPoint computeOffsetFromAbsolute(RenderLayer* layer)
+static LayoutPoint computeOffsetFromAbsolute(RenderLayer* layer)
 {
     TransformState transformState(TransformState::ApplyTransformDirection, FloatPoint());
     layer->renderer()->mapLocalToContainer(0, transformState, ApplyContainerFlip);
     transformState.flatten();
-    return roundedIntPoint(transformState.lastPlanarPoint());
+    return roundedLayoutPoint(transformState.lastPlanarPoint());

[leviw_travelin_and_unemployed, 2014/03/18 22:30:48]

roundedLayoutPoint is actually a no-op and should go away (it's from the
sub-pixel transition). You probably just want to construct a LayoutPoint from
the FloatPoint.

[ajuma, 2014/03/19 19:26:43]

Done.

 }
 
 bool RenderLayerCompositor::updateSquashingAssignment(RenderLayer* layer, SquashingState& squashingState, const CompositingStateTransitionType compositedLayerUpdate)
@@ -668,9 +668,9 @@ bool RenderLayerCompositor::updateSquashingAssignment(RenderLayer* layer, Squash
         ASSERT(!layer->hasCompositedLayerMapping());
         ASSERT(squashingState.hasMostRecentMapping);
 
-        IntPoint offsetFromAbsoluteForSquashedLayer = computeOffsetFromAbsolute(layer);
+        LayoutPoint offsetFromAbsoluteForSquashedLayer = computeOffsetFromAbsolute(layer);
 
-        IntSize offsetFromSquashingCLM(offsetFromAbsoluteForSquashedLayer.x() - squashingState.offsetFromAbsoluteForSquashingCLM.x(),
+        LayoutSize offsetFromSquashingCLM(offsetFromAbsoluteForSquashedLayer.x() - squashingState.offsetFromAbsoluteForSquashingCLM.x(),

[leviw_travelin_and_unemployed, 2014/03/18 22:30:48]

I don't believe this will be guaranteed to be the same sub-pixel offset as
what's currently derived (from its composited ancestor). Either way, it's
important to add an assert that the .fraction() of this offset is the same as
the .fraction() we determine in
CompositedLayerMapping::adjustBoundsForSubPixelAccumulation. (Be aware, I'm in
the process of landing a tweak to that logic in
https://codereview.chromium.org/204053002/).

If you add the assert and validate that we don't hit it, I'm on board with the
rest of this change :)

[ajuma, 2014/03/19 19:26:43]

I've added the assert to CompositedLayerMapping::updateSquashingLayerGeometry. I
ran all the layout tests locally with squashing turned on, and verified that
this assert doesn't get hit.

             offsetFromAbsoluteForSquashedLayer.y() - squashingState.offsetFromAbsoluteForSquashingCLM.y());
 
         bool changedSquashingLayer =
@@ -1140,7 +1140,7 @@ void RenderLayerCompositor::computeCompositingRequirements(RenderLayer* ancestor
         overlapMap->geometryMap().popMappingsToAncestor(ancestorLayer);
 }
 
-void RenderLayerCompositor::SquashingState::updateSquashingStateForNewMapping(CompositedLayerMappingPtr newCompositedLayerMapping, bool hasNewCompositedLayerMapping, IntPoint newOffsetFromAbsoluteForSquashingCLM, RenderLayer* newClippingAncestorForMostRecentMapping)
+void RenderLayerCompositor::SquashingState::updateSquashingStateForNewMapping(CompositedLayerMappingPtr newCompositedLayerMapping, bool hasNewCompositedLayerMapping, LayoutPoint newOffsetFromAbsoluteForSquashingCLM, RenderLayer* newClippingAncestorForMostRecentMapping)
 {
     // The most recent backing is done accumulating any more squashing layers.
     if (hasMostRecentMapping)
@@ -1207,7 +1207,7 @@ void RenderLayerCompositor::assignLayersToBackingsInternal(RenderLayer* layer, S
         // At this point, if the layer is to be "separately" composited, then its backing becomes the most recent in paint-order.
         if (layer->compositingState() == PaintsIntoOwnBacking || layer->compositingState() == HasOwnBackingButPaintsIntoAncestor) {
             ASSERT(!requiresSquashing(layer->compositingReasons()));
-            IntPoint offsetFromAbsoluteForSquashingCLM = computeOffsetFromAbsolute(layer);
+            LayoutPoint offsetFromAbsoluteForSquashingCLM = computeOffsetFromAbsolute(layer);
             squashingState.updateSquashingStateForNewMapping(layer->compositedLayerMapping(), layer->hasCompositedLayerMapping(), offsetFromAbsoluteForSquashingCLM, clippingAncestor);
         }
     }