[New Multicolumn] Render transformed layers into multiple fragments.

Source/core/paint/LayerPainter.cpp

Index: Source/core/paint/LayerPainter.cpp
diff --git a/Source/core/paint/LayerPainter.cpp b/Source/core/paint/LayerPainter.cpp
index 82eb2a148b5e9c95d2d8efdf06e617cb8f66351e..05b6492df76316b2b7cfaa6481e58f8dcf890b2a 100644
--- a/Source/core/paint/LayerPainter.cpp
+++ b/Source/core/paint/LayerPainter.cpp
@@ -85,7 +85,7 @@ void LayerPainter::paintLayer(GraphicsContext* context, const LayerPaintingInfo&
     paintLayerContentsAndReflection(context, paintingInfo, paintFlags);
 }
 
-void LayerPainter::paintLayerContentsAndReflection(GraphicsContext* context, const LayerPaintingInfo& paintingInfo, PaintLayerFlags paintFlags)
+void LayerPainter::paintLayerContentsAndReflection(GraphicsContext* context, const LayerPaintingInfo& paintingInfo, PaintLayerFlags paintFlags, FragmentPolicy fragmentPolicy)
 {
     ASSERT(m_renderLayer.isSelfPaintingLayer() || m_renderLayer.hasSelfPaintingLayerDescendant());
 
@@ -96,7 +96,7 @@ void LayerPainter::paintLayerContentsAndReflection(GraphicsContext* context, con
         m_renderLayer.reflectionInfo()->paint(context, paintingInfo, localPaintFlags | PaintLayerPaintingReflection);
 
     localPaintFlags |= PaintLayerPaintingCompositingAllPhases;
-    paintLayerContents(context, paintingInfo, localPaintFlags);
+    paintLayerContents(context, paintingInfo, localPaintFlags, fragmentPolicy);
 }
 
 class ClipPathHelper {
@@ -165,7 +165,7 @@ private:
     GraphicsContext* m_context;
 };
 
-void LayerPainter::paintLayerContents(GraphicsContext* context, const LayerPaintingInfo& paintingInfo, PaintLayerFlags paintFlags)
+void LayerPainter::paintLayerContents(GraphicsContext* context, const LayerPaintingInfo& paintingInfo, PaintLayerFlags paintFlags, FragmentPolicy fragmentPolicy)
 {
     ASSERT(m_renderLayer.isSelfPaintingLayer() || m_renderLayer.hasSelfPaintingLayerDescendant());
     ASSERT(!(paintFlags & PaintLayerAppliedTransform));
@@ -226,9 +226,12 @@ void LayerPainter::paintLayerContents(GraphicsContext* context, const LayerPaint
     LayerFragments layerFragments;
     if (shouldPaintContent || shouldPaintOutline || isPaintingOverlayScrollbars) {
         // Collect the fragments. This will compute the clip rectangles and paint offsets for each layer fragment.
-        m_renderLayer.collectFragments(layerFragments, localPaintingInfo.rootLayer, localPaintingInfo.paintDirtyRect,
-            (paintFlags & PaintLayerUncachedClipRects) ? UncachedClipRects : PaintingClipRects, IgnoreOverlayScrollbarSize,
-            shouldRespectOverflowClip(paintFlags, m_renderLayer.renderer()), &offsetFromRoot, localPaintingInfo.subPixelAccumulation);
+        ClipRectsCacheSlot cacheSlot = (paintFlags & PaintLayerUncachedClipRects) ? UncachedClipRects : PaintingClipRects;
+        ShouldRespectOverflowClip respectOverflowClip = shouldRespectOverflowClip(paintFlags, m_renderLayer.renderer());
+        if (fragmentPolicy == ForceSingleFragment)
+            m_renderLayer.appendSingleFragmentIgnoringPagination(layerFragments, localPaintingInfo.rootLayer, localPaintingInfo.paintDirtyRect, cacheSlot, IgnoreOverlayScrollbarSize, respectOverflowClip, &offsetFromRoot, localPaintingInfo.subPixelAccumulation);
+        else
+            m_renderLayer.collectFragments(layerFragments, localPaintingInfo.rootLayer, localPaintingInfo.paintDirtyRect, cacheSlot, IgnoreOverlayScrollbarSize, respectOverflowClip, &offsetFromRoot, localPaintingInfo.subPixelAccumulation);
         if (shouldPaintContent)
             shouldPaintContent = atLeastOneFragmentIntersectsDamageRect(layerFragments, localPaintingInfo, paintFlags, offsetFromRoot);
     }
@@ -396,7 +399,7 @@ void LayerPainter::paintFragmentByApplyingTransform(GraphicsContext* context, co
     // Now do a paint with the root layer shifted to be us.
     LayerPaintingInfo transformedPaintingInfo(&m_renderLayer, enclosingIntRect(transform.inverse().mapRect(paintingInfo.paintDirtyRect)), paintingInfo.paintBehavior,
         adjustedSubPixelAccumulation, paintingInfo.paintingRoot);
-    paintLayerContentsAndReflection(context, transformedPaintingInfo, paintFlags);
+    paintLayerContentsAndReflection(context, transformedPaintingInfo, paintFlags, ForceSingleFragment);

[chrishtr, 2015/01/15 23:33:45]

What I mean is that this call stack appears to be present:

paintLayerWithTransform
  -> paintFragmentByApplyingTransform
    -> paintLayerContentsAndReflection
      -> paintLayerContents

paintLayerContents then either collects fragments or collects one fragment based
on the fragment forcing enum parameter.
But paintLayerWithTransform already collected the fragments and is iterating
over them. Why shouldn't the code just pass down the fragment and avoid
recomputing it?

[mstensho (USE GERRIT), 2015/02/03 13:55:01]

paintLayerWithTransform() collects fragment(s) for the pagination layer (the
flow thread), to set up column clipping and column translation (and to figure
out which columns we need to visit). paintLayerContents(), on the other hand,
collects fragment(s) for the layer itself (to set up clipping established by the
layer itself (overflow:hidden, for instance).

I really believe this needs to be done in separate steps. Column clipping needs
to be set before transforming (because the transform is somewhere *inside* the
column contents, and the columns shouldn't be affected by the transform), and
layer clipping is set after transforming the layer.

We could certainly calculate the layer fragment in
paintFragmentByApplyingTransform() and pass it down to paintLayerContents(),
instead of saying ForceSingleFragment and deal with it when we get there, but I
really don't think that would be cleaner. That would mean yet another place
where we "collect" fragments.

 }
 
 void LayerPainter::paintChildren(unsigned childrenToVisit, GraphicsContext* context, const LayerPaintingInfo& paintingInfo, PaintLayerFlags paintFlags)