[New Multicolumn] Minimal support for nested multicol in RenderLayer.

Source/core/rendering/RenderLayer.cpp

Index: Source/core/rendering/RenderLayer.cpp
diff --git a/Source/core/rendering/RenderLayer.cpp b/Source/core/rendering/RenderLayer.cpp
index 4740c6fccc81bdec03aaafa894272e3e713b7de6..4756d0eb073c4adb257518acd784b90b04cd8176 100644
--- a/Source/core/rendering/RenderLayer.cpp
+++ b/Source/core/rendering/RenderLayer.cpp
@@ -453,15 +453,12 @@ static void convertFromFlowThreadToVisualBoundingBoxInAncestor(const RenderLayer
     rect = flowThread->fragmentsBoundingBox(rect);
 
     // Finally, make the visual rectangle relative to |ancestorLayer|.
-    // FIXME: Handle nested fragmentation contexts (crbug.com/423076). For now just give up if there
-    // are different pagination layers involved.
-    if (!ancestorLayer->enclosingPaginationLayer() || ancestorLayer->enclosingPaginationLayer() != paginationLayer) {
-        // The easy case. The ancestor layer is not within the pagination layer.
-        paginationLayer->convertToLayerCoords(ancestorLayer, rect);
+    if (ancestorLayer->enclosingPaginationLayer() != paginationLayer) {
+        rect.moveBy(paginationLayer->visualOffsetFromAncestor(ancestorLayer));
         return;
     }
-    // The ancestor layer is also inside the pagination layer, so we need to subtract the visual
-    // distance from the ancestor layer to the pagination layer.
+    // The ancestor layer is inside the same pagination layer as |layer|, so we need to subtract
+    // the visual distance from the ancestor layer to the pagination layer.
     rect.moveBy(-ancestorLayer->visualOffsetFromAncestor(paginationLayer));
 }
 
@@ -1453,9 +1450,13 @@ void RenderLayer::convertToLayerCoords(const RenderLayer* ancestorLayer, LayoutR
 
 LayoutPoint RenderLayer::visualOffsetFromAncestor(const RenderLayer* ancestorLayer) const
 {
-    RenderLayer* paginationLayer = enclosingPaginationLayer();
     LayoutPoint offset;
-    if (!paginationLayer || paginationLayer == this) {
+    if (ancestorLayer == this)

[chrishtr, 2015/01/07 01:06:08]

Why is this special case needed?

[mstensho (USE GERRIT), 2015/01/07 09:41:47]

In case someone tries to find a layer's offset relative to itself.

Similar stuff in RenderLayer::convertToLayerCoords(). Asserting instead would
cause a lot of failures. Here, for example:

#0  0x0000000002e3a532 in blink::RenderLayer::visualOffsetFromAncestor
(this=0x1330d80103d0, ancestorLayer=0x1330d80103d0) at
../../third_party/WebKit/Source/core/rendering/RenderLayer.cpp:1454
#1  0x0000000002e3f226 in
blink::convertFromFlowThreadToVisualBoundingBoxInAncestor (layer=0x1330d80103d0,
ancestorLayer=0x1330d80103d0, rect=...) at
../../third_party/WebKit/Source/core/rendering/RenderLayer.cpp:462
#2  0x0000000002e396e8 in blink::RenderLayer::fragmentsBoundingBox
(this=0x1330d80103d0, ancestorLayer=0x1330d80103d0) at
../../third_party/WebKit/Source/core/rendering/RenderLayer.cpp:2324
#3  0x0000000002e3f2a2 in
blink::RenderLayer::boundingBoxForCompositingOverlapTest (this=0x1330d80103d0)
at ../../third_party/WebKit/Source/core/rendering/RenderLayer.cpp:2330
#4  0x0000000002f60608 in blink::CompositingInputsUpdater::updateRecursive
(this=0x7fffdc21fb98, layer=0x1330d80103d0,
updateType=blink::CompositingInputsUpdater::ForceUpdate, info=...) at
../../third_party/WebKit/Source/core/rendering/compositing/CompositingInputsUpdater.cpp:112

boundingBoxForCompositingOverlapTest() calls fragmentsBoundingBox(this) (i.e. it
wants the bounding box relatively to itself), and
convertFromFlowThreadToVisualBoundingBoxInAncestor() doesn't bother to check if
|layer| and |ancestorLayer| are the same (and I don't think it should need to,
either).

+        return offset;
+    RenderLayer* paginationLayer = enclosingPaginationLayer();
+    if (paginationLayer == this)
+        paginationLayer = parent()->enclosingPaginationLayer();

[chrishtr, 2015/01/07 01:06:08]

Only two levels of nesting are supported?

[mstensho (USE GERRIT), 2015/01/07 09:41:47]

No, this method is called recursively. :)

+    if (!paginationLayer) {
         convertToLayerCoords(ancestorLayer, offset);
         return offset;
     }
@@ -1466,10 +1467,7 @@ LayoutPoint RenderLayer::visualOffsetFromAncestor(const RenderLayer* ancestorLay
     if (ancestorLayer == paginationLayer)
         return offset;
 
-    // FIXME: Handle nested fragmentation contexts (crbug.com/423076). For now just give up if there
-    // are different pagination layers involved.
-    if (!ancestorLayer->enclosingPaginationLayer() || ancestorLayer->enclosingPaginationLayer() != paginationLayer) {
-        // The easy case. The ancestor layer is not within the pagination layer.
+    if (ancestorLayer->enclosingPaginationLayer() != paginationLayer) {
         offset.moveBy(paginationLayer->visualOffsetFromAncestor(ancestorLayer));
     } else {
         // The ancestor layer is also inside the pagination layer, so we need to subtract the visual
@@ -1555,16 +1553,13 @@ void RenderLayer::collectFragments(LayerFragments& fragments, const RenderLayer*
 
     // Make the dirty rect relative to the fragmentation context (multicol container, etc.).
     RenderFlowThread* enclosingFlowThread = toRenderFlowThread(enclosingPaginationLayer()->renderer());
-    LayoutPoint offsetOfPaginationLayerFromRoot;
-    // FIXME: more work needed if there are nested pagination layers.
-    if (rootLayer != enclosingPaginationLayer() && rootLayer->enclosingPaginationLayer() == enclosingPaginationLayer()) {
-        // The root layer is inside the fragmentation context. So we need to look inside it and find
-        // the visual offset from the fragmentation context.
-        LayoutPoint flowThreadOffset;
-        rootLayer->convertToLayerCoords(enclosingPaginationLayer(), flowThreadOffset);
-        offsetOfPaginationLayerFromRoot = -enclosingFlowThread->flowThreadPointToVisualPoint(flowThreadOffset);
+    LayoutPoint offsetOfPaginationLayerFromRoot; // Visual offset from the root layer to the nearest fragmentation context.
+    if (rootLayer->enclosingPaginationLayer() == enclosingPaginationLayer()) {
+        // The root layer is in the same fragmentation context as this layer, so we need to look
+        // inside it and subtract the offset between the fragmentation context and the root layer.
+        offsetOfPaginationLayerFromRoot = -rootLayer->visualOffsetFromAncestor(enclosingPaginationLayer());
     } else {
-        enclosingPaginationLayer()->convertToLayerCoords(rootLayer, offsetOfPaginationLayerFromRoot);
+        offsetOfPaginationLayerFromRoot = enclosingPaginationLayer()->visualOffsetFromAncestor(rootLayer);
     }
     LayoutRect dirtyRectInFlowThread(dirtyRect);
     dirtyRectInFlowThread.moveBy(-offsetOfPaginationLayerFromRoot);