[New Multicolumn] Actual support for layout of column-span:all.

Source/core/rendering/RenderBox.cpp

Index: Source/core/rendering/RenderBox.cpp
diff --git a/Source/core/rendering/RenderBox.cpp b/Source/core/rendering/RenderBox.cpp
index c7d0398f693495644bb18eba689f214060f9cc47..f9cda51f0481a290320c2987193775a420b1349f 100644
--- a/Source/core/rendering/RenderBox.cpp
+++ b/Source/core/rendering/RenderBox.cpp
@@ -46,6 +46,7 @@
 #include "core/rendering/PaintInfo.h"
 #include "core/rendering/RenderDeprecatedFlexibleBox.h"
 #include "core/rendering/RenderFlexibleBox.h"
+#include "core/rendering/RenderFlowThread.h"
 #include "core/rendering/RenderGeometryMap.h"
 #include "core/rendering/RenderGrid.h"
 #include "core/rendering/RenderInline.h"
@@ -1426,7 +1427,11 @@ LayoutUnit RenderBox::containingBlockLogicalWidthForContent() const
     if (hasOverrideContainingBlockLogicalWidth())
         return overrideContainingBlockContentLogicalWidth();
 
-    RenderBlock* cb = containingBlock();
+    RenderBlock* cb;
+    if (isColumnSpanAll())
+        cb = flowThreadContainingBlock()->containingBlock();
+    else
+        cb = containingBlock();
     return cb->availableLogicalWidth();
 }
 
@@ -1435,7 +1440,11 @@ LayoutUnit RenderBox::containingBlockLogicalHeightForContent(AvailableLogicalHei
     if (hasOverrideContainingBlockLogicalHeight())
         return overrideContainingBlockContentLogicalHeight();
 
-    RenderBlock* cb = containingBlock();
+    RenderBlock* cb;
+    if (isColumnSpanAll())
+        cb = flowThreadContainingBlock()->containingBlock();
+    else
+        cb = containingBlock();
     return cb->availableLogicalHeight(heightType);
 }
 
@@ -4120,7 +4129,11 @@ bool RenderBox::hasUnsplittableScrollingOverflow() const
 
 bool RenderBox::isUnsplittableForPagination() const
 {
-    return isReplaced() || hasUnsplittableScrollingOverflow() || (parent() && isWritingModeRoot());
+    // FIXME: column spanners are only unsplittable (or rather: they do not participate in
+    // fragmentation) in their nearest ancestor multicol container. If there are additional
+    // fragmentation contexts further up in the tree, spanners still need to take those into
+    // account.
+    return isReplaced() || hasUnsplittableScrollingOverflow() || (parent() && isWritingModeRoot()) || isColumnSpanAll();
 }
 
 LayoutUnit RenderBox::lineHeight(bool /*firstLine*/, LineDirectionMode direction, LinePositionMode /*linePositionMode*/) const