[New Multicolumn] balancer confused by content with top margins.

Source/core/rendering/RenderBlockFlow.cpp

Index: Source/core/rendering/RenderBlockFlow.cpp
diff --git a/Source/core/rendering/RenderBlockFlow.cpp b/Source/core/rendering/RenderBlockFlow.cpp
index 1e5e7fd046b940073b8555b2fc13061822a561cd..7e1806454ed73e3d3bdf72ce01d3917d374d03d6 100644
--- a/Source/core/rendering/RenderBlockFlow.cpp
+++ b/Source/core/rendering/RenderBlockFlow.cpp
@@ -228,8 +228,20 @@ void RenderBlockFlow::checkForPaginationLogicalHeightChange(LayoutUnit& pageLogi
         if (!hasSpecifiedPageLogicalHeight && !pageLogicalHeight)
             colInfo->clearForcedBreaks();
     } else if (isRenderFlowThread()) {
-        pageLogicalHeight = 1; // This is just a hack to always make sure we have a page logical height.
-        pageLogicalHeightChanged = toRenderFlowThread(this)->pageLogicalSizeChanged();
+        RenderFlowThread* flowThread = toRenderFlowThread(this);
+
+        // FIXME: This is a hack to always make sure we have a page logical height, if said height
+        // is known. The page logical height thing in LayoutState is meaningless for flow
+        // thread-based pagination (page height isn't necessarily uniform throughout the flow
+        // thread), but as long as it is used universally as a means to determine whether page
+        // height is known or not, we need this. Page height is unknown when column balancing is
+        // enabled and flow thread height is still unknown (i.e. during the first layout pass). When
+        // it's unknown, we need to prevent the pagination code from assuming page breaks everywhere
+        // and thereby eating every top margin. It should be trivial to clean up and get rid of this
+        // hack once the old multicol implementation is gone.
+        pageLogicalHeight = flowThread->isPageLogicalHeightKnown() ? LayoutUnit(1) : LayoutUnit(0);
+
+        pageLogicalHeightChanged = flowThread->pageLogicalSizeChanged();
     }
 }