[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..b43d5e271053c7f5773953cd644851c66238a36c 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);
+
+        // 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();
     }
 }