Handle percent height children of multicol containers correctly.

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 63da47dd6c38ae0151e61c9aaa53724cb3e6ac3a..afb056ec43ddb131de1f56d9e310582633833852 100644
--- a/Source/core/rendering/RenderBox.cpp
+++ b/Source/core/rendering/RenderBox.cpp
@@ -2687,6 +2687,11 @@ LayoutUnit RenderBox::computeContentAndScrollbarLogicalHeightUsing(const Length&
 
 bool RenderBox::skipContainingBlockForPercentHeightCalculation(const RenderBox* containingBlock) const
 {
+    // Flow threads for multicol or paged overflow should be skipped. They are invisible to the DOM,
+    // and percent heights of children should be resolved against the multicol or paged container.
+    if (containingBlock->isRenderFlowThread())
+        return true;
+
     // For quirks mode and anonymous blocks, we skip auto-height containingBlocks when computing percentages.
     // For standards mode, we treat the percentage as auto if it has an auto-height containing block.
     if (!document().inQuirksMode() && !containingBlock->isAnonymousBlock())