[New Multicolumn] Temporarily changing height is scary and can easily be avoided.

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 4f94ffa1c8c1d9ef7fa14ddc6ce582adfd5151c7..53b0b8ecd5357a798c95580acb165a050c544406 100644
--- a/Source/core/rendering/RenderBlockFlow.cpp
+++ b/Source/core/rendering/RenderBlockFlow.cpp
@@ -199,10 +199,11 @@ bool RenderBlockFlow::updateLogicalWidthAndColumnWidth()
 void RenderBlockFlow::checkForPaginationLogicalHeightChange(LayoutUnit& pageLogicalHeight, bool& pageLogicalHeightChanged, bool& hasSpecifiedPageLogicalHeight)
 {
     if (RenderMultiColumnFlowThread* flowThread = multiColumnFlowThread()) {
-        updateLogicalHeight();
-        pageLogicalHeightChanged = contentLogicalHeight() != flowThread->columnHeightAvailable();
-        flowThread->setColumnHeightAvailable(std::max<LayoutUnit>(contentLogicalHeight(), 0));
-        setLogicalHeight(0);
+        LogicalExtentComputedValues computedValues;
+        computeLogicalHeight(LayoutUnit(), logicalTop(), computedValues);
+        LayoutUnit columnHeight = computedValues.m_extent - borderAndPaddingLogicalHeight() - scrollbarLogicalHeight();
+        pageLogicalHeightChanged = columnHeight != flowThread->columnHeightAvailable();
+        flowThread->setColumnHeightAvailable(std::max<LayoutUnit>(columnHeight, 0));
     } else if (hasColumns()) {
         ColumnInfo* colInfo = columnInfo();