Propagate writing-mode from the first region to the flow thread.

Source/core/rendering/RenderNamedFlowThread.cpp

Index: Source/core/rendering/RenderNamedFlowThread.cpp
diff --git a/Source/core/rendering/RenderNamedFlowThread.cpp b/Source/core/rendering/RenderNamedFlowThread.cpp
index 97774fb63f1fe6afed91b3413df0b8252a5eb954..c52fceb253afcfd6dad95ab460a24ba31898b292 100644
--- a/Source/core/rendering/RenderNamedFlowThread.cpp
+++ b/Source/core/rendering/RenderNamedFlowThread.cpp
@@ -90,6 +90,18 @@ void RenderNamedFlowThread::clearContentNodes()
     m_contentNodes.clear();
 }
 
+void RenderNamedFlowThread::updateWritingMode()
+{
+    if (RenderRegion* firstRegion = m_regionList.first()) {
+        if (style()->writingMode() != firstRegion->style()->writingMode()) {
+            // The first region defines the principal writing mode for the entire flow.
+            RefPtr<RenderStyle> newStyle = RenderStyle::clone(style());
+            newStyle->setWritingMode(firstRegion->style()->writingMode());
+            setStyle(newStyle);
+        }
+    }
+}
+
 RenderObject* RenderNamedFlowThread::nextRendererForNode(Node* node) const
 {
     FlowThreadChildList::const_iterator it = m_flowThreadChildList.begin();
@@ -244,6 +256,9 @@ void RenderNamedFlowThread::addRegionToNamedFlowThread(RenderRegion* renderRegio
 
     renderRegion->setIsValid(true);
     addRegionToList(m_regionList, renderRegion);
+
+    if (m_regionList.first() == renderRegion)
+        updateWritingMode();
 }
 
 void RenderNamedFlowThread::addRegionToThread(RenderRegion* renderRegion)
@@ -283,6 +298,7 @@ void RenderNamedFlowThread::removeRegionFromThread(RenderRegion* renderRegion)
     }
 
     ASSERT(m_regionList.contains(renderRegion));
+    bool wasFirst = m_regionList.first() == renderRegion;
     m_regionList.remove(renderRegion);
 
     if (canBeDestroyed())
@@ -291,10 +307,18 @@ void RenderNamedFlowThread::removeRegionFromThread(RenderRegion* renderRegion)
     // After removing all the regions in the flow the following layout needs to dispatch the regionLayoutUpdate event
     if (m_regionList.isEmpty())
         setDispatchRegionLayoutUpdateEvent(true);
+    else if (wasFirst)
+        updateWritingMode();
 
     invalidateRegions();
 }
 
+void RenderNamedFlowThread::regionChangedWritingMode(RenderRegion* region)
+{
+    if (m_regionList.first() == region)
+        updateWritingMode();
+}
+
 void RenderNamedFlowThread::computeOversetStateForRegions(LayoutUnit oldClientAfterEdge)
 {
     LayoutUnit height = oldClientAfterEdge;