[New Multicolumn] Make positionForPoint() work.

Source/core/layout/LayoutMultiColumnSet.cpp

Index: Source/core/layout/LayoutMultiColumnSet.cpp
diff --git a/Source/core/layout/LayoutMultiColumnSet.cpp b/Source/core/layout/LayoutMultiColumnSet.cpp
index 54f8af4520021e936bdacd86e8d0b160f55c23ce..d8c8f0fe9092b401a37856ee530d11e3bbbcf4aa 100644
--- a/Source/core/layout/LayoutMultiColumnSet.cpp
+++ b/Source/core/layout/LayoutMultiColumnSet.cpp
@@ -26,6 +26,7 @@
 #include "config.h"
 #include "core/layout/LayoutMultiColumnSet.h"
 
+#include "core/editing/PositionWithAffinity.h"
 #include "core/layout/LayoutMultiColumnFlowThread.h"
 #include "core/layout/MultiColumnFragmentainerGroup.h"
 #include "core/paint/MultiColumnSetPainter.h"
@@ -59,6 +60,12 @@ const MultiColumnFragmentainerGroup& LayoutMultiColumnSet::fragmentainerGroupAtF
     return m_fragmentainerGroups.first();
 }
 
+const MultiColumnFragmentainerGroup& LayoutMultiColumnSet::fragmentainerGroupAtVisualPoint(const LayoutPoint&) const
+{
+    // FIXME: implement this, once we have support for multiple rows.
+    return m_fragmentainerGroups.first();
+}
+
 LayoutUnit LayoutMultiColumnSet::pageLogicalHeight() const
 {
     // FIXME: pageLogicalHeight() needs to take a flow thread offset parameter, so that we can
@@ -185,6 +192,15 @@ void LayoutMultiColumnSet::computeLogicalHeight(LayoutUnit, LayoutUnit logicalTo
     computedValues.m_position = logicalTop;
 }
 
+PositionWithAffinity LayoutMultiColumnSet::positionForPoint(const LayoutPoint& point)
+{
+    // Convert the visual point to a flow thread point.
+    const MultiColumnFragmentainerGroup& row = fragmentainerGroupAtVisualPoint(point);
+    LayoutPoint flowThreadPoint = row.visualPointToFlowThreadPoint(point + row.offsetFromColumnSet());
+    // Then drill into the flow thread, where we'll find the actual content.
+    return flowThread()->positionForPoint(flowThreadPoint);
+}
+
 LayoutUnit LayoutMultiColumnSet::columnGap() const
 {
     LayoutBlockFlow* parentBlock = multiColumnBlockFlow();