Support for mapping from outer/visual to flowthread coord space in nested multicol.

third_party/WebKit/Source/core/layout/LayoutMultiColumnSet.cpp

Index: third_party/WebKit/Source/core/layout/LayoutMultiColumnSet.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutMultiColumnSet.cpp b/third_party/WebKit/Source/core/layout/LayoutMultiColumnSet.cpp
index 913047561c4eb51a5f1f050d837115a10e4b89db..bdd1b420752905914632dc80a1250697276b2af8 100644
--- a/third_party/WebKit/Source/core/layout/LayoutMultiColumnSet.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutMultiColumnSet.cpp
@@ -64,10 +64,15 @@ unsigned LayoutMultiColumnSet::fragmentainerGroupIndexAtFlowThreadOffset(LayoutU
     return m_fragmentainerGroups.size() - 1;
 }
 
-const MultiColumnFragmentainerGroup& LayoutMultiColumnSet::fragmentainerGroupAtVisualPoint(const LayoutPoint&) const
+const MultiColumnFragmentainerGroup& LayoutMultiColumnSet::fragmentainerGroupAtVisualPoint(const LayoutPoint& visualPoint) const
 {
-    // FIXME: implement this, once we have support for multiple rows.
-    return m_fragmentainerGroups.first();
+    LayoutUnit blockOffset = isHorizontalWritingMode() ? visualPoint.y() : visualPoint.x();
+    for (unsigned index = 0; index < m_fragmentainerGroups.size(); index++) {
+        const auto& row = m_fragmentainerGroups[index];
+        if (row.logicalTop() + row.logicalHeight() > blockOffset)
+            return row;
+    }
+    return m_fragmentainerGroups.last();

[eae, 2016/04/22 19:35:39]

Do we need to assert that there is at least one entry in m_fragmentainerGroups?

[mstensho (USE GERRIT), 2016/04/22 19:46:00]

I was thinking that it really shouldn't be necessary, but I see now that
fragmentainerGroupIndexAtFlowThreadOffset() (the method above) does it, so I
think it makes sense to add it here too, for the sake of consistency and lurking
dragons.

Done.

 }
 
 LayoutUnit LayoutMultiColumnSet::pageLogicalHeightForOffset(LayoutUnit offsetInFlowThread) const