[New Multicolumn] Make positionForPoint() work.

Source/core/layout/MultiColumnFragmentainerGroup.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef MultiColumnFragmentainerGroup_h
 #define MultiColumnFragmentainerGroup_h
 
 #include "core/layout/LayoutMultiColumnFlowThread.h"
 
 namespace blink {
@@ skipping 42 matching lines @@
     void resetColumnHeight();
     void addContentRun(LayoutUnit endOffsetInFlowThread);
     void updateMinimumColumnHeight(LayoutUnit height) { m_minimumColumnHeight = std::max(height, m_minimumColumnHeight); }
     void recordSpaceShortage(LayoutUnit);
     bool recalculateColumnHeight(BalancedColumnHeightCalculation calculationMode);
 
     void expandToEncompassFlowThreadOverflow();
 
     LayoutSize flowThreadTranslationAtOffset(LayoutUnit offsetInFlowThread) const;
     LayoutUnit columnLogicalTopForOffset(LayoutUnit offsetInFlowThread) const;
+    LayoutPoint visualPointToFlowThreadPoint(const LayoutPoint& visualPoint) const;
     void collectLayerFragments(LayerFragments&, const LayoutRect& layerBoundingBox, const LayoutRect& dirtyRect) const;
     LayoutRect calculateOverflow() const;
 
     // The "CSS actual" value of column-count. This includes overflowing columns, if any.
     unsigned actualColumnCount() const;
 
 private:
     LayoutUnit heightAdjustedForRowOffset(LayoutUnit height) const;
     LayoutUnit calculateMaxColumnHeight() const;
     void setAndConstrainColumnHeight(LayoutUnit);
 
     // Return the index of the content run with the currently tallest columns, taking all implicit
     // breaks assumed so far into account.
     unsigned findRunWithTallestColumns() const;
 
     // Given the current list of content runs, make assumptions about where we need to insert
     // implicit breaks (if there's room for any at all; depending on the number of explicit breaks),
     // and store the results. This is needed in order to balance the columns.
     void distributeImplicitBreaks();
 
     LayoutUnit calculateColumnHeight(BalancedColumnHeightCalculation) const;
 
     LayoutRect columnRectAt(unsigned columnIndex) const;
+    LayoutUnit logicalTopInFlowThreadAt(unsigned columnIndex) const { return m_logicalTopInFlowThread + columnIndex * m_columnHeight; }
     LayoutRect flowThreadPortionRectAt(unsigned columnIndex) const;
     LayoutRect flowThreadPortionOverflowRect(const LayoutRect& flowThreadPortion, unsigned columnIndex, unsigned columnCount, LayoutUnit columnGap) const;
 
     enum ColumnIndexCalculationMode {
         ClampToExistingColumns, // Stay within the range of already existing columns.
         AssumeNewColumns // Allow column indices outside the range of already existing columns.
     };
     unsigned columnIndexAtOffset(LayoutUnit offsetInFlowThread, ColumnIndexCalculationMode = ClampToExistingColumns) const;
 
+    // Return the column that the specified visual point belongs to. Only the coordinate on the
+    // column progression axis is relevant. Every point belongs to a column, even if said point is
+    // not inside any of the columns.
+    unsigned columnIndexAtVisualPoint(const LayoutPoint& visualPoint) const;

[Julien - ping for review, 2015/03/09 15:49:03]

It's clunky that we need to convert the point into flow-thread coordinate and
then ask for the column index.

Do you intent to reuse this code in the future? If not, it seems like just
converting a point to a column index would do the job inside one function.

[mstensho (USE GERRIT), 2015/03/12 11:43:27]

This method doesn't convert to flow-thread coordinates. I have no plans for
other uses of this method, but doesn't it really serve a purpose on its own? I
feel that visualPointToFlowThreadPoint() is heavy enough as it is.

+
     LayoutMultiColumnSet& m_columnSet;
 
     LayoutUnit m_logicalTop;
     LayoutUnit m_logicalTopInFlowThread;
     LayoutUnit m_logicalBottomInFlowThread;
 
     LayoutUnit m_columnHeight;
 
     // The following variables are used when balancing the column set.
     LayoutUnit m_maxColumnHeight; // Maximum column height allowed.
@@ skipping 41 matching lines @@
     // Remove all fragmentainer groups but the first one.
     void deleteExtraGroups();
 
 private:
     LayoutMultiColumnSet& m_columnSet;
 };
 
 } // namespace blink
 
 #endif // MultiColumnFragmentainerGroup_h