Make LayoutFlowThread::fragmentsBoundingBox() faster.

Source/core/layout/MultiColumnFragmentainerGroup.cpp

Index: Source/core/layout/MultiColumnFragmentainerGroup.cpp
diff --git a/Source/core/layout/MultiColumnFragmentainerGroup.cpp b/Source/core/layout/MultiColumnFragmentainerGroup.cpp
index 12e4cb0f45a5f463fceb12c834dd0cce5e81a944..668ca251716199381f8c1a5f403b4e417ea6aacf 100644
--- a/Source/core/layout/MultiColumnFragmentainerGroup.cpp
+++ b/Source/core/layout/MultiColumnFragmentainerGroup.cpp
@@ -175,6 +175,37 @@ LayoutPoint MultiColumnFragmentainerGroup::visualPointToFlowThreadPoint(const La
     return LayoutPoint(localPoint.x(), localPoint.y() + logicalTopInFlowThreadAt(columnIndex));
 }
 
+LayoutRect MultiColumnFragmentainerGroup::fragmentsBoundingBox(const LayoutRect& boundingBoxInFlowThread) const
+{
+    // Find the start and end column intersected by the bounding box.
+    LayoutRect flippedBoundingBoxInFlowThread(boundingBoxInFlowThread);
+    LayoutFlowThread* flowThread = m_columnSet.flowThread();
+    flowThread->flipForWritingMode(flippedBoundingBoxInFlowThread);
+    bool isHorizontalWritingMode = m_columnSet.isHorizontalWritingMode();
+    LayoutUnit boundingBoxLogicalTop = isHorizontalWritingMode ? flippedBoundingBoxInFlowThread.y() : flippedBoundingBoxInFlowThread.x();
+    LayoutUnit boundingBoxLogicalBottom = isHorizontalWritingMode ? flippedBoundingBoxInFlowThread.maxY() : flippedBoundingBoxInFlowThread.maxX();
+    if (boundingBoxLogicalBottom <= logicalTopInFlowThread() || boundingBoxLogicalTop >= logicalBottomInFlowThread())
+        return LayoutRect(); // The bounding box doesn't intersect this fragmentainer group.
+    unsigned startColumn;
+    unsigned endColumn;
+    columnIntervalForBlockRangeInFlowThread(boundingBoxLogicalTop, boundingBoxLogicalBottom, startColumn, endColumn);
+
+    LayoutRect startColumnFlowThreadOverflowPortion = flowThreadPortionOverflowRectAt(startColumn);
+    flowThread->flipForWritingMode(startColumnFlowThreadOverflowPortion);
+    LayoutRect startColumnRect(boundingBoxInFlowThread);
+    startColumnRect.intersect(startColumnFlowThreadOverflowPortion);
+    startColumnRect.move(flowThreadTranslationAtOffset(logicalTopInFlowThreadAt(startColumn)));
+    if (startColumn == endColumn)
+        return startColumnRect; // It all takes place in one column. We're done.
+
+    LayoutRect endColumnFlowThreadOverflowPortion = flowThreadPortionOverflowRectAt(endColumn);
+    flowThread->flipForWritingMode(endColumnFlowThreadOverflowPortion);
+    LayoutRect endColumnRect(boundingBoxInFlowThread);
+    endColumnRect.intersect(endColumnFlowThreadOverflowPortion);
+    endColumnRect.move(flowThreadTranslationAtOffset(logicalTopInFlowThreadAt(endColumn)));
+    return unionRect(startColumnRect, endColumnRect);
+}
+
 void MultiColumnFragmentainerGroup::collectLayerFragments(DeprecatedPaintLayerFragments& fragments, const LayoutRect& layerBoundingBox, const LayoutRect& dirtyRect) const
 {
     // |layerBoundingBox| is in the flow thread coordinate space, relative to the top/left edge of