Translate the clip rect correctly when iterating over fragmentainers.

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

 // Copyright 2016 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.
 
 #include "core/layout/FragmentainerIterator.h"
 
 #include "core/layout/LayoutMultiColumnSet.h"
 
 namespace blink {
 
@@ skipping 81 matching lines @@
 }
 
 bool FragmentainerIterator::setFragmentainersOfInterest()
 {
     const MultiColumnFragmentainerGroup& group = currentGroup();
 
     // Figure out the start and end fragmentainers for the block range we're interested in. We
     // might not have to walk the entire fragmentainer group.
     group.columnIntervalForBlockRangeInFlowThread(m_logicalTopInFlowThread, m_logicalBottomInFlowThread, m_currentFragmentainerIndex, m_endFragmentainerIndex);
 
-    // Now intersect with the fragmentainers that actually intersect with the clip rect, to narrow
-    // it down even further.
+    // Now intersect with the fragmentainers that actually intersect with the visual clip rect, to
+    // narrow it down even further. The clip rect needs to be relative to the current fragmentainer
+    // group.
+    LayoutRect clipRect = m_clipRectInMulticolContainer;
+    LayoutSize offset = group.flowThreadTranslationAtOffset(group.logicalTopInFlowThread(), CoordinateSpaceConversion::Visual);
+    clipRect.move(-offset);
     unsigned firstFragmentainerInClipRect, lastFragmentainerInClipRect;
-    group.columnIntervalForVisualRect(m_clipRectInMulticolContainer, firstFragmentainerInClipRect, lastFragmentainerInClipRect);
+    group.columnIntervalForVisualRect(clipRect, firstFragmentainerInClipRect, lastFragmentainerInClipRect);
     // If the two fragmentainer intervals are disjoint, there's nothing of interest in this
     // fragmentainer group.
     if (firstFragmentainerInClipRect > m_endFragmentainerIndex || lastFragmentainerInClipRect < m_currentFragmentainerIndex)
         return false;
     if (m_currentFragmentainerIndex < firstFragmentainerInClipRect)
         m_currentFragmentainerIndex = firstFragmentainerInClipRect;
     if (m_endFragmentainerIndex > lastFragmentainerInClipRect)
         m_endFragmentainerIndex = lastFragmentainerInClipRect;
     DCHECK(m_endFragmentainerIndex >= m_currentFragmentainerIndex);
     return true;
 }
 
 void FragmentainerIterator::updateOutput()
 {
     const MultiColumnFragmentainerGroup& group = currentGroup();
 
     // Set the physical translation offset.
     LayoutUnit fragmentainerLogicalTopInFlowThread = group.logicalTopInFlowThread() + m_currentFragmentainerIndex * group.logicalHeight();
     m_paginationOffset = group.flowThreadTranslationAtOffset(fragmentainerLogicalTopInFlowThread, CoordinateSpaceConversion::Visual);
 
     // Set the overflow clip rect that corresponds to the fragmentainer.
     m_clipRectInFlowThread = group.flowThreadPortionOverflowRectAt(m_currentFragmentainerIndex);
 
     // Flip it into a physical rectangle.
     m_flowThread.flipForWritingMode(m_clipRectInFlowThread);
 }
 
 } // namespace blink