Fix fixed-background offset issue

Source/core/paint/BlockPainter.cpp

 // Copyright 2014 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 "config.h"
 #include "core/paint/BlockPainter.h"
 
 #include "core/editing/DragCaretController.h"
 #include "core/editing/FrameSelection.h"
 #include "core/frame/Settings.h"
@@ skipping 155 matching lines @@
 
     {
         Optional<ScrollRecorder> scrollRecorder;
         Optional<PaintInfo> scrolledPaintInfo;
         if (m_layoutBlock.hasOverflowClip()) {
             IntSize scrollOffset = m_layoutBlock.scrolledContentOffset();
             if (m_layoutBlock.layer()->scrollsOverflow() || !scrollOffset.isZero()) {
                 scrollRecorder.emplace(*paintInfo.context, m_layoutBlock, paintPhase, scrollOffset);
                 scrolledPaintInfo.emplace(paintInfo);
                 scrolledPaintInfo->rect.move(scrollOffset);
+                scrolledPaintInfo->accumulatedScrollOffsetSincePaintContainer += scrollOffset;
             }
         }
 
         // We're done. We don't bother painting any children.
         if (paintPhase == PaintPhaseBlockBackground || paintInfo.paintRootBackgroundOnly())
             return;
 
         const PaintInfo& contentsPaintInfo = scrolledPaintInfo ? *scrolledPaintInfo : paintInfo;
 
         if (paintPhase != PaintPhaseSelfOutline)
@@ skipping 86 matching lines @@
 
         // We don't paint our own background, but we do let the kids paint their backgrounds.
         PaintInfo paintInfoForChild(paintInfo);
         paintInfoForChild.phase = newPhase;
         paintInfoForChild.updatePaintingRootForChildren(&m_layoutBlock);
         m_layoutBlock.paintChildren(paintInfoForChild, paintOffset);
     }
 }
 
 } // namespace blink