Fixed pos objects are offset from the top of the page in paged media

third_party/WebKit/Source/core/paint/PaintLayerPainter.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 "core/paint/PaintLayerPainter.h"
 
 #include "core/frame/LocalFrame.h"
 #include "core/layout/LayoutInline.h"
 #include "core/layout/LayoutView.h"
 #include "core/paint/ClipPathClipper.h"
@@ skipping 568 matching lines @@
   PaintLayer* paginationLayer = m_paintLayer.enclosingPaginationLayer();
   PaintLayerFragments fragments;
   // TODO(crbug.com/619094): Figure out the correct behaviour for fixed position objects
   // in paged media with vertical writing modes.
   if (isFixedPosObjectInPagedMedia && view->isHorizontalWritingMode()) {
     // "For paged media, boxes with fixed positions are repeated on every page."
     // - https://www.w3.org/TR/2011/REC-CSS2-20110607/visuren.html#fixed-positioning
     unsigned pages =
         ceilf(view->documentRect().height() / view->pageLogicalHeight());
     LayoutPoint paginationOffset;
+
+    // The fixed position object is offset from the top of the page, so remove any scroll offset.

[Xianzhu, 2016/10/06 22:40:42]

Nit: Wrap at 80 chars. (Though there are other comments exceeding 80 chars, some
ones may be working on fixing them, so we should not introduce new ones.)

+    LayoutPoint offsetFromRoot;
+    m_paintLayer.convertToLayerCoords(paintingInfo.rootLayer, offsetFromRoot);
+    paginationOffset -= offsetFromRoot - m_paintLayer.location();
+
     for (unsigned i = 0; i < pages; i++) {
       PaintLayerFragment fragment;
       fragment.backgroundRect = paintingInfo.paintDirtyRect;
       fragment.paginationOffset = paginationOffset;
       fragments.append(fragment);
       paginationOffset += LayoutPoint(LayoutUnit(), view->pageLogicalHeight());
     }
   } else if (paginationLayer) {
     // FIXME: This is a mess. Look closely at this code and the code in Layer and fix any
     // issues in it & refactor to make it obvious from code structure what it does and that it's
@@ skipping 447 matching lines @@
 
   PaintLayerPaintingInfo paintingInfo(&m_paintLayer,
                                       LayoutRect(enclosingIntRect(damageRect)),
                                       paintFlags, LayoutSize());
   paintLayer(context, paintingInfo, PaintLayerPaintingOverlayScrollbars);
 
   m_paintLayer.setContainsDirtyOverlayScrollbars(false);
 }
 
 }  // namespace blink