Display fixed position objects on every page of 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/FrameView.h"
 #include "core/frame/Settings.h"
 #include "core/layout/ClipPathOperation.h"
 #include "core/layout/LayoutBlock.h"
@@ skipping 436 matching lines @@
 
     ClipRect ancestorBackgroundClipRect;
     if (parentLayer) {
         // Calculate the clip rectangle that the ancestors establish.
         ClipRectsContext clipRectsContext(paintingInfo.rootLayer, (paintFlags & PaintLayerUncachedClipRects) ? UncachedClipRects : PaintingClipRects, IgnoreOverlayScrollbarSize);
         if (shouldRespectOverflowClip(paintFlags, m_paintLayer.layoutObject()) == IgnoreOverflowClip)
             clipRectsContext.setIgnoreOverflowClip();
         ancestorBackgroundClipRect = m_paintLayer.clipper().backgroundClipRect(clipRectsContext);
     }
 
+    LayoutObject* object = m_paintLayer.layoutObject();
+    LayoutView* view = object->view();
+    bool isFixedPosObjectInPagedMedia = object->style()->position() == FixedPosition && view->pageLogicalHeight();

[Xianzhu, 2016/03/21 19:37:18]

We should exclude the case that the view is not the container of the fixed-pos
element (e.g. if the fixed-pos element is under a transformed element).

We may need to add another condition here: ... && view ==
object->containerForFixedPosition().

     PaintLayer* paginationLayer = m_paintLayer.enclosingPaginationLayer();
     PaintLayerFragments fragments;
-    if (paginationLayer) {
+    if (isFixedPosObjectInPagedMedia) {
+        // "For paged media, boxes with fixed positions are repeated on every page."

[Xianzhu, 2016/03/21 19:37:18]

Nit: include the link of source of the above statement.

+        ASSERT(view->firstChild() && view->firstChild()->isLayoutBlock());
+        int pages = toLayoutBlock(view->firstChild())->logicalHeight() / view->pageLogicalHeight();
+        LayoutPoint paginationOffset;
+        for (int i = 0; i <= pages; i++) {
+            PaintLayerFragment fragment;
+            fragment.paginationOffset = paginationOffset;
+            fragments.append(fragment);
+            paginationOffset += LayoutPoint(0, 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
         // correct.
         ClipRectsCacheSlot cacheSlot = (paintFlags & PaintLayerUncachedClipRects) ? UncachedClipRects : PaintingClipRects;
         ShouldRespectOverflowClipType respectOverflowClip = shouldRespectOverflowClip(paintFlags, m_paintLayer.layoutObject());
         // Calculate the transformed bounding box in the current coordinate space, to figure out
         // which fragmentainers (e.g. columns) we need to visit.
         LayoutRect transformedExtent = PaintLayer::transparencyClipBox(&m_paintLayer, paginationLayer, PaintLayer::PaintingTransparencyClipBox, PaintLayer::RootOfTransparencyClipBox, paintingInfo.subPixelAccumulation, paintingInfo.getGlobalPaintFlags());
         // FIXME: we don't check if paginationLayer is within paintingInfo.rootLayer here.
         paginationLayer->collectFragments(fragments, paintingInfo.rootLayer, paintingInfo.paintDirtyRect, cacheSlot, IgnoreOverlayScrollbarSize, respectOverflowClip, 0, paintingInfo.subPixelAccumulation, &transformedExtent);
     } else {
         // We don't need to collect any fragments in the regular way here. We have already
         // calculated a clip rectangle for the ancestry if it was needed, and clipping this
         // layer is something that can be done further down the path, when the transform has
         // been applied.
         PaintLayerFragment fragment;
         fragment.backgroundRect = paintingInfo.paintDirtyRect;
         fragments.append(fragment);
     }
 
     bool needsScope = fragments.size() > 1;
     PaintResult result = FullyPainted;
     for (const auto& fragment : fragments) {
         Optional<ScopeRecorder> scopeRecorder;
         if (needsScope)
             scopeRecorder.emplace(context);
         Optional<LayerClipRecorder> clipRecorder;
-        if (parentLayer) {
+        if (parentLayer && !isFixedPosObjectInPagedMedia) {

[Xianzhu, 2016/03/21 19:37:18]

I think we should still clip (especially css clip) fixed-pos elements.

For example:
<div style="clip: rect(0, 100px, 5000px, 0)>
  <div style="position: fixed; ..."></div>
</div>

I think in this case, whether the fixed-pos element appears in all pages depends
on how big the clip is.

             ClipRect clipRectForFragment(ancestorBackgroundClipRect);
             clipRectForFragment.moveBy(fragment.paginationOffset);
             clipRectForFragment.intersect(fragment.backgroundRect);
             if (clipRectForFragment.isEmpty())
                 continue;
             if (needsToClip(paintingInfo, clipRectForFragment)) {
                 if (m_paintLayer.layoutObject()->isPositioned() && clipRectForFragment.isClippedByClipCss())
                     UseCounter::count(m_paintLayer.layoutObject()->document(), UseCounter::ClipCssOfPositionedElement);
                 clipRecorder.emplace(context, *parentLayer->layoutObject(), DisplayItem::ClipLayerParent, clipRectForFragment, &paintingInfo, fragment.paginationOffset, paintFlags);
             }
@@ skipping 269 matching lines @@
     if (!m_paintLayer.containsDirtyOverlayScrollbars())
         return;
 
     PaintLayerPaintingInfo paintingInfo(&m_paintLayer, LayoutRect(enclosingIntRect(damageRect)), paintFlags, LayoutSize());
     paintLayer(context, paintingInfo, PaintLayerPaintingOverlayScrollbars);
 
     m_paintLayer.setContainsDirtyOverlayScrollbars(false);
 }
 
 } // namespace blink