Be more restrictive about forcing relayout of children for pagination.

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

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2005 Allan Sandfeld Jensen (kde@carewolf.com)
  *           (C) 2005, 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010 Apple Inc.
  *               All rights reserved.
  * Copyright (C) 2013 Adobe Systems Incorporated. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 688 matching lines @@
                    m_frameRect.height().toFloat());
 }
 
 void LayoutBox::updateLayerTransformAfterLayout() {
   // Transform-origin depends on box size, so we need to update the layer
   // transform after layout.
   if (hasLayer())
     layer()->updateTransformationMatrix();
 }
 
+LayoutUnit LayoutBox::logicalHeightIncludingOverflow() const {
+  if (!m_overflow)
+    return logicalHeight();
+  LayoutRect overflow = layoutOverflowRect();
+  if (style()->isHorizontalWritingMode())
+    return overflow.maxY();
+  return overflow.maxX();
+}
+
 LayoutUnit LayoutBox::constrainLogicalWidthByMinMax(LayoutUnit logicalWidth,
                                                     LayoutUnit availableWidth,
                                                     LayoutBlock* cb) const {
   const ComputedStyle& styleToUse = styleRef();
   if (!styleToUse.logicalMaxWidth().isMaxSizeNone())
     logicalWidth =
         std::min(logicalWidth,
                  computeLogicalWidthUsing(MaxSize, styleToUse.logicalMaxWidth(),
                                           availableWidth, cb));
   return std::max(logicalWidth, computeLogicalWidthUsing(
@@ skipping 3976 matching lines @@
     if (scrollableArea->hasHorizontalScrollbar() &&
         !scrollableArea->layerForHorizontalScrollbar())
       return true;
     if (scrollableArea->hasVerticalScrollbar() &&
         !scrollableArea->layerForVerticalScrollbar())
       return true;
   }
   return false;
 }
 
+void LayoutBox::updateFragmentationInfoForChild(LayoutBox& child) {
+  LayoutState* layoutState = view()->layoutState();
+  DCHECK(layoutState->isPaginated());
+  child.setOffsetToNextPage(LayoutUnit());
+  if (!pageLogicalHeightForOffset(child.logicalTop()))
+    return;
+
+  LayoutUnit logicalTop = child.logicalTop();
+  LayoutUnit logicalHeight = child.logicalHeightIncludingOverflow();
+  LayoutUnit spaceLeft =
+      pageRemainingLogicalHeightForOffset(logicalTop, AssociateWithLatterPage);
+  if (spaceLeft < logicalHeight)
+    child.setOffsetToNextPage(spaceLeft);
+}
+
 void LayoutBox::markChildForPaginationRelayoutIfNeeded(
     LayoutBox& child,
     SubtreeLayoutScope& layoutScope) {
   DCHECK(!child.needsLayout());
   LayoutState* layoutState = view()->layoutState();
-  if (layoutState->paginationStateChanged()) {
+  // TODO(mstensho): Should try to get this to work for floats too, instead of
+  // just marking and bailing here.
+  if (layoutState->paginationStateChanged() || child.isFloating()) {
     layoutScope.setChildNeedsLayout(&child);
     return;
   }
   if (!layoutState->isPaginated())
     return;
 
-  if (layoutState->pageLogicalHeightChanged() ||
-      (layoutState->pageLogicalHeight() &&
-       layoutState->pageLogicalOffset(child, child.logicalTop()) !=
-           child.pageLogicalOffset()))
-    layoutScope.setChildNeedsLayout(&child);
+  LayoutUnit logicalTop = child.logicalTop();
+  if (LayoutUnit pageLogicalHeight = pageLogicalHeightForOffset(logicalTop)) {
+    // Figure out if we really need to force re-layout of the child. We only
+    // need to do this if there's a chance that we need to recalculate
+    // pagination struts inside.
+    LayoutUnit remainingSpace = pageRemainingLogicalHeightForOffset(
+        logicalTop, AssociateWithLatterPage);
+    LayoutUnit logicalHeight = child.logicalHeightIncludingOverflow();
+    if ((!child.offsetToNextPage() && logicalHeight <= remainingSpace) ||
+        child.offsetToNextPage() == remainingSpace) {
+      // We don't need to relayout this child, either because the child wasn't
+      // previously fragmented, and won't be fragmented now either, or because
+      // it would fragment at the exact same position as before.
+      //
+      // We want to skip layout of this child, but we need to ask the flow
+      // thread for permission first. We currently cannot skip over objects
+      // containing column spanners.
+      LayoutFlowThread* flowThread = child.flowThreadContainingBlock();
+      if (!flowThread || flowThread->canSkipLayout(child))
+        return;
+    }
+  }
+
+  layoutScope.setChildNeedsLayout(&child);
 }
 
 void LayoutBox::markOrthogonalWritingModeRoot() {
   ASSERT(frameView());
   frameView()->addOrthogonalWritingModeRoot(*this);
 }
 
 void LayoutBox::unmarkOrthogonalWritingModeRoot() {
   ASSERT(frameView());
   frameView()->removeOrthogonalWritingModeRoot(*this);
@@ skipping 546 matching lines @@
   // meant to be called on the object currently being laid out.
   ASSERT(!isLayoutBlock());
 
   // In case this box doesn't establish a layout state, try the containing
   // block.
   LayoutBlock* containerBlock = containingBlock();
   ASSERT(layoutState->layoutObject() == containerBlock);
   return containerBlock->offsetFromLogicalTopOfFirstPage() + logicalTop();
 }
 
-void LayoutBox::setPageLogicalOffset(LayoutUnit offset) {
+void LayoutBox::setOffsetToNextPage(LayoutUnit offset) {
   if (!m_rareData && !offset)
     return;
-  ensureRareData().m_pageLogicalOffset = offset;
+  ensureRareData().m_offsetToNextPage = offset;
 }
 
 void LayoutBox::logicalExtentAfterUpdatingLogicalWidth(
     const LayoutUnit& newLogicalTop,
     LayoutBox::LogicalExtentComputedValues& computedValues) {
   // FIXME: None of this is right for perpendicular writing-mode children.
   LayoutUnit oldLogicalWidth = logicalWidth();
   LayoutUnit oldLogicalLeft = logicalLeft();
   LayoutUnit oldMarginLeft = marginLeft();
   LayoutUnit oldMarginRight = marginRight();
@@ skipping 293 matching lines @@
   LayoutRect rect = frameRect();
 
   LayoutBlock* block = containingBlock();
   if (block)
     block->adjustChildDebugRect(rect);
 
   return rect;
 }
 
 }  // namespace blink