Stay put at the top of the current page when inserting a forced break.

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

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 1434 matching lines @@
 
         if (marginInfo.margin())
             marginInfo.setHasMarginAfterQuirk(hasMarginAfterQuirk(&child));
     }
 
     // If margins would pull us past the top of the next page, then we need to pull back and pretend like the margins
     // collapsed into the page edge.
     LayoutState* layoutState = view()->layoutState();
     if (layoutState->isPaginated() && isPageLogicalHeightKnown(beforeCollapseLogicalTop) && logicalTop > beforeCollapseLogicalTop) {
         LayoutUnit oldLogicalTop = logicalTop;
-        logicalTop = std::min(logicalTop, nextPageLogicalTop(beforeCollapseLogicalTop, AssociateWithLatterPage));
+        logicalTop = std::min(logicalTop, nextPageLogicalTop(beforeCollapseLogicalTop));
         setLogicalHeight(logicalHeight() + (logicalTop - oldLogicalTop));
     }
 
     if (previousBlockFlow) {
         // If |child| is a self-collapsing block it may have collapsed into a previous sibling and although it hasn't reduced the height of the parent yet
         // any floats from the parent will now overhang.
         LayoutUnit oldLogicalHeight = logicalHeight();
         setLogicalHeight(logicalTop);
         if (!previousBlockFlow->avoidsFloats() && (previousBlockFlow->logicalTop() + previousBlockFlow->lowestFloatLogicalBottom()) > logicalTop)
             addOverhangingFloats(previousBlockFlow, false);
@@ skipping 204 matching lines @@
 
         // Collapse the result with our current margins.
         if (!discardMarginBefore)
             logicalTopEstimate += std::max(marginInfo.positiveMargin(), positiveMarginBefore) - std::max(marginInfo.negativeMargin(), negativeMarginBefore);
     }
 
     // Adjust logicalTopEstimate down to the next page if the margins are so large that we don't fit on the current
     // page.
     LayoutState* layoutState = view()->layoutState();
     if (layoutState->isPaginated() && isPageLogicalHeightKnown(logicalHeight()) && logicalTopEstimate > logicalHeight())
-        logicalTopEstimate = std::min(logicalTopEstimate, nextPageLogicalTop(logicalHeight(), AssociateWithLatterPage));
+        logicalTopEstimate = std::min(logicalTopEstimate, nextPageLogicalTop(logicalHeight()));
 
     logicalTopEstimate += getClearDelta(&child, logicalTopEstimate);
 
     estimateWithoutPagination = logicalTopEstimate;
 
     if (layoutState->isPaginated()) {
         if (!layoutInfo.isAtFirstInFlowChild()) {
             // Estimate the need for a forced break in front of this child. The final break policy
             // at this class A break point isn't known until we have laid out the children of
             // |child|. There may be forced break-before values set on first-children inside that
@@ skipping 189 matching lines @@
         return childStyle.marginAfterCollapse() == MarginCollapseSeparate;
     if (child.isHorizontalWritingMode() == isHorizontalWritingMode())
         return childStyle.marginBeforeCollapse() == MarginCollapseSeparate;
 
     // FIXME: See |mustDiscardMarginBeforeForChild| above.
     return false;
 }
 
 LayoutUnit LayoutBlockFlow::applyForcedBreak(LayoutUnit logicalOffset, EBreak breakValue)
 {
+    if (!isForcedFragmentainerBreakValue(breakValue))
+        return logicalOffset;
     // TODO(mstensho): honor breakValue. There are different types of forced breaks. We currently
     // just assume that we want to break to the top of the next fragmentainer of the fragmentation
     // context we're in. However, we may want to find the next left or right page - even if we're
     // inside a multicol container when printing.
-    if (isForcedFragmentainerBreakValue(breakValue))
-        return nextPageLogicalTop(logicalOffset, AssociateWithFormerPage);
-    return logicalOffset;
+    LayoutUnit pageLogicalHeight = pageLogicalHeightForOffset(logicalOffset);
+    if (!pageLogicalHeight)
+        return logicalOffset; // Page height is still unknown, so we cannot insert forced breaks.
+    LayoutUnit remainingLogicalHeight = pageRemainingLogicalHeightForOffset(logicalOffset, AssociateWithLatterPage);
+    if (remainingLogicalHeight == pageLogicalHeight)
+        return logicalOffset; // Don't break if we're already at the block start of a fragmentainer.
+    return logicalOffset + remainingLogicalHeight;
 }
 
 void LayoutBlockFlow::setBreakBefore(EBreak breakValue)
 {
     if (breakValue != BreakAuto && !isBreakBetweenControllable(breakValue))
         breakValue = BreakAuto;
     if (breakValue == BreakAuto && !m_rareData)
         return;
     ensureRareData().m_breakBefore = breakValue;
 }
@@ skipping 1853 matching lines @@
 
     return LayoutBlock::invalidatePaintIfNeeded(paintInvalidationState);
 }
 
 void LayoutBlockFlow::invalidateDisplayItemClients(PaintInvalidationReason invalidationReason) const
 {
     BlockFlowPaintInvalidator(*this).invalidateDisplayItemClients(invalidationReason);
 }
 
 } // namespace blink