Avoid unnecessary relayout of floats when not paginated.

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 4701 matching lines @@
     return;
 
   LayoutUnit logicalTop = child.logicalTop();
   LayoutUnit logicalHeight = child.logicalHeightIncludingOverflow();
   LayoutUnit spaceLeft =
       pageRemainingLogicalHeightForOffset(logicalTop, AssociateWithLatterPage);
   if (spaceLeft < logicalHeight)
     child.setOffsetToNextPage(spaceLeft);
 }
 
+bool LayoutBox::childNeedsRelayoutForPagination(const LayoutBox& child) const {
+  // TODO(mstensho): Should try to get this to work for floats too, instead of
+  // just marking and bailing here.
+  if (child.isFloating())
+    return true;
+  LayoutUnit logicalTop = child.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.
+  if (LayoutUnit pageLogicalHeight = pageLogicalHeightForOffset(logicalTop)) {
+    LayoutUnit logicalHeight = child.logicalHeightIncludingOverflow();
+    LayoutUnit remainingSpace = pageRemainingLogicalHeightForOffset(
+        logicalTop, AssociateWithLatterPage);
+    if (child.offsetToNextPage()) {
+      // We need to relayout unless we're going to break at the exact same
+      // location as before.
+      if (child.offsetToNextPage() != remainingSpace)

[eae, 2016/11/01 16:55:10]

Is checking the break offset sufficient here? Don't you need to compare the
start offset as well?

[mstensho (USE GERRIT), 2016/11/01 17:54:26]

We no longer store the start offset, as of
https://codereview.chromium.org/2462643002/ . The break offset is all we need.
How far we are from the top of the page isn't really that interesting. What
matters is where we break.

+        return true;
+    } else if (logicalHeight > remainingSpace) {
+      // Last time we laid out this child, we didn't need to break, but now we
+      // have to. So we need to relayout.
+      return true;
+    }
+  } else {
+    return true;
+  }
+
+  // It seems that we can 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();
+  return flowThread && !flowThread->canSkipLayout(child);
+}
+
 void LayoutBox::markChildForPaginationRelayoutIfNeeded(
     LayoutBox& child,
     SubtreeLayoutScope& layoutScope) {
   DCHECK(!child.needsLayout());
   LayoutState* layoutState = view()->layoutState();
-  // TODO(mstensho): Should try to get this to work for floats too, instead of
-  // just marking and bailing here.
-  if (layoutState->paginationStateChanged() || child.isFloating()) {
+
+  if (layoutState->paginationStateChanged() ||
+      (layoutState->isPaginated() && childNeedsRelayoutForPagination(child)))
     layoutScope.setChildNeedsLayout(&child);
-    return;
-  }
-  if (!layoutState->isPaginated())
-    return;
-
-  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 863 matching lines @@
   LayoutRect rect = frameRect();
 
   LayoutBlock* block = containingBlock();
   if (block)
     block->adjustChildDebugRect(rect);
 
   return rect;
 }
 
 }  // namespace blink