ColumnBalancer: need to examine lines that protrude into the flow thread portion.

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

 // Copyright 2015 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/layout/ColumnBalancer.h"
 
 #include "core/layout/LayoutMultiColumnFlowThread.h"
 #include "core/layout/LayoutMultiColumnSet.h"
 #include "core/layout/api/LineLayoutBlockFlow.h"
 
@@ skipping 23 matching lines @@
   // block flow with inline children, there may be interesting floats to examine
   // here.
   traverseChildren(box);
 }
 
 void ColumnBalancer::traverseLines(const LayoutBlockFlow& blockFlow) {
   for (const RootInlineBox* line = blockFlow.firstRootBox(); line;
        line = line->nextRootBox()) {
     LayoutUnit lineTopInFlowThread =
         m_flowThreadOffset + line->lineTopWithLeading();
-    if (lineTopInFlowThread < logicalTopInFlowThread())
-      continue;
+    if (lineTopInFlowThread < logicalTopInFlowThread()) {
+      // If the line is fully about the flow thread portion range we're working
+      // with, we can skip it. If its logical top is outside the range, but its
+      // logical bottom protrudes into the range, we need to examine it.
+      LayoutUnit lineBottom = line->lineBottomWithLeading();
+      if (m_flowThreadOffset + lineBottom <= logicalTopInFlowThread())
+        continue;
+    }
     if (lineTopInFlowThread >= logicalBottomInFlowThread())
       break;
     examineLine(*line);
   }
 }
 
 void ColumnBalancer::traverseChildren(const LayoutObject& object) {
   // The break-after value from the previous in-flow block-level object to be
   // joined with the break-before value of the next in-flow block-level sibling.
   EBreak previousBreakAfterValue = BreakAuto;
@@ skipping 170 matching lines @@
   for (unsigned i = 1; i < minimumLineCount && firstLine->prevRootBox(); i++)
     firstLine = firstLine->prevRootBox();
   return lastLine.lineBottomWithLeading() - firstLine->lineTopWithLeading();
 }
 
 void InitialColumnHeightFinder::examineLine(const RootInlineBox& line) {
   LayoutUnit lineTop = line.lineTopWithLeading();
   LayoutUnit lineTopInFlowThread = flowThreadOffset() + lineTop;
   LayoutUnit minimumLogialHeight =
       columnLogicalHeightRequirementForLine(line.block().styleRef(), line);
+  if (lineTopInFlowThread < LayoutUnit())
+    minimumLogialHeight += lineTopInFlowThread;
   m_tallestUnbreakableLogicalHeight =
       std::max(m_tallestUnbreakableLogicalHeight, minimumLogialHeight);
   ASSERT(
       isFirstAfterBreak(lineTopInFlowThread) || !line.paginationStrut() ||
       !isLogicalTopWithinBounds(lineTopInFlowThread - line.paginationStrut()));
   if (isFirstAfterBreak(lineTopInFlowThread) &&
       m_lastBreakSeen != lineTopInFlowThread) {
     m_lastBreakSeen = lineTopInFlowThread;
     recordStrutBeforeOffset(lineTopInFlowThread, line.paginationStrut());
   }
@@ skipping 236 matching lines @@
   if (group.columnLogicalTopForOffset(lineTopInFlowThread) !=
       group.columnLogicalTopForOffset(lineBottomWithOverflow)) {
     LayoutUnit shortage =
         lineBottomWithOverflow -
         group.columnLogicalTopForOffset(lineBottomWithOverflow);
     recordSpaceShortage(shortage);
   }
 }
 
 }  // namespace blink