Need to examine the *bottom* of fragmented content.

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

Index: third_party/WebKit/Source/core/layout/LayoutMultiColumnFlowThread.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutMultiColumnFlowThread.cpp b/third_party/WebKit/Source/core/layout/LayoutMultiColumnFlowThread.cpp
index 4731477579ec2afa4d170a3d00e8a856ae78947f..88dc399fb1500e19036e636afa4c79ae83b9cd3f 100644
--- a/third_party/WebKit/Source/core/layout/LayoutMultiColumnFlowThread.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutMultiColumnFlowThread.cpp
@@ -452,7 +452,7 @@ FragmentationContext* LayoutMultiColumnFlowThread::enclosingFragmentationContext
     return view()->fragmentationContext();
 }
 
-void LayoutMultiColumnFlowThread::appendNewFragmentainerGroupIfNeeded(LayoutUnit offsetInFlowThread)
+void LayoutMultiColumnFlowThread::appendNewFragmentainerGroupIfNeeded(LayoutUnit bottomOffsetInFlowThread)
 {
     if (!isPageLogicalHeightKnown()) {
         // If we have no clue about the height of the multicol container, bail. This situation
@@ -462,7 +462,11 @@ void LayoutMultiColumnFlowThread::appendNewFragmentainerGroupIfNeeded(LayoutUnit
         // Its height is indefinite for now.
         return;
     }
-    LayoutMultiColumnSet* columnSet = columnSetAtBlockOffset(offsetInFlowThread);
+    // TODO(mstensho): bottomOffsetInFlowThread is an endpoint-exclusive offset, i.e. the offset
+    // just after the bottom of some object. So, ideally, columnSetAtBlockOffset() should be
+    // informed about this (i.e. take a PageBoundaryRule argument). This is not the only place with
+    // this issue; see also pageRemainingLogicalHeightForOffset().
+    LayoutMultiColumnSet* columnSet = columnSetAtBlockOffset(bottomOffsetInFlowThread);
     if (columnSet->isInitialHeightCalculated()) {
         // We only insert additional fragmentainer groups in the initial layout pass. We only want
         // to balance columns in the last fragmentainer group (if we need to balance at all), so we
@@ -470,7 +474,7 @@ void LayoutMultiColumnFlowThread::appendNewFragmentainerGroupIfNeeded(LayoutUnit
         return;
     }
 
-    if (!columnSet->hasFragmentainerGroupForColumnAt(offsetInFlowThread)) {
+    if (!columnSet->hasFragmentainerGroupForColumnAt(bottomOffsetInFlowThread)) {
         FragmentationContext* enclosingFragmentationContext = this->enclosingFragmentationContext();
         if (!enclosingFragmentationContext)
             return; // Not nested. We'll never need more rows than the one we already have then.
@@ -480,7 +484,7 @@ void LayoutMultiColumnFlowThread::appendNewFragmentainerGroupIfNeeded(LayoutUnit
         // multicol container. That in turn may mean that we've run out of columns there too.
         const MultiColumnFragmentainerGroup& newRow = columnSet->appendNewFragmentainerGroup();
         if (LayoutMultiColumnFlowThread* enclosingFlowThread = enclosingFragmentationContext->associatedFlowThread())
-            enclosingFlowThread->appendNewFragmentainerGroupIfNeeded(newRow.blockOffsetInEnclosingFragmentationContext());
+            enclosingFlowThread->appendNewFragmentainerGroupIfNeeded(newRow.blockOffsetInEnclosingFragmentationContext() + newRow.logicalHeight());
     }
 }
 
@@ -936,7 +940,7 @@ void LayoutMultiColumnFlowThread::layout()
     m_lastSetWorkedOn = nullptr;
 }
 
-void LayoutMultiColumnFlowThread::contentWasLaidOut(LayoutUnit logicalTopInFlowThreadAfterPagination)
+void LayoutMultiColumnFlowThread::contentWasLaidOut(LayoutUnit logicalBottomInFlowThreadAfterPagination)
 {
     // Check if we need another fragmentainer group. If we've run out of columns in the last
     // fragmentainer group (column row), we need to insert another fragmentainer group to hold more
@@ -952,7 +956,7 @@ void LayoutMultiColumnFlowThread::contentWasLaidOut(LayoutUnit logicalTopInFlowT
     bool mayBeNested = multiColumnBlockFlow()->isInsideFlowThread() || view()->fragmentationContext();
     if (!mayBeNested)
         return;
-    appendNewFragmentainerGroupIfNeeded(logicalTopInFlowThreadAfterPagination);
+    appendNewFragmentainerGroupIfNeeded(logicalBottomInFlowThreadAfterPagination);
 }
 
 }