Avoid stretching the parent of a column spanner.

Source/core/layout/LayoutPagedFlowThread.cpp

 // Copyright 2014 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 "config.h"
 #include "core/layout/LayoutPagedFlowThread.h"
 
+#include "core/layout/LayoutMultiColumnSet.h"
+
 namespace blink {
 
 LayoutPagedFlowThread* LayoutPagedFlowThread::createAnonymous(Document& document, const ComputedStyle& parentStyle)
 {
     LayoutPagedFlowThread* LayoutObject = new LayoutPagedFlowThread();
     LayoutObject->setDocumentForAnonymous(&document);
     LayoutObject->setStyle(ComputedStyle::createAnonymousStyleWithDisplay(parentStyle, BLOCK));
     return LayoutObject;
 }
 
+int LayoutPagedFlowThread::pageCount()
+{
+    if (LayoutMultiColumnSet* columnSet = firstMultiColumnSet())
+        return columnSet->actualColumnCount();
+    return 1;
+}
+
 bool LayoutPagedFlowThread::needsNewWidth() const
 {
     return progressionIsInline() != pagedBlockFlow()->style()->hasInlinePaginationAxis();
 }
 
 void LayoutPagedFlowThread::updateLogicalWidth()
 {
     // As long as we inherit from LayoutMultiColumnFlowThread, we need to bypass its implementation
     // here. We're not split into columns, so the flow thread width will just be whatever is
     // available in the containing block.
     LayoutFlowThread::updateLogicalWidth();
 }
 
 void LayoutPagedFlowThread::layout()
 {
+    ASSERT(firstMultiColumnBox() == lastMultiColumnBox()); // There should either be zero or one of those for paged layout.
     setProgressionIsInline(pagedBlockFlow()->style()->hasInlinePaginationAxis());
     LayoutMultiColumnFlowThread::layout();
+
+    LayoutMultiColumnSet* columnSet = firstMultiColumnSet();
+    if (!columnSet)
+        return;
+    LayoutUnit pageLogicalHeight = columnSet->pageLogicalHeight();
+    if (!pageLogicalHeight)
+        return; // Page height not calculated yet. Happens in the first layout pass when height is auto.
+    // Ensure uniform page height. We don't want the last page to be shorter than the others,
+    // or it'll be impossible to scroll that whole page into view.
+    LayoutUnit paddedLogicalBottomInFlowThread = pageLogicalHeight * pageCount();
+    ASSERT(paddedLogicalBottomInFlowThread >= columnSet->logicalBottomInFlowThread());
+    columnSet->endFlow(paddedLogicalBottomInFlowThread);
 }
 
 } // namespace blink