Avoid stretching the parent of a column spanner.

Source/core/layout/LayoutPagedFlowThread.cpp

Index: Source/core/layout/LayoutPagedFlowThread.cpp
diff --git a/Source/core/layout/LayoutPagedFlowThread.cpp b/Source/core/layout/LayoutPagedFlowThread.cpp
index f43233e9143cdf268ce84e2809cb31cb5c9415d1..43105d001bc2e2daa969ca01a0d9fa6d4922b733 100644
--- a/Source/core/layout/LayoutPagedFlowThread.cpp
+++ b/Source/core/layout/LayoutPagedFlowThread.cpp
@@ -5,6 +5,8 @@
 #include "config.h"
 #include "core/layout/LayoutPagedFlowThread.h"
 
+#include "core/layout/LayoutMultiColumnSet.h"
+
 namespace blink {
 
 LayoutPagedFlowThread* LayoutPagedFlowThread::createAnonymous(Document& document, const ComputedStyle& parentStyle)
@@ -15,6 +17,13 @@ LayoutPagedFlowThread* LayoutPagedFlowThread::createAnonymous(Document& document
     return LayoutObject;
 }
 
+int LayoutPagedFlowThread::pageCount()
+{
+    if (LayoutMultiColumnSet* columnSet = firstMultiColumnSet())
+        return columnSet->actualColumnCount();
+    return 1;
+}
+
 bool LayoutPagedFlowThread::needsNewWidth() const
 {
     return progressionIsInline() != pagedBlockFlow()->style()->hasInlinePaginationAxis();
@@ -30,8 +39,21 @@ void LayoutPagedFlowThread::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