Jump to the next outer column when an inner column is too short.

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

 /*
  * Copyright (C) 2012 Apple Inc.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 79 matching lines @@
         remainingLogicalHeight = intMod(remainingLogicalHeight, pageLogicalHeight);
     }
     return remainingLogicalHeight;
 }
 
 bool LayoutMultiColumnSet::isPageLogicalHeightKnown() const
 {
     return firstFragmentainerGroup().logicalHeight();
 }
 
+LayoutUnit LayoutMultiColumnSet::nextLogicalTopForUnbreakableContent(LayoutUnit flowThreadOffset, LayoutUnit contentLogicalHeight) const
+{
+    ASSERT(pageLogicalTopForOffset(flowThreadOffset) == flowThreadOffset); // That's all we bother to handle.

[leviw_travelin_and_unemployed, 2015/11/25 19:26:34]

Nit: This comment doesn't help me.

[mstensho (USE GERRIT), 2015/11/25 20:08:17]

Removed.

"|flowThreadOffset| is expected to be at the exact top of a column." says
LayoutFlowThread::nextLogicalTopForUnbreakableContent(). But that's exactly what
the assertion checks, so the comment is redundant.

+    LayoutMultiColumnFlowThread* enclosingFlowThread = multiColumnFlowThread()->enclosingFlowThread();
+    if (!enclosingFlowThread) {
+        // If there's no enclosing fragmentation context, there'll ever be only one row, and all
+        // columns there will have the same height.
+        return flowThreadOffset;
+    }
+
+    if (pageLogicalHeightForOffset(flowThreadOffset) >= contentLogicalHeight)

[leviw_travelin_and_unemployed, 2015/11/25 19:26:34]

This makes sense, but it appears in the current code it'll never be hit?
LayoutBlock::calculatePaginationStrutToFitContent has the same check.

[mstensho (USE GERRIT), 2015/11/25 20:08:17]

Right. No need to be that robust. Let's add an assertion instead.

[leviw_travelin_and_unemployed, 2015/11/25 22:00:49]

I was hoping you'd say that ;)

+        return flowThreadOffset;
+
+    // There's a likelihood for subsequent rows to be taller than the first one.
+    // TODO(mstensho): if we're doubly nested (e.g. multicol in multicol in multicol), we need to
+    // look beyond the first row here.

[leviw_travelin_and_unemployed, 2015/11/25 19:26:34]

http://i.imgur.com/EqQOTR9.jpg

[mstensho (USE GERRIT), 2015/11/25 20:08:17]

http://renholdsnytt.no/var/askmedia/storage/images/media/gloriapark/diverse/z...

[leviw_travelin_and_unemployed, 2015/11/25 22:00:49]

HA!

+    const MultiColumnFragmentainerGroup& firstRow = firstFragmentainerGroup();
+    LayoutUnit firstRowLogicalBottomInFlowThread = firstRow.logicalTopInFlowThread() + firstRow.logicalHeight() * usedColumnCount();
+    if (flowThreadOffset >= firstRowLogicalBottomInFlowThread)
+        return flowThreadOffset; // We're not in the first row. Give up.
+    LayoutUnit newLogicalHeight = enclosingFlowThread->pageLogicalHeightForOffset(firstRowLogicalBottomInFlowThread);
+    if (contentLogicalHeight > newLogicalHeight) {
+        // The next outer column or page doesn't have enough space either. Give up and stay where
+        // we are.
+        return flowThreadOffset;
+    }
+    return firstRowLogicalBottomInFlowThread;
+}
+
 LayoutMultiColumnSet* LayoutMultiColumnSet::nextSiblingMultiColumnSet() const
 {
     for (LayoutObject* sibling = nextSibling(); sibling; sibling = sibling->nextSibling()) {
         if (sibling->isLayoutMultiColumnSet())
             return toLayoutMultiColumnSet(sibling);
     }
     return nullptr;
 }
 
 LayoutMultiColumnSet* LayoutMultiColumnSet::previousSiblingMultiColumnSet() const
@@ skipping 240 matching lines @@
 
 LayoutRect LayoutMultiColumnSet::flowThreadPortionRect() const
 {
     LayoutRect portionRect(LayoutUnit(), logicalTopInFlowThread(), pageLogicalWidth(), logicalHeightInFlowThread());
     if (!isHorizontalWritingMode())
         return portionRect.transposedRect();
     return portionRect;
 }
 
 }