[New Multicolumn] Let a spanner's containing block be the multicol container.

Source/core/rendering/RenderMultiColumnSpannerPlaceholder.cpp

Index: Source/core/rendering/RenderMultiColumnSpannerPlaceholder.cpp
diff --git a/Source/core/rendering/RenderMultiColumnSpannerPlaceholder.cpp b/Source/core/rendering/RenderMultiColumnSpannerPlaceholder.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..bdef00e0c3a78a93f72e3e20c153993feba1fae9
--- /dev/null
+++ b/Source/core/rendering/RenderMultiColumnSpannerPlaceholder.cpp
@@ -0,0 +1,75 @@
+// 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/rendering/RenderMultiColumnSpannerPlaceholder.h"
+
+namespace blink {
+
+RenderMultiColumnSpannerPlaceholder* RenderMultiColumnSpannerPlaceholder::createAnonymous(RenderStyle* parentStyle, RenderBox* rendererInFlowThread)
+{
+    RenderMultiColumnSpannerPlaceholder* newSpanner = new RenderMultiColumnSpannerPlaceholder(rendererInFlowThread);
+    Document& document = rendererInFlowThread->document();
+    newSpanner->setDocumentForAnonymous(&document);
+    RefPtr<RenderStyle> newStyle = RenderStyle::createAnonymousStyleWithDisplay(parentStyle, BLOCK);
+    newSpanner->setStyle(newStyle);
+    return newSpanner;
+}
+
+RenderMultiColumnSpannerPlaceholder::RenderMultiColumnSpannerPlaceholder(RenderBox* rendererInFlowThread)
+    : RenderBox(0)
+    , m_rendererInFlowThread(rendererInFlowThread)
+{
+}
+
+void RenderMultiColumnSpannerPlaceholder::spannerWillBeRemoved()
+{
+    ASSERT(m_rendererInFlowThread);
+    RenderBox* renderer = m_rendererInFlowThread;
+    m_rendererInFlowThread = 0;
+    flowThread()->flowThreadDescendantWillBeRemoved(renderer);
+    // |this| should be destroyed by now.
+}
+
+void RenderMultiColumnSpannerPlaceholder::willBeRemovedFromTree()
+{
+    if (m_rendererInFlowThread)
+        m_rendererInFlowThread->setSpannerPlaceholder(0);
+    RenderBox::willBeRemovedFromTree();
+}
+
+bool RenderMultiColumnSpannerPlaceholder::needsPreferredWidthsRecalculation() const
+{
+    return m_rendererInFlowThread->needsPreferredWidthsRecalculation();
+}
+
+void RenderMultiColumnSpannerPlaceholder::layout()
+{
+    ASSERT(needsLayout());
+
+    // FIXME: actual spanner positioning isn't implemented yet. Just set it to 0,0 for consistency
+    // (in case the spanner used to be something else that was laid out properly).
+    m_rendererInFlowThread->setLogicalTop(LayoutUnit());
+    m_rendererInFlowThread->setLogicalLeft(LayoutUnit());
+
+    // Lay out the actual column-span:all element.
+    m_rendererInFlowThread->layoutIfNeeded();
+
+    clearNeedsLayout();
+}
+
+void RenderMultiColumnSpannerPlaceholder::invalidateTreeIfNeeded(const PaintInvalidationState& paintInvalidationState)
+{
+    if (paintInvalidationStateIsDirty())
+        clearPaintInvalidationState(paintInvalidationState);
+    PaintInvalidationState newPaintInvalidationState(paintInvalidationState, *this, paintInvalidationState.paintInvalidationContainer());
+    m_rendererInFlowThread->invalidateTreeIfNeeded(newPaintInvalidationState);

[Julien - ping for review, 2014/12/11 19:09:38]

A normal implementation would call into the base class to generate the
invalidations, clear the flags and make sure the children generate the
invalidations.

We reuse a RenderBox but it seems like this object could be anything and not
just a RenderObject, is it possible this kind of weird functions points at this
flaw in the design?

[mstensho (USE GERRIT), 2014/12/11 21:01:14]

There'll be no children, but yes, I really think we should let the base class do
whatever it wants to do.

Done.
What flaw? Mine? :) Sorry, I don't quite understand.

[Julien - ping for review, 2014/12/11 23:45:44]

The fact that we need such fake overridden function may point to a fault in the
design. That's what I tried to say but re-reading the comment, I am less
convinced (and my usual argument for that is RenderScrollbar, which is a
RenderBox, but I was convinced it's not crazy).

+}
+
+const char* RenderMultiColumnSpannerPlaceholder::renderName() const
+{
+    return "RenderMultiColumnSpannerPlaceholder";
+}
+
+}