[New Multicolumn] Support for paged overflow.

Source/core/rendering/RenderBlockFlow.cpp

Index: Source/core/rendering/RenderBlockFlow.cpp
diff --git a/Source/core/rendering/RenderBlockFlow.cpp b/Source/core/rendering/RenderBlockFlow.cpp
index e5fadb246635972590f86ef26c5dc2cece8f79f8..d65f741ff88c6f6488d9b67be75c46320848aff7 100644
--- a/Source/core/rendering/RenderBlockFlow.cpp
+++ b/Source/core/rendering/RenderBlockFlow.cpp
@@ -39,6 +39,7 @@
 #include "core/rendering/RenderFlowThread.h"
 #include "core/rendering/RenderLayer.h"
 #include "core/rendering/RenderMultiColumnFlowThread.h"
+#include "core/rendering/RenderPagedFlowThread.h"
 #include "core/rendering/RenderText.h"
 #include "core/rendering/RenderView.h"
 #include "core/rendering/line/LineWidth.h"
@@ -1861,7 +1862,7 @@ void RenderBlockFlow::styleDidChange(StyleDifference diff, const RenderStyle* ol
     }
 
     if (diff.needsFullLayout() || !oldStyle)
-        createOrDestroyMultiColumnFlowThreadIfNeeded();
+        createOrDestroyMultiColumnFlowThreadIfNeeded(oldStyle);
 }
 
 void RenderBlockFlow::updateStaticInlinePositionForChild(RenderBox* child, LayoutUnit logicalTop)
@@ -2779,25 +2780,49 @@ RootInlineBox* RenderBlockFlow::createRootInlineBox()
     return new RootInlineBox(*this);
 }
 
-void RenderBlockFlow::createOrDestroyMultiColumnFlowThreadIfNeeded()
+static inline bool isPagedOverflow(const RenderBlockFlow* renderer, const RenderStyle* style)
+{
+    return style->isOverflowPaged() && renderer->node() != renderer->document().viewportDefiningElement();

[rune, 2014/06/18 22:09:05]

"this" is always passed as renderer. Why not make this a class method?

+}
+
+void RenderBlockFlow::createOrDestroyMultiColumnFlowThreadIfNeeded(const RenderStyle* oldStyle)
 {
     if (!document().regionBasedColumnsEnabled())
         return;
 
-    bool needsFlowThread = style()->specifiesColumns();
-    if (needsFlowThread != static_cast<bool>(multiColumnFlowThread())) {
-        if (needsFlowThread) {
-            RenderMultiColumnFlowThread* flowThread = RenderMultiColumnFlowThread::createAnonymous(document(), style());
-            addChild(flowThread);
-            flowThread->populate();
-            RenderBlockFlowRareData& rareData = ensureRareData();
-            ASSERT(!rareData.m_multiColumnFlowThread);
-            rareData.m_multiColumnFlowThread = flowThread;
-        } else {
+    // Paged overflow trumps multicol in this implementation. Ideally, it should be possible to have
+    // both paged overflow and multicol on the same element, but then we need two flow
+    // threads. Anyway, this is nothing to worry about until we can actually nest multicol properly
+    // inside other fragmentation contexts.
+    bool shouldBePaged = isPagedOverflow(this, style());
+    bool shouldBeMulticol = !shouldBePaged && style()->specifiesColumns();
+

[rune, 2014/06/18 22:09:05]

We can get rid of this code duplication. Also we can have an enumerated type
instead of two booleans.

How about (modulo naming):

FlowThreadType { None, MultiColumn, Paged }

FlowThreadType flowThreadType(const RenderStyle* style) {
    if (isPagedOverflow(style))
        return Paged;
    if (style->specifiesColumns())
        return MultiColumn;
    return None;
}

and then  compare new/old FlowThreadType.

+    if (multiColumnFlowThread()) {
+        ASSERT(oldStyle);
+        bool wasPaged = isPagedOverflow(this, oldStyle);
+        bool wasMulticol = !wasPaged && oldStyle->specifiesColumns();
+        if (wasMulticol != shouldBeMulticol || wasPaged != shouldBePaged) {
+            // If we're no longer to be multicol/paged, destroy the flow thread. Also destroy it
+            // when switching between multicol and paged, since that affects the column set
+            // structure (multicol containers may have spanners, paged containers may not).
             multiColumnFlowThread()->evacuateAndDestroy();
             ASSERT(!multiColumnFlowThread());
         }
     }
+    if ((shouldBeMulticol || shouldBePaged) && !multiColumnFlowThread()) {
+        RenderMultiColumnFlowThread* flowThread;
+        if (shouldBePaged) {
+            // Paged overflow is currently done using the multicol implementation.
+            flowThread = RenderPagedFlowThread::createAnonymous(document(), style());
+        } else {
+            flowThread = RenderMultiColumnFlowThread::createAnonymous(document(), style());
+        }
+        addChild(flowThread);
+        flowThread->populate();
+        RenderBlockFlowRareData& rareData = ensureRareData();
+        ASSERT(!rareData.m_multiColumnFlowThread);
+        rareData.m_multiColumnFlowThread = flowThread;
+    }
 }
 
 RenderBlockFlow::RenderBlockFlowRareData& RenderBlockFlow::ensureRareData()