Region based multicol: support explicit column breaks

Source/core/rendering/RenderMultiColumnBlock.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 23 matching lines @@
 
 namespace WebCore {
 
 RenderMultiColumnBlock::RenderMultiColumnBlock(Element* element)
     : RenderBlockFlow(element)
     , m_flowThread(0)
     , m_columnCount(1)
     , m_columnWidth(0)
     , m_columnHeightAvailable(0)
     , m_inBalancingPass(false)
+    , m_needsRebalancing(false)
 {
 }
 
 void RenderMultiColumnBlock::styleDidChange(StyleDifference diff, const RenderStyle* oldStyle)
 {
     RenderBlock::styleDidChange(diff, oldStyle);
     for (RenderBox* child = firstChildBox(); child; child = child->nextSiblingBox())
         child->setStyle(RenderStyle::createAnonymousStyleWithDisplay(style(), BLOCK));
 }
 
@@ skipping 36 matching lines @@
 void RenderMultiColumnBlock::checkForPaginationLogicalHeightChange(LayoutUnit& /*pageLogicalHeight*/, bool& /*pageLogicalHeightChanged*/, bool& /*hasSpecifiedPageLogicalHeight*/)
 {
     // We don't actually update any of the variables. We just subclassed to adjust our column height.
     updateLogicalHeight();
     m_columnHeightAvailable = max<LayoutUnit>(contentLogicalHeight(), 0);
     setLogicalHeight(0);
 }
 
 bool RenderMultiColumnBlock::relayoutForPagination(bool, LayoutUnit, LayoutStateMaintainer& statePusher)
 {
-    if (m_inBalancingPass || !requiresBalancing())
+    if (m_inBalancingPass || !m_needsRebalancing)
         return false;
+    m_needsRebalancing = false;
     m_inBalancingPass = true; // Prevent re-entering this method (and recursion into layout).
 
     bool needsRelayout;
     bool neededRelayout = false;
     bool firstPass = true;
     do {
         // Column heights may change here because of balancing. We may have to do multiple layout
         // passes, depending on how the contents is fitted to the changed column heights. In most
         // cases, laying out again twice or even just once will suffice. Sometimes we need more
         // passes than that, though, but the number of retries should not exceed the number of
         // columns, unless we have a bug.
         needsRelayout = false;
         for (RenderBox* childBox = firstChildBox(); childBox; childBox = childBox->nextSiblingBox()) {
             if (childBox != m_flowThread && childBox->isRenderMultiColumnSet()) {
                 RenderMultiColumnSet* multicolSet = toRenderMultiColumnSet(childBox);
-                if (multicolSet->calculateBalancedHeight(firstPass)) {
+                if (multicolSet->recalculateBalancedHeight(firstPass)) {
                     multicolSet->setChildNeedsLayout(MarkOnlyThis);
                     needsRelayout = true;
                 }
             }
         }
 
         if (needsRelayout) {
             // Layout again. Column balancing resulted in a new height.
             neededRelayout = true;
             m_flowThread->setChildNeedsLayout(MarkOnlyThis);
@@ skipping 37 matching lines @@
             shouldInvalidateRegions = true;
         }
     }
 
     if (shouldInvalidateRegions)
         m_flowThread->invalidateRegions();
 
     if (relayoutChildren)
         layoutScope.setChildNeedsLayout(m_flowThread);
 
+    if (requiresBalancing()) {
+        // At the end of multicol layout, relayoutForPagination() is called unconditionally, but if
+        // no children are to be laid out (e.g. fixed width with layout already being up-to-date),
+        // we want to prevent it from doing any work, so that the column balancing machinery doesn't
+        // kick in and trigger additional unnecessary layout passes. Actually, it's not just a good
+        // idea in general to not waste time on balancing content that hasn't been re-laid out; we
+        // are actually required to guarantee this. The calculation of implicit breaks needs to be
+        // preceded by a proper layout pass, since it's layout that sets up content runs, and the
+        // runs get deleted right after every pass.
+        m_needsRebalancing = shouldInvalidateRegions || m_flowThread->needsLayout();
+    }
+
     setLogicalTopForChild(m_flowThread, borderBefore() + paddingBefore());
     m_flowThread->layoutIfNeeded();
     determineLogicalLeftPositionForChild(m_flowThread);
 
     return m_flowThread;
 }
 
 const char* RenderMultiColumnBlock::renderName() const
 {
     if (isFloating())
         return "RenderMultiColumnBlock (floating)";
     if (isOutOfFlowPositioned())
         return "RenderMultiColumnBlock (positioned)";
     if (isAnonymousBlock())
         return "RenderMultiColumnBlock (anonymous)";
     // FIXME: Temporary hack while the new generated content system is being implemented.
     if (isPseudoElement())
         return "RenderMultiColumnBlock (generated)";
     if (isAnonymous())
         return "RenderMultiColumnBlock (generated)";
     if (isRelPositioned())
         return "RenderMultiColumnBlock (relative positioned)";
     return "RenderMultiColumnBlock";
 }
 
 }