[New Multicolumn] Preparatory work for nested multicol support.

Source/core/rendering/RenderMultiColumnSet.cpp

Index: Source/core/rendering/RenderMultiColumnSet.cpp
diff --git a/Source/core/rendering/RenderMultiColumnSet.cpp b/Source/core/rendering/RenderMultiColumnSet.cpp
index 7d53c06537eb2e609e40a57a550c6038e808daee..0ed1a9dba234316f28173a28844b46913c0cb36b 100644
--- a/Source/core/rendering/RenderMultiColumnSet.cpp
+++ b/Source/core/rendering/RenderMultiColumnSet.cpp
@@ -26,6 +26,7 @@
 #include "config.h"
 #include "core/rendering/RenderMultiColumnSet.h"
 
+#include "core/layout/MultiColumnFragmentainerGroup.h"
 #include "core/paint/MultiColumnSetPainter.h"
 #include "core/rendering/RenderMultiColumnFlowThread.h"
 
@@ -33,10 +34,7 @@ namespace blink {
 
 RenderMultiColumnSet::RenderMultiColumnSet(RenderFlowThread* flowThread)
     : RenderRegion(0, flowThread)
-    , m_columnHeight(0)
-    , m_maxColumnHeight(RenderFlowThread::maxLogicalHeight())
-    , m_minSpaceShortage(RenderFlowThread::maxLogicalHeight())
-    , m_minimumColumnHeight(0)
+    , m_fragmentainerGroups(*this)
 {
 }
 
@@ -49,6 +47,25 @@ RenderMultiColumnSet* RenderMultiColumnSet::createAnonymous(RenderFlowThread& fl
     return renderer;
 }
 
+MultiColumnFragmentainerGroup& RenderMultiColumnSet::fragmentainerGroupAtFlowThreadOffset(LayoutUnit)
+{
+    // FIXME: implement this, once we have support for multiple rows.
+    return m_fragmentainerGroups.first();
+}
+
+const MultiColumnFragmentainerGroup& RenderMultiColumnSet::fragmentainerGroupAtFlowThreadOffset(LayoutUnit) const
+{
+    // FIXME: implement this, once we have support for multiple rows.
+    return m_fragmentainerGroups.first();
+}
+
+LayoutUnit RenderMultiColumnSet::pageLogicalHeight() const
+{
+    // FIXME: pageLogicalHeight() needs to take a flow thread offset parameter, so that we can
+    // locate the right row.
+    return firstFragmentainerGroup().logicalHeight();
+}
+
 RenderMultiColumnSet* RenderMultiColumnSet::nextSiblingMultiColumnSet() const
 {
     for (LayoutObject* sibling = nextSibling(); sibling; sibling = sibling->nextSibling()) {
@@ -67,24 +84,14 @@ RenderMultiColumnSet* RenderMultiColumnSet::previousSiblingMultiColumnSet() cons
     return 0;
 }
 
-void RenderMultiColumnSet::setLogicalTopInFlowThread(LayoutUnit logicalTop)
+LayoutUnit RenderMultiColumnSet::logicalTopInFlowThread() const
 {
-    LayoutRect rect = flowThreadPortionRect();
-    if (isHorizontalWritingMode())
-        rect.setY(logicalTop);
-    else
-        rect.setX(logicalTop);
-    setFlowThreadPortionRect(rect);
+    return firstFragmentainerGroup().logicalTopInFlowThread();
 }
 
-void RenderMultiColumnSet::setLogicalBottomInFlowThread(LayoutUnit logicalBottom)
+LayoutUnit RenderMultiColumnSet::logicalBottomInFlowThread() const
 {
-    LayoutRect rect = flowThreadPortionRect();
-    if (isHorizontalWritingMode())
-        rect.shiftMaxYEdgeTo(logicalBottom);
-    else
-        rect.shiftMaxXEdgeTo(logicalBottom);
-    setFlowThreadPortionRect(rect);
+    return lastFragmentainerGroup().logicalBottomInFlowThread();
 }
 
 bool RenderMultiColumnSet::heightIsAuto() const
@@ -105,245 +112,79 @@ bool RenderMultiColumnSet::heightIsAuto() const
 
 LayoutSize RenderMultiColumnSet::flowThreadTranslationAtOffset(LayoutUnit blockOffset) const
 {
-    unsigned columnIndex = columnIndexAtOffset(blockOffset);
-    LayoutRect portionRect(flowThreadPortionRectAt(columnIndex));
-    flipForWritingMode(portionRect);
-    LayoutRect columnRect(columnRectAt(columnIndex));
-    flipForWritingMode(columnRect);
-    return toLayoutPoint(contentBoxOffset()) + columnRect.location() - portionRect.location();
-}
-
-LayoutUnit RenderMultiColumnSet::heightAdjustedForSetOffset(LayoutUnit height) const
-{
-    // Adjust for the top offset within the content box of the multicol container (containing
-    // block), unless this is the first set. We know that the top offset for the first set will be
-    // zero, but if the multicol container has non-zero top border or padding, the set's top offset
-    // (initially being 0 and relative to the border box) will be negative until it has been laid
-    // out. Had we used this bogus offset, we would calculate the wrong height, and risk performing
-    // a wasted layout iteration. Of course all other sets (if any) have this problem in the first
-    // layout pass too, but there's really nothing we can do there until the flow thread has been
-    // laid out anyway.
-    if (previousSiblingMultiColumnSet()) {
-        RenderBlockFlow* multicolBlock = multiColumnBlockFlow();
-        LayoutUnit contentLogicalTop = logicalTop() - multicolBlock->borderAndPaddingBefore();
-        height -= contentLogicalTop;
-    }
-    return max(height, LayoutUnit(1)); // Let's avoid zero height, as that would probably cause an infinite amount of columns to be created.
+    const MultiColumnFragmentainerGroup& row = fragmentainerGroupAtFlowThreadOffset(blockOffset);
+    return row.offsetFromColumnSet() + row.flowThreadTranslationAtOffset(blockOffset);
 }
 
-LayoutUnit RenderMultiColumnSet::pageLogicalTopForOffset(LayoutUnit offset) const
+void RenderMultiColumnSet::updateMinimumColumnHeight(LayoutUnit offsetInFlowThread, LayoutUnit height)
 {
-    unsigned columnIndex = columnIndexAtOffset(offset, AssumeNewColumns);
-    return logicalTopInFlowThread() + columnIndex * pageLogicalHeight();
+    fragmentainerGroupAtFlowThreadOffset(offsetInFlowThread).updateMinimumColumnHeight(height);
 }
 
-void RenderMultiColumnSet::setAndConstrainColumnHeight(LayoutUnit newHeight)
-{
-    m_columnHeight = newHeight;
-    if (m_columnHeight > m_maxColumnHeight)
-        m_columnHeight = m_maxColumnHeight;
-    // FIXME: the height may also be affected by the enclosing pagination context, if any.
-}
-
-unsigned RenderMultiColumnSet::findRunWithTallestColumns() const
+LayoutUnit RenderMultiColumnSet::pageLogicalTopForOffset(LayoutUnit offset) const
 {
-    unsigned indexWithLargestHeight = 0;
-    LayoutUnit largestHeight;
-    LayoutUnit previousOffset = logicalTopInFlowThread();
-    size_t runCount = m_contentRuns.size();
-    ASSERT(runCount);
-    for (size_t i = 0; i < runCount; i++) {
-        const ContentRun& run = m_contentRuns[i];
-        LayoutUnit height = run.columnLogicalHeight(previousOffset);
-        if (largestHeight < height) {
-            largestHeight = height;
-            indexWithLargestHeight = i;
-        }
-        previousOffset = run.breakOffset();
-    }
-    return indexWithLargestHeight;
-}
-
-void RenderMultiColumnSet::distributeImplicitBreaks()
-{
-#if ENABLE(ASSERT)
-    // There should be no implicit breaks assumed at this point.
-    for (unsigned i = 0; i < m_contentRuns.size(); i++)
-        ASSERT(!m_contentRuns[i].assumedImplicitBreaks());
-#endif // ENABLE(ASSERT)
-
-    // Insert a final content run to encompass all content. This will include overflow if this is
-    // the last set.
-    addContentRun(logicalBottomInFlowThread());
-    unsigned columnCount = m_contentRuns.size();
-
-    // If there is room for more breaks (to reach the used value of column-count), imagine that we
-    // insert implicit breaks at suitable locations. At any given time, the content run with the
-    // currently tallest columns will get another implicit break "inserted", which will increase its
-    // column count by one and shrink its columns' height. Repeat until we have the desired total
-    // number of breaks. The largest column height among the runs will then be the initial column
-    // height for the balancer to use.
-    while (columnCount < usedColumnCount()) {
-        unsigned index = findRunWithTallestColumns();
-        m_contentRuns[index].assumeAnotherImplicitBreak();
-        columnCount++;
-    }
-}
-
-LayoutUnit RenderMultiColumnSet::calculateColumnHeight(BalancedHeightCalculation calculationMode) const
-{
-    if (calculationMode == GuessFromFlowThreadPortion) {
-        // Initial balancing. Start with the lowest imaginable column height. We use the tallest
-        // content run (after having "inserted" implicit breaks), and find its start offset (by
-        // looking at the previous run's end offset, or, if there's no previous run, the set's start
-        // offset in the flow thread).
-        unsigned index = findRunWithTallestColumns();
-        LayoutUnit startOffset = index > 0 ? m_contentRuns[index - 1].breakOffset() : logicalTopInFlowThread();
-        return std::max<LayoutUnit>(m_contentRuns[index].columnLogicalHeight(startOffset), m_minimumColumnHeight);
-    }
-
-    if (actualColumnCount() <= usedColumnCount()) {
-        // With the current column height, the content fits without creating overflowing columns. We're done.
-        return m_columnHeight;
-    }
-
-    if (m_contentRuns.size() >= usedColumnCount()) {
-        // Too many forced breaks to allow any implicit breaks. Initial balancing should already
-        // have set a good height. There's nothing more we should do.
-        return m_columnHeight;
-    }
-
-    // If the initial guessed column height wasn't enough, stretch it now. Stretch by the lowest
-    // amount of space shortage found during layout.
-
-    ASSERT(m_minSpaceShortage > 0); // We should never _shrink_ the height!
-    ASSERT(m_minSpaceShortage != RenderFlowThread::maxLogicalHeight()); // If this happens, we probably have a bug.
-    if (m_minSpaceShortage == RenderFlowThread::maxLogicalHeight())
-        return m_columnHeight; // So bail out rather than looping infinitely.
-
-    return m_columnHeight + m_minSpaceShortage;
+    return fragmentainerGroupAtFlowThreadOffset(offset).columnLogicalTopForOffset(offset);
 }
 
 void RenderMultiColumnSet::addContentRun(LayoutUnit endOffsetFromFirstPage)
 {
     if (!heightIsAuto())
         return;
-    if (!m_contentRuns.isEmpty() && endOffsetFromFirstPage <= m_contentRuns.last().breakOffset())
-        return;
-    // Append another item as long as we haven't exceeded used column count. What ends up in the
-    // overflow area shouldn't affect column balancing.
-    if (m_contentRuns.size() < usedColumnCount())
-        m_contentRuns.append(ContentRun(endOffsetFromFirstPage));
+    fragmentainerGroupAtFlowThreadOffset(endOffsetFromFirstPage).addContentRun(endOffsetFromFirstPage);
 }
 
-bool RenderMultiColumnSet::recalculateColumnHeight(BalancedHeightCalculation calculationMode)
+bool RenderMultiColumnSet::recalculateColumnHeight(BalancedColumnHeightCalculation calculationMode)
 {
-    LayoutUnit oldColumnHeight = m_columnHeight;
-
-    m_maxColumnHeight = calculateMaxColumnHeight();
-
-    if (heightIsAuto()) {
-        if (calculationMode == GuessFromFlowThreadPortion) {
-            // Post-process the content runs and find out where the implicit breaks will occur.
-            distributeImplicitBreaks();
-        }
-        LayoutUnit newColumnHeight = calculateColumnHeight(calculationMode);
-        setAndConstrainColumnHeight(newColumnHeight);
-        // After having calculated an initial column height, the multicol container typically needs at
-        // least one more layout pass with a new column height, but if a height was specified, we only
-        // need to do this if we think that we need less space than specified. Conversely, if we
-        // determined that the columns need to be as tall as the specified height of the container, we
-        // have already laid it out correctly, and there's no need for another pass.
-    } else {
-        // The position of the column set may have changed, in which case height available for
-        // columns may have changed as well.
-        setAndConstrainColumnHeight(m_columnHeight);
-    }
-
-    // We can get rid of the content runs now, if we haven't already done so. They are only needed
-    // to calculate the initial balanced column height. In fact, we have to get rid of them before
-    // the next layout pass, since each pass will rebuild this.
-    m_contentRuns.clear();
-
-    if (m_columnHeight == oldColumnHeight)
-        return false; // No change. We're done.
-
-    m_minSpaceShortage = RenderFlowThread::maxLogicalHeight();
-    return true; // Need another pass.
+    bool changed = false;
+    for (auto& group : m_fragmentainerGroups)
+        changed = group.recalculateColumnHeight(calculationMode) || changed;
+    return changed;
 }
 
-void RenderMultiColumnSet::recordSpaceShortage(LayoutUnit spaceShortage)
+void RenderMultiColumnSet::recordSpaceShortage(LayoutUnit offsetInFlowThread, LayoutUnit spaceShortage)
 {
-    if (spaceShortage >= m_minSpaceShortage)
-        return;
-
-    // The space shortage is what we use as our stretch amount. We need a positive number here in
-    // order to get anywhere.
-    ASSERT(spaceShortage > 0);
-
-    m_minSpaceShortage = spaceShortage;
+    fragmentainerGroupAtFlowThreadOffset(offsetInFlowThread).recordSpaceShortage(spaceShortage);
 }
 
 void RenderMultiColumnSet::resetColumnHeight()
 {
-    // Nuke previously stored minimum column height. Contents may have changed for all we know.
-    m_minimumColumnHeight = 0;
-
-    m_maxColumnHeight = calculateMaxColumnHeight();
-
-    LayoutUnit oldColumnHeight = pageLogicalHeight();
-
-    if (heightIsAuto())
-        m_columnHeight = 0;
-    else
-        setAndConstrainColumnHeight(heightAdjustedForSetOffset(multiColumnFlowThread()->columnHeightAvailable()));
+    m_fragmentainerGroups.deleteExtraGroups();
+    m_fragmentainerGroups.first().resetColumnHeight();
+}
 
-    if (pageLogicalHeight() != oldColumnHeight)
-        setChildNeedsLayout(MarkOnlyThis);
+void RenderMultiColumnSet::beginFlow(LayoutUnit offsetInFlowThread)
+{
+    // At this point layout is exactly at the beginning of this set. Store block offset from flow
+    // thread start.
+    m_fragmentainerGroups.first().setLogicalTopInFlowThread(offsetInFlowThread);
+}
 
-    // Content runs are only needed in the initial layout pass, in order to find an initial column
-    // height, and should have been deleted afterwards. We're about to rebuild the content runs, so
-    // the list needs to be empty.
-    ASSERT(m_contentRuns.isEmpty());
+void RenderMultiColumnSet::endFlow(LayoutUnit offsetInFlowThread)
+{
+    // At this point layout is exactly at the end of this set. Store block offset from flow thread
+    // start. This set is now considered "flowed", although we may have to revisit it later (with
+    // beginFlow()), e.g. if a subtree in the flow thread has to be laid out over again because the
+    // initial margin collapsing estimates were wrong.
+    m_fragmentainerGroups.last().setLogicalBottomInFlowThread(offsetInFlowThread);
 }
 
 void RenderMultiColumnSet::expandToEncompassFlowThreadContentsIfNeeded()
 {
     ASSERT(multiColumnFlowThread()->lastMultiColumnSet() == this);
-    LayoutRect rect(flowThreadPortionRect());
-
-    // Get the offset within the flow thread in its block progression direction. Then get the
-    // flow thread's remaining logical height including its overflow and expand our rect
-    // to encompass that remaining height and overflow. The idea is that we will generate
-    // additional columns and pages to hold that overflow, since people do write bad
-    // content like <body style="height:0px"> in multi-column layouts.
-    bool isHorizontal = flowThread()->isHorizontalWritingMode();
-    LayoutUnit logicalTopOffset = isHorizontal ? rect.y() : rect.x();
-    LayoutRect layoutRect = flowThread()->layoutOverflowRect();
-    LayoutUnit logicalHeightWithOverflow = (isHorizontal ? layoutRect.maxY() : layoutRect.maxX()) - logicalTopOffset;
-    setFlowThreadPortionRect(LayoutRect(rect.x(), rect.y(), isHorizontal ? rect.width() : logicalHeightWithOverflow, isHorizontal ? logicalHeightWithOverflow : rect.height()));
+    // FIXME: this may result in the need for creating additional rows, since there may not be
+    // enough space remaining in the currently last row.
+    m_fragmentainerGroups.last().expandToEncompassFlowThreadOverflow();
 }
 
 void RenderMultiColumnSet::computeLogicalHeight(LayoutUnit, LayoutUnit logicalTop, LogicalExtentComputedValues& computedValues) const
 {
-    computedValues.m_extent = m_columnHeight;
+    LayoutUnit logicalHeight;
+    for (const auto& group : m_fragmentainerGroups)
+        logicalHeight += group.logicalHeight();
+    computedValues.m_extent = logicalHeight;
     computedValues.m_position = logicalTop;
 }
 
-LayoutUnit RenderMultiColumnSet::calculateMaxColumnHeight() const
-{
-    RenderBlockFlow* multicolBlock = multiColumnBlockFlow();
-    const LayoutStyle& multicolStyle = multicolBlock->styleRef();
-    LayoutUnit availableHeight = multiColumnFlowThread()->columnHeightAvailable();
-    LayoutUnit maxColumnHeight = availableHeight ? availableHeight : RenderFlowThread::maxLogicalHeight();
-    if (!multicolStyle.logicalMaxHeight().isMaxSizeNone()) {
-        LayoutUnit logicalMaxHeight = multicolBlock->computeContentLogicalHeight(multicolStyle.logicalMaxHeight(), -1);
-        if (logicalMaxHeight != -1 && maxColumnHeight > logicalMaxHeight)
-            maxColumnHeight = logicalMaxHeight;
-    }
-    return heightAdjustedForSetOffset(maxColumnHeight);
-}
-
 LayoutUnit RenderMultiColumnSet::columnGap() const
 {
     RenderBlockFlow* parentBlock = multiColumnBlockFlow();
@@ -354,110 +195,9 @@ LayoutUnit RenderMultiColumnSet::columnGap() const
 
 unsigned RenderMultiColumnSet::actualColumnCount() const
 {
-    // We must always return a value of 1 or greater. Column count = 0 is a meaningless situation,
-    // and will confuse and cause problems in other parts of the code.
-    if (!pageLogicalHeight())
-        return 1;
-
-    // Our portion rect determines our column count. We have as many columns as needed to fit all the content.
-    LayoutUnit logicalHeightInColumns = flowThread()->isHorizontalWritingMode() ? flowThreadPortionRect().height() : flowThreadPortionRect().width();
-    if (!logicalHeightInColumns)
-        return 1;
-
-    unsigned count = ceil(logicalHeightInColumns.toFloat() / pageLogicalHeight().toFloat());
-    ASSERT(count >= 1);
-    return count;
-}
-
-LayoutRect RenderMultiColumnSet::columnRectAt(unsigned index) const
-{
-    LayoutUnit colLogicalWidth = pageLogicalWidth();
-    LayoutUnit colLogicalHeight = pageLogicalHeight();
-    LayoutUnit colLogicalTop = borderBefore() + paddingBefore();
-    LayoutUnit colLogicalLeft = borderAndPaddingLogicalLeft();
-    LayoutUnit colGap = columnGap();
-
-    if (multiColumnFlowThread()->progressionIsInline()) {
-        if (style()->isLeftToRightDirection())
-            colLogicalLeft += index * (colLogicalWidth + colGap);
-        else
-            colLogicalLeft += contentLogicalWidth() - colLogicalWidth - index * (colLogicalWidth + colGap);
-    } else {
-        colLogicalTop += index * (colLogicalHeight + colGap);
-    }
-
-    if (isHorizontalWritingMode())
-        return LayoutRect(colLogicalLeft, colLogicalTop, colLogicalWidth, colLogicalHeight);
-    return LayoutRect(colLogicalTop, colLogicalLeft, colLogicalHeight, colLogicalWidth);
-}
-
-unsigned RenderMultiColumnSet::columnIndexAtOffset(LayoutUnit offset, ColumnIndexCalculationMode mode) const
-{
-    LayoutRect portionRect(flowThreadPortionRect());
-
-    // Handle the offset being out of range.
-    LayoutUnit flowThreadLogicalTop = isHorizontalWritingMode() ? portionRect.y() : portionRect.x();
-    if (offset < flowThreadLogicalTop)
-        return 0;
-    // If we're laying out right now, we cannot constrain against some logical bottom, since it
-    // isn't known yet. Otherwise, just return the last column if we're past the logical bottom.
-    if (mode == ClampToExistingColumns) {
-        LayoutUnit flowThreadLogicalBottom = isHorizontalWritingMode() ? portionRect.maxY() : portionRect.maxX();
-        if (offset >= flowThreadLogicalBottom)
-            return actualColumnCount() - 1;
-    }
-
-    if (LayoutUnit pageLogicalHeight = this->pageLogicalHeight())
-        return (offset - flowThreadLogicalTop).toFloat() / pageLogicalHeight.toFloat();
-
-    return 0;
-}
-
-LayoutRect RenderMultiColumnSet::flowThreadPortionRectAt(unsigned index) const
-{
-    LayoutRect portionRect = flowThreadPortionRect();
-    if (isHorizontalWritingMode())
-        portionRect = LayoutRect(portionRect.x(), portionRect.y() + index * pageLogicalHeight(), portionRect.width(), pageLogicalHeight());
-    else
-        portionRect = LayoutRect(portionRect.x() + index * pageLogicalHeight(), portionRect.y(), pageLogicalHeight(), portionRect.height());
-    return portionRect;
-}
-
-LayoutRect RenderMultiColumnSet::flowThreadPortionOverflowRect(const LayoutRect& portionRect, unsigned index, unsigned colCount, LayoutUnit colGap) const
-{
-    // This function determines the portion of the flow thread that paints for the column. Along the inline axis, columns are
-    // unclipped at outside edges (i.e., the first and last column in the set), and they clip to half the column
-    // gap along interior edges.
-    //
-    // In the block direction, we will not clip overflow out of the top of the first column, or out of the bottom of
-    // the last column. This applies only to the true first column and last column across all column sets.
-    //
-    // FIXME: Eventually we will know overflow on a per-column basis, but we can't do this until we have a painting
-    // mode that understands not to paint contents from a previous column in the overflow area of a following column.
-    // This problem applies to regions and pages as well and is not unique to columns.
-    bool isFirstColumn = !index;
-    bool isLastColumn = index == colCount - 1;
-    bool isLeftmostColumn = style()->isLeftToRightDirection() ? isFirstColumn : isLastColumn;
-    bool isRightmostColumn = style()->isLeftToRightDirection() ? isLastColumn : isFirstColumn;
-
-    // Calculate the overflow rectangle, based on the flow thread's, clipped at column logical
-    // top/bottom unless it's the first/last column.
-    LayoutRect overflowRect = overflowRectForFlowThreadPortion(portionRect, isFirstColumn && isFirstRegion(), isLastColumn && isLastRegion());
-
-    // Avoid overflowing into neighboring columns, by clipping in the middle of adjacent column
-    // gaps. Also make sure that we avoid rounding errors.
-    if (isHorizontalWritingMode()) {
-        if (!isLeftmostColumn)
-            overflowRect.shiftXEdgeTo(portionRect.x() - colGap / 2);
-        if (!isRightmostColumn)
-            overflowRect.shiftMaxXEdgeTo(portionRect.maxX() + colGap - colGap / 2);
-    } else {
-        if (!isLeftmostColumn)
-            overflowRect.shiftYEdgeTo(portionRect.y() - colGap / 2);
-        if (!isRightmostColumn)
-            overflowRect.shiftMaxYEdgeTo(portionRect.maxY() + colGap - colGap / 2);
-    }
-    return overflowRect;
+    // FIXME: remove this method. It's a meaningless question to ask the set "how many columns do
+    // you actually have?", since that may vary for each row.
+    return firstFragmentainerGroup().actualColumnCount();
 }
 
 void RenderMultiColumnSet::paintObject(const PaintInfo& paintInfo, const LayoutPoint& paintOffset)
@@ -467,125 +207,21 @@ void RenderMultiColumnSet::paintObject(const PaintInfo& paintInfo, const LayoutP
 
 void RenderMultiColumnSet::collectLayerFragments(LayerFragments& fragments, const LayoutRect& layerBoundingBox, const LayoutRect& dirtyRect)
 {
-    // |layerBoundingBox| is in the flow thread coordinate space, relative to the top/left edge of
-    // the flow thread, but note that it has been converted with respect to writing mode (so that
-    // it's visual/physical in that sense).
-    //
-    // |dirtyRect| is visual, relative to the multicol container.
-    //
-    // Then there's the output from this method - the stuff we put into the list of fragments. The
-    // fragment.paginationOffset point is the actual visual translation required to get from a
-    // location in the flow thread to a location in a given column. The fragment.paginationClip
-    // rectangle, on the other hand, is in flow thread coordinates.
-    //
-    // All other rectangles in this method are sized physically, and the inline direction coordinate
-    // is physical too, but the block direction coordinate is "logical top". This is the same as
-    // e.g. RenderBox::frameRect(). These rectangles also pretend that there's only one long column,
-    // i.e. they are for the flow thread.
-
-    // Put the layer bounds into flow thread-local coordinates by flipping it first. Since we're in
-    // a renderer, most rectangles are represented this way.
-    LayoutRect layerBoundsInFlowThread(layerBoundingBox);
-    flowThread()->flipForWritingMode(layerBoundsInFlowThread);
-
-    // Now we can compare with the flow thread portions owned by each column. First let's
-    // see if the rect intersects our flow thread portion at all.
-    LayoutRect clippedRect(layerBoundsInFlowThread);
-    clippedRect.intersect(RenderRegion::flowThreadPortionOverflowRect());
-    if (clippedRect.isEmpty())
-        return;
-
-    // Now we know we intersect at least one column. Let's figure out the logical top and logical
-    // bottom of the area we're checking.
-    LayoutUnit layerLogicalTop = isHorizontalWritingMode() ? layerBoundsInFlowThread.y() : layerBoundsInFlowThread.x();
-    LayoutUnit layerLogicalBottom = (isHorizontalWritingMode() ? layerBoundsInFlowThread.maxY() : layerBoundsInFlowThread.maxX()) - 1;
-
-    // Figure out the start and end columns and only check within that range so that we don't walk the
-    // entire column set.
-    unsigned startColumn = columnIndexAtOffset(layerLogicalTop);
-    unsigned endColumn = columnIndexAtOffset(layerLogicalBottom);
-
-    LayoutUnit colLogicalWidth = pageLogicalWidth();
-    LayoutUnit colGap = columnGap();
-    unsigned colCount = actualColumnCount();
-
-    RenderMultiColumnFlowThread* flowThread = multiColumnFlowThread();
-    bool progressionIsInline = flowThread->progressionIsInline();
-    bool leftToRight = style()->isLeftToRightDirection();
-
-    LayoutUnit initialBlockOffset = logicalTop() - flowThread->logicalTop();
-
-    for (unsigned i = startColumn; i <= endColumn; i++) {
-        // Get the portion of the flow thread that corresponds to this column.
-        LayoutRect flowThreadPortion = flowThreadPortionRectAt(i);
-
-        // Now get the overflow rect that corresponds to the column.
-        LayoutRect flowThreadOverflowPortion = flowThreadPortionOverflowRect(flowThreadPortion, i, colCount, colGap);
-
-        // In order to create a fragment we must intersect the portion painted by this column.
-        LayoutRect clippedRect(layerBoundsInFlowThread);
-        clippedRect.intersect(flowThreadOverflowPortion);
-        if (clippedRect.isEmpty())
-            continue;
-
-        // We also need to intersect the dirty rect. We have to apply a translation and shift based off
-        // our column index.
-        LayoutPoint translationOffset;
-        LayoutUnit inlineOffset = progressionIsInline ? i * (colLogicalWidth + colGap) : LayoutUnit();
-        if (!leftToRight)
-            inlineOffset = -inlineOffset;
-        translationOffset.setX(inlineOffset);
-        LayoutUnit blockOffset;
-        if (progressionIsInline) {
-            blockOffset = initialBlockOffset + (isHorizontalWritingMode() ? -flowThreadPortion.y() : -flowThreadPortion.x());
-        } else {
-            // Column gap can apply in the block direction for page fragmentainers.
-            // There is currently no spec which calls for column-gap to apply
-            // for page fragmentainers at all, but it's applied here for compatibility
-            // with the old multicolumn implementation.
-            blockOffset = i * colGap;
-        }
-        if (isFlippedBlocksWritingMode(style()->writingMode()))
-            blockOffset = -blockOffset;
-        translationOffset.setY(blockOffset);
-        if (!isHorizontalWritingMode())
-            translationOffset = translationOffset.transposedPoint();
-        // FIXME: The translation needs to include the multicolumn set's content offset within the
-        // multicolumn block as well. This won't be an issue until we start creating multiple multicolumn sets.
-
-        // Shift the dirty rect to be in flow thread coordinates with this translation applied.
-        LayoutRect translatedDirtyRect(dirtyRect);
-        translatedDirtyRect.moveBy(-translationOffset);
-
-        // See if we intersect the dirty rect.
-        clippedRect = layerBoundingBox;
-        clippedRect.intersect(translatedDirtyRect);
-        if (clippedRect.isEmpty())
-            continue;
-
-        // Something does need to paint in this column. Make a fragment now and supply the physical translation
-        // offset and the clip rect for the column with that offset applied.
-        LayerFragment fragment;
-        fragment.paginationOffset = translationOffset;
-
-        LayoutRect flippedFlowThreadOverflowPortion(flowThreadOverflowPortion);
-        // Flip it into more a physical (Layer-style) rectangle.
-        flowThread->flipForWritingMode(flippedFlowThreadOverflowPortion);
-        fragment.paginationClip = flippedFlowThreadOverflowPortion;
-        fragments.append(fragment);
-    }
+    for (const auto& group : m_fragmentainerGroups)
+        group.collectLayerFragments(fragments, layerBoundingBox, dirtyRect);
 }
 
 void RenderMultiColumnSet::addOverflowFromChildren()
 {
-    unsigned colCount = actualColumnCount();
-    if (!colCount)
-        return;
-
-    LayoutRect lastRect = columnRectAt(colCount - 1);
-    addLayoutOverflow(lastRect);
+    LayoutRect overflowRect;
+    for (const auto& group : m_fragmentainerGroups) {
+        LayoutRect rect = group.calculateOverflow();
+        rect.move(group.offsetFromColumnSet());
+        overflowRect.unite(rect);
+    }
+    addLayoutOverflow(overflowRect);
     if (!hasOverflowClip())
-        addVisualOverflow(lastRect);
+        addVisualOverflow(overflowRect);
 }
 
 const char* RenderMultiColumnSet::renderName() const
@@ -630,4 +266,12 @@ void RenderMultiColumnSet::detachRegion()
     }
 }
 
+LayoutRect RenderMultiColumnSet::flowThreadPortionRect() const
+{
+    LayoutRect portionRect(LayoutUnit(), logicalTopInFlowThread(), pageLogicalWidth(), logicalHeightInFlowThread());
+    if (!isHorizontalWritingMode())
+        return portionRect.transposedRect();
+    return portionRect;
+}
+
 }