bool MultiColumnFragmentainerGroup::recalculateColumnHeight(
LayoutMultiColumnSet& columnSet) {
LayoutUnit oldColumnHeight = m_columnHeight;
m_maxColumnHeight = calculateMaxColumnHeight();
// Only the last row may have auto height, and thus be balanced. There are no
// good reasons to balance the preceding rows, and that could potentially lead
// to an insane number of layout passes as well.
if (isLastGroup() && columnSet.heightIsAuto()) {
LayoutUnit newColumnHeight;
if (!columnSet.isInitialHeightCalculated()) {
// Initial balancing: Start with the lowest imaginable column height. Also
// calculate the height of the tallest piece of unbreakable content.
// Columns should never get any shorter than that (unless constrained by
// max-height). Propagate this to our containing column set, in case there
// is an outer multicol container that also needs to balance. After having
// calculated the initial column height, the multicol container needs
// another layout pass with the column height that we just calculated.
InitialColumnHeightFinder initialHeightFinder(
columnSet, logicalTopInFlowThread(), logicalBottomInFlowThread());
LayoutUnit tallestUnbreakableLogicalHeight =
initialHeightFinder.tallestUnbreakableLogicalHeight();
columnSet.propagateTallestUnbreakableLogicalHeight(
tallestUnbreakableLogicalHeight);
newColumnHeight =
std::max(initialHeightFinder.initialMinimalBalancedHeight(),
tallestUnbreakableLogicalHeight);
} else {
// Rebalancing: After having laid out again, we'll need to rebalance if
// the height wasn't enough and we're allowed to stretch it, and then
// re-lay out. There are further details on the column balancing
// machinery in ColumnBalancer and its derivates.
newColumnHeight = rebalanceColumnHeightIfNeeded();
}
setAndConstrainColumnHeight(newColumnHeight);
} 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);
}
if (m_columnHeight == oldColumnHeight)
return false; // No change. We're done.
return true; // Need another pass.
}
void LayoutMultiColumnFlowThread::layoutColumns(SubtreeLayoutScope& layoutScope)
{
// Since we ended up here, it means that the multicol container (our parent) needed
// layout. Since contents of the multicol container are diverted to the flow thread, the flow
// thread needs layout as well.
layoutScope.setChildNeedsLayout(this);
m_needsColumnHeightsRecalculation = false;
m_blockOffsetInEnclosingFragmentationContext = enclosingFragmentationContext() ? multiColumnBlockFlow()->offsetFromLogicalTopOfFirstPage() : LayoutUnit();
for (LayoutBox* columnBox = firstMultiColumnBox(); columnBox; columnBox = columnBox->nextSiblingMultiColumnBox()) {
if (!columnBox->isLayoutMultiColumnSet()) {
ASSERT(columnBox->isLayoutMultiColumnSpannerPlaceholder()); // no other type is expected.
m_needsColumnHeightsRecalculation = true;
continue;
}
LayoutMultiColumnSet* columnSet = toLayoutMultiColumnSet(columnBox);
layoutScope.setChildNeedsLayout(columnSet);
if (!m_inBalancingPass) {
// This is the initial layout pass. We need to reset the column height, because contents
// typically have changed.
columnSet->resetColumnHeight();
}
if (!m_needsColumnHeightsRecalculation)
m_needsColumnHeightsRecalculation = columnSet->heightIsAuto();
// Since column sets are regular block flow objects, and their position is changed in
// regular block layout code (with no means for the multicol code to notice unless we add
// hooks there), store the previous position now. If it changes in the imminent layout
// pass, we may have to rebalance its columns.
columnSet->storeOldPosition();
}
invalidateColumnSets();
layout();
}