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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at */
/* rendering object for css3 multi-column layout */
#include "nsColumnSetFrame.h"
#include "mozilla/ColumnUtils.h"
#include "mozilla/Logging.h"
#include "mozilla/PresShell.h"
#include "mozilla/StaticPrefs_layout.h"
#include "mozilla/ToString.h"
#include "nsCSSRendering.h"
#include "nsDisplayList.h"
#include "nsIFrameInlines.h"
#include "nsLayoutUtils.h"
using namespace mozilla;
using namespace mozilla::layout;
// To see this log, use $ MOZ_LOG=ColumnSet:4 ./mach run
static LazyLogModule sColumnSetLog("ColumnSet");
#define COLUMN_SET_LOG(msg, ...) \
MOZ_LOG(sColumnSetLog, LogLevel::Debug, (msg, ##__VA_ARGS__))
class nsDisplayColumnRule : public nsPaintedDisplayItem {
nsDisplayColumnRule(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame)
: nsPaintedDisplayItem(aBuilder, aFrame) {
nsRect GetBounds(nsDisplayListBuilder* aBuilder, bool* aSnap) const override {
*aSnap = false;
// We just return the frame's ink-overflow rect, which is guaranteed to
// contain all the column-rule areas. It's not worth calculating the exact
// union of those areas since it would only lead to performance improvements
// during painting in rare edge cases.
return mFrame->InkOverflowRect() + ToReferenceFrame();
bool CreateWebRenderCommands(
mozilla::wr::DisplayListBuilder& aBuilder,
mozilla::wr::IpcResourceUpdateQueue& aResources,
const StackingContextHelper& aSc,
mozilla::layers::RenderRootStateManager* aManager,
nsDisplayListBuilder* aDisplayListBuilder) override;
void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) override;
nsTArray<nsCSSBorderRenderer> mBorderRenderers;
void nsDisplayColumnRule::Paint(nsDisplayListBuilder* aBuilder,
gfxContext* aCtx) {
mBorderRenderers, aCtx, GetPaintRect(aBuilder, aCtx), ToReferenceFrame());
for (auto iter = mBorderRenderers.begin(); iter != mBorderRenderers.end();
iter++) {
bool nsDisplayColumnRule::CreateWebRenderCommands(
mozilla::wr::DisplayListBuilder& aBuilder,
mozilla::wr::IpcResourceUpdateQueue& aResources,
const StackingContextHelper& aSc,
mozilla::layers::RenderRootStateManager* aManager,
nsDisplayListBuilder* aDisplayListBuilder) {
RefPtr dt = gfxPlatform::GetPlatform()->ScreenReferenceDrawTarget();
if (!dt || !dt->IsValid()) {
return false;
gfxContext screenRefCtx(dt);
bool dummy;
mBorderRenderers, &screenRefCtx, GetBounds(aDisplayListBuilder, &dummy),
if (mBorderRenderers.IsEmpty()) {
return true;
for (auto& renderer : mBorderRenderers) {
renderer.CreateWebRenderCommands(this, aBuilder, aResources, aSc);
return true;
// The maximum number of columns we support.
static constexpr int32_t kMaxColumnCount = 1000;
* Tracking issues:
* XXX cursor movement around the top and bottom of colums seems to make the
* editor lose the caret.
* XXX should we support CSS columns applied to table elements?
nsContainerFrame* NS_NewColumnSetFrame(PresShell* aPresShell,
ComputedStyle* aStyle,
nsFrameState aStateFlags) {
nsColumnSetFrame* it =
new (aPresShell) nsColumnSetFrame(aStyle, aPresShell->GetPresContext());
return it;
nsColumnSetFrame::nsColumnSetFrame(ComputedStyle* aStyle,
nsPresContext* aPresContext)
: nsContainerFrame(aStyle, aPresContext, kClassID),
void nsColumnSetFrame::ForEachColumnRule(
const std::function<void(const nsRect& lineRect)>& aSetLineRect,
const nsPoint& aPt) const {
nsIFrame* child = mFrames.FirstChild();
if (!child) return; // no columns
nsIFrame* nextSibling = child->GetNextSibling();
if (!nextSibling) return; // 1 column only - this means no gap to draw on
const nsStyleColumn* colStyle = StyleColumn();
nscoord ruleWidth = colStyle->GetColumnRuleWidth();
if (!ruleWidth) return;
WritingMode wm = GetWritingMode();
bool isVertical = wm.IsVertical();
bool isRTL = wm.IsBidiRTL();
nsRect contentRect = GetContentRectRelativeToSelf() + aPt;
nsSize ruleSize = isVertical ? nsSize(contentRect.width, ruleWidth)
: nsSize(ruleWidth, contentRect.height);
while (nextSibling) {
// The frame tree goes RTL in RTL.
// The |prevFrame| and |nextFrame| frames here are the visually preceding
// (left/above) and following (right/below) frames, not in logical writing-
// mode direction.
nsIFrame* prevFrame = isRTL ? nextSibling : child;
nsIFrame* nextFrame = isRTL ? child : nextSibling;
// Each child frame's position coordinates is actually relative to this
// nsColumnSetFrame.
// linePt will be at the top-left edge to paint the line.
nsPoint linePt;
if (isVertical) {
nscoord edgeOfPrev = prevFrame->GetRect().YMost() + aPt.y;
nscoord edgeOfNext = nextFrame->GetRect().Y() + aPt.y;
linePt = nsPoint(contentRect.x,
(edgeOfPrev + edgeOfNext - ruleSize.height) / 2);
} else {
nscoord edgeOfPrev = prevFrame->GetRect().XMost() + aPt.x;
nscoord edgeOfNext = nextFrame->GetRect().X() + aPt.x;
linePt = nsPoint((edgeOfPrev + edgeOfNext - ruleSize.width) / 2,
aSetLineRect(nsRect(linePt, ruleSize));
child = nextSibling;
nextSibling = nextSibling->GetNextSibling();
void nsColumnSetFrame::CreateBorderRenderers(
nsTArray<nsCSSBorderRenderer>& aBorderRenderers, gfxContext* aCtx,
const nsRect& aDirtyRect, const nsPoint& aPt) {
WritingMode wm = GetWritingMode();
bool isVertical = wm.IsVertical();
const nsStyleColumn* colStyle = StyleColumn();
StyleBorderStyle ruleStyle;
// Per spec, inset => ridge and outset => groove
if (colStyle->mColumnRuleStyle == StyleBorderStyle::Inset) {
ruleStyle = StyleBorderStyle::Ridge;
} else if (colStyle->mColumnRuleStyle == StyleBorderStyle::Outset) {
ruleStyle = StyleBorderStyle::Groove;
} else {
ruleStyle = colStyle->mColumnRuleStyle;
nscoord ruleWidth = colStyle->GetColumnRuleWidth();
if (!ruleWidth) {
nscolor ruleColor =
nsPresContext* pc = PresContext();
// In order to re-use a large amount of code, we treat the column rule as a
// border. We create a new border style object and fill in all the details of
// the column rule as the left border. PaintBorder() does all the rendering
// for us, so we not only save an enormous amount of code but we'll support
// all the line styles that we support on borders!
nsStyleBorder border;
Sides skipSides;
if (isVertical) {
border.SetBorderWidth(eSideTop, ruleWidth, pc->AppUnitsPerDevPixel());
border.SetBorderStyle(eSideTop, ruleStyle);
border.mBorderTopColor = StyleColor::FromColor(ruleColor);
skipSides |= mozilla::SideBits::eLeftRight;
skipSides |= mozilla::SideBits::eBottom;
} else {
border.SetBorderWidth(eSideLeft, ruleWidth, pc->AppUnitsPerDevPixel());
border.SetBorderStyle(eSideLeft, ruleStyle);
border.mBorderLeftColor = StyleColor::FromColor(ruleColor);
skipSides |= mozilla::SideBits::eTopBottom;
skipSides |= mozilla::SideBits::eRight;
// If we use box-decoration-break: slice (the default), the border
// renderers will require clipping if we have continuations (see the
// aNeedsClip parameter to ConstructBorderRenderer in nsCSSRendering).
// Since it doesn't matter which box-decoration-break we use since
// we're only drawing borders (and not border-images), use 'clone'.
border.mBoxDecorationBreak = StyleBoxDecorationBreak::Clone;
[&](const nsRect& aLineRect) {
// Assert that we're not drawing a border-image here; if we were, we
// couldn't ignore the ImgDrawResult that PaintBorderWithStyleBorder
// returns.
gfx::DrawTarget* dt = aCtx ? aCtx->GetDrawTarget() : nullptr;
bool borderIsEmpty = false;
Maybe<nsCSSBorderRenderer> br =
pc, dt, this, aDirtyRect, aLineRect, border, Style(),
&borderIsEmpty, skipSides);
if (br.isSome()) {
static uint32_t ColumnBalancingDepth(const ReflowInput& aReflowInput,
uint32_t aMaxDepth) {
uint32_t depth = 0;
for (const ReflowInput* ri = aReflowInput.mParentReflowInput;
ri && depth < aMaxDepth; ri = ri->mParentReflowInput) {
if (ri->mFlags.mIsColumnBalancing) {
return depth;
nsColumnSetFrame::ReflowConfig nsColumnSetFrame::ChooseColumnStrategy(
const ReflowInput& aReflowInput, bool aForceAuto = false) const {
const nsStyleColumn* colStyle = StyleColumn();
nscoord availContentISize = aReflowInput.AvailableISize();
if (aReflowInput.ComputedISize() != NS_UNCONSTRAINEDSIZE) {
availContentISize = aReflowInput.ComputedISize();
nscoord colBSize = aReflowInput.AvailableBSize();
nscoord colGap =
ColumnUtils::GetColumnGap(this, aReflowInput.ComputedISize());
int32_t numColumns =
? 0
: std::min(colStyle->mColumnCount.AsInteger(), kMaxColumnCount);
// If column-fill is set to 'balance' or we have a column-span sibling, then
// we want to balance the columns.
bool isBalancing = (colStyle->mColumnFill == StyleColumnFill::Balance ||
HasColumnSpanSiblings()) &&
if (isBalancing) {
const uint32_t kMaxNestedColumnBalancingDepth = 2;
const uint32_t balancingDepth =
ColumnBalancingDepth(aReflowInput, kMaxNestedColumnBalancingDepth);
if (balancingDepth == kMaxNestedColumnBalancingDepth) {
isBalancing = false;
numColumns = 1;
nscoord colISize;
// In vertical writing-mode, "column-width" (inline size) will actually be
// physical height, but its CSS name is still column-width.
if (colStyle->mColumnWidth.IsLength()) {
colISize =
NS_ASSERTION(colISize >= 0, "negative column width");
// Reduce column count if necessary to make columns fit in the
// available width. Compute max number of columns that fit in
// availContentISize, satisfying colGap*(maxColumns - 1) +
// colISize*maxColumns <= availContentISize
if (availContentISize != NS_UNCONSTRAINEDSIZE && colGap + colISize > 0 &&
numColumns > 0) {
// This expression uses truncated rounding, which is what we
// want
int32_t maxColumns =
(availContentISize + colGap) / (colGap + colISize));
numColumns = std::max(1, std::min(numColumns, maxColumns));
} else if (numColumns > 0 && availContentISize != NS_UNCONSTRAINEDSIZE) {
nscoord iSizeMinusGaps = availContentISize - colGap * (numColumns - 1);
colISize = iSizeMinusGaps / numColumns;
} else {
// Take care of the situation where there's only one column but it's
// still too wide
colISize = std::max(1, std::min(colISize, availContentISize));
nscoord expectedISizeLeftOver = 0;
availContentISize != NS_UNCONSTRAINEDSIZE) {
// distribute leftover space
// First, determine how many columns will be showing if the column
// count is auto
if (numColumns <= 0) {
// choose so that colGap*(nominalColumnCount - 1) +
// colISize*nominalColumnCount is nearly availContentISize
// make sure to round down
if (colGap + colISize > 0) {
numColumns = (availContentISize + colGap) / (colGap + colISize);
// The number of columns should never exceed kMaxColumnCount.
numColumns = std::min(kMaxColumnCount, numColumns);
if (numColumns <= 0) {
numColumns = 1;
// Compute extra space and divide it among the columns
nscoord extraSpace =
std::max(0, availContentISize -
(colISize * numColumns + colGap * (numColumns - 1)));
nscoord extraToColumns = extraSpace / numColumns;
colISize += extraToColumns;
expectedISizeLeftOver = extraSpace - (extraToColumns * numColumns);
if (isBalancing) {
if (numColumns <= 0) {
// Hmm, auto column count, column width or available width is unknown,
// and balancing is required. Let's just use one column then.
numColumns = 1;
colBSize = std::min(mLastBalanceBSize, colBSize);
} else {
// CSS Fragmentation spec says, "To guarantee progress, fragmentainers are
// assumed to have a minimum block size of 1px regardless of their used
// Note: we don't enforce the minimum block-size during balancing because
// this affects the result. If a balancing column container or its
// next-in-flows has zero block-size, it eventually gives up balancing, and
// ends up here.
colBSize = std::max(colBSize, nsPresContext::CSSPixelsToAppUnits(1));
ReflowConfig config;
config.mUsedColCount = numColumns;
config.mColISize = colISize;
config.mExpectedISizeLeftOver = expectedISizeLeftOver;
config.mColGap = colGap;
config.mColBSize = colBSize;
config.mIsBalancing = isBalancing;
config.mForceAuto = aForceAuto;
config.mKnownFeasibleBSize = NS_UNCONSTRAINEDSIZE;
config.mKnownInfeasibleBSize = 0;
"%s: this=%p, mUsedColCount=%d, mColISize=%d, "
"mExpectedISizeLeftOver=%d, mColGap=%d, mColBSize=%d, mIsBalancing=%d",
__func__, this, config.mUsedColCount, config.mColISize,
config.mExpectedISizeLeftOver, config.mColGap, config.mColBSize,
return config;
static void MarkPrincipalChildrenDirty(nsIFrame* aFrame) {
for (nsIFrame* childFrame : aFrame->PrincipalChildList()) {
static void MoveChildTo(nsIFrame* aChild, LogicalPoint aOrigin, WritingMode aWM,
const nsSize& aContainerSize) {
if (aChild->GetLogicalPosition(aWM, aContainerSize) == aOrigin) {
aChild->SetPosition(aWM, aOrigin, aContainerSize);
nscoord nsColumnSetFrame::GetMinISize(gfxContext* aRenderingContext) {
nscoord iSize = 0;
if (mFrames.FirstChild()) {
// We want to ignore this in the case that we're size contained
// because our children should not contribute to our
// intrinsic size.
iSize = mFrames.FirstChild()->GetMinISize(aRenderingContext);
const nsStyleColumn* colStyle = StyleColumn();
if (colStyle->mColumnWidth.IsLength()) {
nscoord colISize =
// As available width reduces to zero, we reduce our number of columns
// to one, and don't enforce the column width, so just return the min
// of the child's min-width with any specified column width.
iSize = std::min(iSize, colISize);
} else {
"column-count and column-width can't both be auto");
// As available width reduces to zero, we still have mColumnCount columns,
// so compute our minimum size based on the number of columns and their gaps
// and minimum per-column size.
nscoord colGap = ColumnUtils::GetColumnGap(this, NS_UNCONSTRAINEDSIZE);
iSize = ColumnUtils::IntrinsicISize(colStyle->mColumnCount.AsInteger(),
colGap, iSize);
// XXX count forced column breaks here? Maybe we should return the child's
// min-width times the minimum number of columns.
return iSize;
nscoord nsColumnSetFrame::GetPrefISize(gfxContext* aRenderingContext) {
// Our preferred width is our desired column width, if specified, otherwise
// the child's preferred width, times the number of columns, plus the width
// of any required column gaps
// XXX what about forced column breaks here?
const nsStyleColumn* colStyle = StyleColumn();
nscoord colISize;
if (colStyle->mColumnWidth.IsLength()) {
colISize =
} else if (mFrames.FirstChild()) {
// We want to ignore this in the case that we're size contained
// because our children should not contribute to our
// intrinsic size.
colISize = mFrames.FirstChild()->GetPrefISize(aRenderingContext);
} else {
colISize = 0;
// If column-count is auto, assume one column.
uint32_t numColumns =
colStyle->mColumnCount.IsAuto() ? 1 : colStyle->mColumnCount.AsInteger();
nscoord colGap = ColumnUtils::GetColumnGap(this, NS_UNCONSTRAINEDSIZE);
return ColumnUtils::IntrinsicISize(numColumns, colGap, colISize);
nsColumnSetFrame::ColumnBalanceData nsColumnSetFrame::ReflowColumns(
ReflowOutput& aDesiredSize, const ReflowInput& aReflowInput,
nsReflowStatus& aStatus, const ReflowConfig& aConfig,
bool aUnboundedLastColumn) {
ColumnBalanceData colData;
bool allFit = true;
WritingMode wm = GetWritingMode();
const bool isRTL = wm.IsBidiRTL();
const bool shrinkingBSize = mLastBalanceBSize > aConfig.mColBSize;
const bool changingBSize = mLastBalanceBSize != aConfig.mColBSize;
"%s: Doing column reflow pass: mLastBalanceBSize=%d,"
" mColBSize=%d, RTL=%d, mUsedColCount=%d,"
" mColISize=%d, mColGap=%d",
__func__, mLastBalanceBSize, aConfig.mColBSize, isRTL,
aConfig.mUsedColCount, aConfig.mColISize, aConfig.mColGap);
if (changingBSize) {
mLastBalanceBSize = aConfig.mColBSize;
// XXX Seems like this could fire if incremental reflow pushed the column
// set down so we reflow incrementally with a different available height.
// We need a way to do an incremental reflow and be sure availableHeight
// changes are taken account of! Right now I think block frames with
// absolute children might exit early.
aKidReason != eReflowReason_Incremental,
"incremental reflow should not have changed the balance height");
nsRect contentRect(0, 0, 0, 0);
OverflowAreas overflowRects;
nsIFrame* child = mFrames.FirstChild();
LogicalPoint childOrigin(wm, 0, 0);
// In vertical-rl mode, columns will not be correctly placed if the
// reflowInput's ComputedWidth() is UNCONSTRAINED (in which case we'll get
// a containerSize.width of zero here). In that case, the column positions
// will be adjusted later, after our correct contentSize is known.
// When column-span is enabled, containerSize.width is always constrained.
// However, for RTL, we need to adjust the column positions as well after our
// correct containerSize is known.
nsSize containerSize = aReflowInput.ComputedSizeAsContainerIfConstrained();
const nscoord computedBSize =
nscoord contentBEnd = 0;
bool reflowNext = false;
while (child) {
const bool reflowLastColumnWithUnconstrainedAvailBSize =
aUnboundedLastColumn && colData.mColCount == aConfig.mUsedColCount &&
// We need to reflow the child (column) ...
bool reflowChild =
// if we are told to do so;
aReflowInput.ShouldReflowAllKids() ||
// if the child is dirty;
child->IsSubtreeDirty() ||
// if it's the last child because we need to obtain the block-end
// margin;
!child->GetNextSibling() ||
// if the next column is dirty, because the next column's first line(s)
// might be pullable back to this column;
child->GetNextSibling()->IsSubtreeDirty() ||
// if this is the last column and we are supposed to assign unbounded
// block-size to it, because that could change the available block-size
// from the last time we reflowed it and we should try to pull all the
// content from its next sibling (Note that it might be the last column,
// but not be the last child because the desired number of columns has
// changed.)
// If column-fill is auto (not the default), then we might need to
// move content between columns for any change in column block-size.
// The same is true if we have a non-'auto' computed block-size.
// FIXME: It's not clear to me why it's *ever* valid to have
// reflowChild be false when changingBSize is true, since it
// seems like a child broken over multiple columns might need to
// change the size of the fragment in each column.
if (!reflowChild && changingBSize &&
(StyleColumn()->mColumnFill == StyleColumnFill::Auto ||
reflowChild = true;
// If we need to pull up content from the prev-in-flow then this is not just
// a block-size shrink. The prev in flow will have set the dirty bit.
// Check the overflow rect YMost instead of just the child's content
// block-size. The child may have overflowing content that cares about the
// available block-size boundary. (It may also have overflowing content that
// doesn't care about the available block-size boundary, but if so, too bad,
// this optimization is defeated.) We want scrollable overflow here since
// this is a calculation that affects layout.
if (!reflowChild && shrinkingBSize) {
switch (wm.GetBlockDir()) {
case WritingMode::BlockDir::TB:
if (child->ScrollableOverflowRect().YMost() > aConfig.mColBSize) {
reflowChild = true;
case WritingMode::BlockDir::LR:
if (child->ScrollableOverflowRect().XMost() > aConfig.mColBSize) {
reflowChild = true;
case WritingMode::BlockDir::RL:
// XXX not sure how to handle this, so for now just don't attempt
// the optimization
reflowChild = true;
MOZ_ASSERT_UNREACHABLE("unknown block direction");
nscoord childContentBEnd = 0;
if (!reflowNext && !reflowChild) {
// This child does not need to be reflowed, but we may need to move it
MoveChildTo(child, childOrigin, wm, containerSize);
// If this is the last frame then make sure we get the right status
nsIFrame* kidNext = child->GetNextSibling();
if (kidNext) {
if (kidNext->HasAnyStateBits(NS_FRAME_IS_OVERFLOW_CONTAINER)) {
} else {
} else {
aStatus = mLastFrameStatus;
childContentBEnd = nsLayoutUtils::CalculateContentBEnd(wm, child);
COLUMN_SET_LOG("%s: Skipping child #%d %p: status=%s", __func__,
colData.mColCount, child, ToString(aStatus).c_str());
} else {
LogicalSize availSize(wm, aConfig.mColISize, aConfig.mColBSize);
if (reflowLastColumnWithUnconstrainedAvailBSize) {
"%s: Reflowing last column with unconstrained block-size. Change "
"available block-size from %d to %d",
__func__, aConfig.mColBSize, availSize.BSize(wm));
if (reflowNext) {
LogicalSize kidCBSize(wm, availSize.ISize(wm), computedBSize);
ReflowInput kidReflowInput(PresContext(), aReflowInput, child, availSize,
kidReflowInput.mFlags.mIsTopOfPage = [&]() {
const bool isNestedMulticolOrPaginated =
if (isNestedMulticolOrPaginated) {
if (aConfig.mForceAuto) {
// If we are forced to fill columns sequentially, force fit the
// content whether we are at top of page or not.
return true;
if (aReflowInput.mFlags.mIsTopOfPage) {
// If this is the last balancing reflow, we want to force fit
// content to avoid infinite loops.
return !aConfig.mIsBalancing || aConfig.mIsLastBalancingReflow;
// If we are a not at the top of page, we shouldn't force fit content.
// This is because our ColumnSetWrapperFrame can be pushed to the next
// column or page and reflowed again with a potentially larger
// available block-size.
return false;
// We are a top-level multicol in non-paginated context. Force fit the
// content only if we are not balancing columns.
return !aConfig.mIsBalancing;
kidReflowInput.mFlags.mTableIsSplittable = false;
kidReflowInput.mFlags.mIsColumnBalancing = aConfig.mIsBalancing;
kidReflowInput.mFlags.mIsInLastColumnBalancingReflow =
kidReflowInput.mBreakType = ReflowInput::BreakType::Column;
// We need to reflow any float placeholders, even if our column block-size
// hasn't changed.
kidReflowInput.mFlags.mMustReflowPlaceholders = !changingBSize;
"%s: Reflowing child #%d %p: availSize=(%d,%d), kidCBSize=(%d,%d), "
"child's mIsTopOfPage=%d",
__func__, colData.mColCount, child, availSize.ISize(wm),
availSize.BSize(wm), kidCBSize.ISize(wm), kidCBSize.BSize(wm),
// Note if the column's next in flow is not being changed by this
// incremental reflow. This may allow the current column to avoid trying
// to pull lines from the next column.
if (child->GetNextSibling() && !HasAnyStateBits(NS_FRAME_IS_DIRTY) &&
!child->GetNextSibling()->HasAnyStateBits(NS_FRAME_IS_DIRTY)) {
kidReflowInput.mFlags.mNextInFlowUntouched = true;
ReflowOutput kidDesiredSize(wm);
// XXX it would be cool to consult the float manager for the
// previous block to figure out the region of floats from the
// previous column that extend into this column, and subtract
// that region from the new float manager. So you could stick a
// really big float in the first column and text in following
// columns would flow around it.
"-moz-column-content has no margin!");
ReflowChild(child, PresContext(), kidDesiredSize, kidReflowInput, wm,
childOrigin, containerSize, ReflowChildFlags::Default,
if (colData.mColCount == 1 && aStatus.IsInlineBreakBefore()) {
COLUMN_SET_LOG("%s: Content in the first column reports break-before!",
allFit = false;
reflowNext = aStatus.NextInFlowNeedsReflow();
// The carried-out block-end margin of column content might be non-zero
// when we try to find the best column balancing block size, but it should
// never affect the size column set nor be further carried out. Set it to
// zero.
// FIXME: For some types of fragmentation, we should carry the margin into
// the next column. Also see
// FIXME: This should never happen for the last column, since it should be
// a margin root; see nsBlockFrame::IsMarginRoot(). However, sometimes the
// last column has an empty continuation while searching for the best
// column balancing bsize, which prevents the last column from being a
// margin root.
NS_FRAME_TRACE_REFLOW_OUT("Column::Reflow", aStatus);
FinishReflowChild(child, PresContext(), kidDesiredSize, &kidReflowInput,
wm, childOrigin, containerSize,
childContentBEnd = nsLayoutUtils::CalculateContentBEnd(wm, child);
if (childContentBEnd > aConfig.mColBSize) {
allFit = false;
if (childContentBEnd > availSize.BSize(wm)) {
colData.mMaxOverflowingBSize =
std::max(childContentBEnd, colData.mMaxOverflowingBSize);
"%s: Reflowed child #%d %p: status=%s, desiredSize=(%d,%d), "
"childContentBEnd=%d, CarriedOutBEndMargin=%d (ignored)",
__func__, colData.mColCount, child, ToString(aStatus).c_str(),
kidDesiredSize.ISize(wm), kidDesiredSize.BSize(wm), childContentBEnd,
contentRect.UnionRect(contentRect, child->GetRect());
ConsiderChildOverflow(overflowRects, child);
contentBEnd = std::max(contentBEnd, childContentBEnd);
colData.mLastBSize = childContentBEnd;
colData.mSumBSize += childContentBEnd;
// Build a continuation column if necessary
nsIFrame* kidNextInFlow = child->GetNextInFlow();
if (aStatus.IsFullyComplete()) {
NS_ASSERTION(!kidNextInFlow, "next in flow should have been deleted");
child = nullptr;
// Make sure that the column has a next-in-flow. If not, we must
// create one to hold the overflowing stuff, even if we're just
// going to put it on our overflow list and let *our*
// next in flow handle it.
if (!kidNextInFlow) {
"We have to create a continuation, but the block doesn't "
"want us to reflow it?");
// We need to create a continuing column
kidNextInFlow = CreateNextInFlow(child);
// Make sure we reflow a next-in-flow when it switches between being
// normal or overflow container
if (aStatus.IsOverflowIncomplete()) {
if (!kidNextInFlow->HasAnyStateBits(NS_FRAME_IS_OVERFLOW_CONTAINER)) {
reflowNext = true;
} else if (kidNextInFlow->HasAnyStateBits(NS_FRAME_IS_OVERFLOW_CONTAINER)) {
reflowNext = true;
// We have reached the maximum number of columns. If we are balancing, stop
// this reflow and continue finding the optimal balancing block-size.
// Otherwise, i.e. we are not balancing, stop this reflow and let the parent
// of our multicol container create a next-in-flow if all of the following
// conditions are met.
// 1) We fill columns sequentially by the request of the style, not by our
// internal needs, i.e. aConfig.mForceAuto is false.
// We don't want to stop this reflow when we force fill the columns
// sequentially. We usually go into this mode when giving up balancing, and
// this is the last resort to fit all our children by creating overflow
// columns.
// 2) In a fragmented context, our multicol container still has block-size
// left for its next-in-flow, i.e.
// aReflowInput.mFlags.mColumnSetWrapperHasNoBSizeLeft is false.
// Note that in a continuous context, i.e. our multicol container's
// available block-size is unconstrained, if it has a fixed block-size
// mColumnSetWrapperHasNoBSizeLeft is always true because nothing stops it
// from applying all its block-size in the first-in-flow. Otherwise, i.e.
// our multicol container has an unconstrained block-size, we shouldn't be
// here because all our children should fit in the very first column even if
// mColumnSetWrapperHasNoBSizeLeft is false.
// According to the definition of mColumnSetWrapperHasNoBSizeLeft, if the
// bit is *not* set, either our multicol container has unconstrained
// block-size, or it has a constrained block-size and has block-size left
// for its next-in-flow. In either cases, the parent of our multicol
// container can create a next-in-flow for the container that guaranteed to
// have non-zero block-size for the container's children.
// Put simply, if either one of the above conditions is not met, we are
// going to create more overflow columns until all our children are fit.
if (colData.mColCount >= aConfig.mUsedColCount &&
(aConfig.mIsBalancing ||
(!aConfig.mForceAuto &&
!aReflowInput.mFlags.mColumnSetWrapperHasNoBSizeLeft))) {
NS_ASSERTION(aConfig.mIsBalancing ||
aReflowInput.AvailableBSize() != NS_UNCONSTRAINEDSIZE,
"Why are we here if we have unlimited block-size to fill "
"columns sequentially.");
// No more columns allowed here. Stop.
// Move any of our leftover columns to our overflow list. Our
// next-in-flow will eventually pick them up.
nsFrameList continuationColumns = mFrames.TakeFramesAfter(child);
if (continuationColumns.NotEmpty()) {
child = nullptr;
COLUMN_SET_LOG("%s: We are not going to create overflow columns.",
if (PresContext()->HasPendingInterrupt()) {
// Stop the loop now while |child| still points to the frame that bailed
// out. We could keep going here and condition a bunch of the code in
// this loop on whether there's an interrupt, or even just keep going and
// trying to reflow the blocks (even though we know they'll interrupt
// right after their first line), but stopping now is conceptually the
// simplest (and probably fastest) thing.
// Advance to the next column
child = child->GetNextSibling();
if (child) {
childOrigin.I(wm) += aConfig.mColISize + aConfig.mColGap;
COLUMN_SET_LOG("%s: Next childOrigin.iCoord=%d", __func__,
if (PresContext()->CheckForInterrupt(this) &&
HasAnyStateBits(NS_FRAME_IS_DIRTY)) {
// Mark all our kids starting with |child| dirty
// Note that this is a CheckForInterrupt call, not a HasPendingInterrupt,
// because we might have interrupted while reflowing |child|, and since
// we're about to add a dirty bit to |child| we need to make sure that
// |this| is scheduled to have dirty bits marked on it and its ancestors.
// Otherwise, when we go to mark dirty bits on |child|'s ancestors we'll
// bail out immediately, since it'll already have a dirty bit.
for (; child; child = child->GetNextSibling()) {
colData.mMaxBSize = contentBEnd;
LogicalSize contentSize = LogicalSize(wm, contentRect.Size());
contentSize.BSize(wm) = std::max(contentSize.BSize(wm), contentBEnd);
mLastFrameStatus = aStatus;
if (computedBSize != NS_UNCONSTRAINEDSIZE && !HasColumnSpanSiblings()) {
"Available block-size should be constrained because it's "
"restricted by the computed block-size when our reflow "
"input is created in nsBlockFrame::ReflowBlockFrame()!");
// If a) our parent ColumnSetWrapper has constrained block-size
// (nsBlockFrame::ReflowBlockFrame() applies the block-size constraint
// when creating a ReflowInput for ColumnSetFrame child); and b) we are the
// sole ColumnSet or the last ColumnSet continuation split by column-spans
// in a ColumnSetWrapper, extend our block-size to consume the available
// block-size so that the column-rules are drawn to the content block-end
// edge of the multicol container.
contentSize.BSize(wm) =
std::max(contentSize.BSize(wm), aReflowInput.AvailableBSize());
aDesiredSize.SetSize(wm, contentSize);
aDesiredSize.mOverflowAreas = overflowRects;
// In vertical-rl mode, make a second pass if necessary to reposition the
// columns with the correct container width. (In other writing modes,
// correct containerSize was not required for column positioning so we don't
// need this fixup.)
// RTL column positions also depend on ColumnSet's actual contentSize. We need
// this fixup, too.
if ((wm.IsVerticalRL() || isRTL) &&
containerSize.width != contentSize.Width(wm)) {
const nsSize finalContainerSize = aDesiredSize.PhysicalSize();
OverflowAreas overflowRects;
for (nsIFrame* child : mFrames) {
// Get the logical position as set previously using a provisional or
// dummy containerSize, and reset with the correct container size.
child->SetPosition(wm, child->GetLogicalPosition(wm, containerSize),
ConsiderChildOverflow(overflowRects, child);
aDesiredSize.mOverflowAreas = overflowRects;
colData.mFeasible = allFit && aStatus.IsFullyComplete();
"%s: Done column reflow pass: %s, mMaxBSize=%d, mSumBSize=%d, "
"mLastBSize=%d, mMaxOverflowingBSize=%d",
__func__, colData.mFeasible ? "Feasible :)" : "Infeasible :(",
colData.mMaxBSize, colData.mSumBSize, colData.mLastBSize,
return colData;
void nsColumnSetFrame::DrainOverflowColumns() {
// First grab the prev-in-flows overflows and reparent them to this
// frame.
nsPresContext* presContext = PresContext();
nsColumnSetFrame* prev = static_cast<nsColumnSetFrame*>(GetPrevInFlow());
if (prev) {
AutoFrameListPtr overflows(presContext, prev->StealOverflowFrames());
if (overflows) {
nsContainerFrame::ReparentFrameViewList(*overflows, prev, this);
mFrames.InsertFrames(this, nullptr, std::move(*overflows));
// Now pull back our own overflows and append them to our children.
// We don't need to reparent them since we're already their parent.
AutoFrameListPtr overflows(presContext, StealOverflowFrames());
if (overflows) {
// We're already the parent for these frames, so no need to set
// their parent again.
mFrames.AppendFrames(nullptr, std::move(*overflows));
void nsColumnSetFrame::FindBestBalanceBSize(const ReflowInput& aReflowInput,
nsPresContext* aPresContext,
ReflowConfig& aConfig,
ColumnBalanceData aColData,
ReflowOutput& aDesiredSize,
bool aUnboundedLastColumn,
nsReflowStatus& aStatus) {
"Why are we here if we are not balancing columns?");
const nscoord availableContentBSize = aReflowInput.AvailableBSize();
// Termination of the algorithm below is guaranteed because
// aConfig.knownFeasibleBSize - aConfig.knownInfeasibleBSize decreases in
// every iteration.
int32_t iterationCount = 1;
// We set this flag when we detect that we may contain a frame
// that can break anywhere (thus foiling the linear decrease-by-one
// search)
bool maybeContinuousBreakingDetected = false;
bool possibleOptimalBSizeDetected = false;
// This is the extra block-size added to the optimal column block-size
// estimation which is calculated in the while-loop by dividing
// aColData.mSumBSize into N columns.
// The constant is arbitrary. We use a half of line-height first. In case a
// column container uses *zero* (or a very small) line-height, use a half of
// default line-height 1140/2 = 570 app units as the minimum value. Otherwise
// we might take more than necessary iterations before finding a feasible
// block-size.
nscoord extraBlockSize = std::max(570, aReflowInput.GetLineHeight() / 2);
// We use divide-by-N to estimate the optimal column block-size only if the
// last column's available block-size is unbounded.
bool foundFeasibleBSizeCloserToBest = !aUnboundedLastColumn;
// Stop the binary search when the difference of the feasible and infeasible
// block-size is within this gap. Here we use one device pixel.
const int32_t gapToStop = aPresContext->DevPixelsToAppUnits(1);
while (!aPresContext->HasPendingInterrupt()) {
nscoord lastKnownFeasibleBSize = aConfig.mKnownFeasibleBSize;
// Record what we learned from the last reflow
if (aColData.mFeasible) {
// mMaxBSize is feasible. Also, mLastBalanceBSize is feasible.
aConfig.mKnownFeasibleBSize =
std::min(aConfig.mKnownFeasibleBSize, aColData.mMaxBSize);
aConfig.mKnownFeasibleBSize =
std::min(aConfig.mKnownFeasibleBSize, mLastBalanceBSize);
// Furthermore, no block-size less than the block-size of the last
// column can ever be feasible. (We might be able to reduce the
// block-size of a non-last column by moving content to a later column,
// but we can't do that with the last column.)
if (aColData.mColCount == aConfig.mUsedColCount) {
aConfig.mKnownInfeasibleBSize =
std::max(aConfig.mKnownInfeasibleBSize, aColData.mLastBSize - 1);
} else {
aConfig.mKnownInfeasibleBSize =
std::max(aConfig.mKnownInfeasibleBSize, mLastBalanceBSize);
// If a column didn't fit in its available block-size, then its current
// block-size must be the minimum block-size for unbreakable content in
// the column, and therefore no smaller block-size can be feasible.
aConfig.mKnownInfeasibleBSize = std::max(
aConfig.mKnownInfeasibleBSize, aColData.mMaxOverflowingBSize - 1);
if (aUnboundedLastColumn) {
// The last column is unbounded, so all content got reflowed, so the
// mMaxBSize is feasible.
aConfig.mKnownFeasibleBSize =
std::min(aConfig.mKnownFeasibleBSize, aColData.mMaxBSize);
"Last column should be complete if the available "
"block-size is unconstrained!");
"%s: this=%p, mKnownInfeasibleBSize=%d, mKnownFeasibleBSize=%d",
__func__, this, aConfig.mKnownInfeasibleBSize,
if (aConfig.mKnownInfeasibleBSize >= aConfig.mKnownFeasibleBSize - 1) {
// aConfig.mKnownFeasibleBSize is where we want to be. This can happen in
// the very first iteration when a column container solely has a tall
// unbreakable child that overflows the container.
if (aConfig.mKnownInfeasibleBSize >= availableContentBSize) {
// There's no feasible block-size to fit our contents. We may need to
// reflow one more time after this loop.
const nscoord gap =
aConfig.mKnownFeasibleBSize - aConfig.mKnownInfeasibleBSize;
if (gap <= gapToStop && possibleOptimalBSizeDetected) {
// We detected a possible optimal block-size in the last iteration. If it
// is infeasible, we may need to reflow one more time after this loop.
if (lastKnownFeasibleBSize - aConfig.mKnownFeasibleBSize == 1) {
// We decreased the feasible block-size by one twip only. This could
// indicate that there is a continuously breakable child frame
// that we are crawling through.
maybeContinuousBreakingDetected = true;
nscoord nextGuess = aConfig.mKnownInfeasibleBSize + gap / 2;
if (aConfig.mKnownFeasibleBSize - nextGuess < extraBlockSize &&
!maybeContinuousBreakingDetected) {
// We're close to our target, so just try shrinking just the
// minimum amount that will cause one of our columns to break
// differently.
nextGuess = aConfig.mKnownFeasibleBSize - 1;
} else if (!foundFeasibleBSizeCloserToBest) {
// Make a guess by dividing mSumBSize into N columns and adding
// extraBlockSize to try to make it on the feasible side.
nextGuess = aColData.mSumBSize / aConfig.mUsedColCount + extraBlockSize;
// Sanitize it
nextGuess = clamped(nextGuess, aConfig.mKnownInfeasibleBSize + 1,
aConfig.mKnownFeasibleBSize - 1);
// We keep doubling extraBlockSize in every iteration until we find a
// feasible guess.
extraBlockSize *= 2;
} else if (aConfig.mKnownFeasibleBSize == NS_UNCONSTRAINEDSIZE) {
// This can happen when we had a next-in-flow so we didn't
// want to do an unbounded block-size measuring step. Let's just increase
// from the infeasible block-size by some reasonable amount.
nextGuess = aConfig.mKnownInfeasibleBSize * 2 + extraBlockSize;
} else if (gap <= gapToStop) {
// Floor nextGuess to the greatest multiple of gapToStop below or equal to
// mKnownFeasibleBSize.
nextGuess = aConfig.mKnownFeasibleBSize / gapToStop * gapToStop;
possibleOptimalBSizeDetected = true;
// Don't bother guessing more than our block-size constraint.
nextGuess = std::min(availableContentBSize, nextGuess);
COLUMN_SET_LOG("%s: Choosing next guess=%d, iteration=%d", __func__,
nextGuess, iterationCount);
aConfig.mColBSize = nextGuess;
aUnboundedLastColumn = false;
aColData =
ReflowColumns(aDesiredSize, aReflowInput, aStatus, aConfig, false);
if (!foundFeasibleBSizeCloserToBest && aColData.mFeasible) {
foundFeasibleBSizeCloserToBest = true;
if (!aColData.mFeasible && !aPresContext->HasPendingInterrupt()) {
// We need to reflow one more time at the feasible block-size to
// get a valid layout.
if (aConfig.mKnownInfeasibleBSize >= availableContentBSize) {
aConfig.mColBSize = availableContentBSize;
if (mLastBalanceBSize == availableContentBSize) {
// If we end up here, we have a constrained available content
// block-size, and our last column's block-size exceeds it. Also, if
// this is the first balancing iteration, the last column is given
// unconstrained available block-size, so it has a fully complete
// reflow status. Therefore, we always want to reflow again at the
// available content block-size to get a valid layout and a correct
// reflow status (likely an *incomplete* status) so that our column
// container can be fragmented if needed.
if (aReflowInput.mFlags.mColumnSetWrapperHasNoBSizeLeft) {
// If our column container has a constrained block-size (either in a
// paginated context or in a nested column container), and is going
// to consume all its computed block-size in this fragment, then our
// column container has no block-size left to contain our
// next-in-flows. We have to give up balancing, and create our
// own overflow columns.
// We don't want to create overflow columns immediately when our
// content doesn't fit since this changes our reflow status from
// incomplete to complete. Valid reasons include 1) the outer column
// container might do column balancing, and it can enlarge the
// available content block-size so that the nested one could fit its
// content in next balancing iteration; or 2) the outer column
// container is filling columns sequentially, and may have more
// inline-size to create more column boxes for the nested column
// container's next-in-flows.
aConfig = ChooseColumnStrategy(aReflowInput, true);
} else {
aConfig.mColBSize = aConfig.mKnownFeasibleBSize;
// This is our last attempt to reflow. If our column container's available
// block-size is unconstrained, make sure that the last column is
// allowed to have arbitrary block-size here, even though we were
// balancing. Otherwise we'd have to split, and it's not clear what we'd
// do with that.
COLUMN_SET_LOG("%s: Last attempt to call ReflowColumns", __func__);
aConfig.mIsLastBalancingReflow = true;
const bool forceUnboundedLastColumn =
aReflowInput.mParentReflowInput->AvailableBSize() ==
ReflowColumns(aDesiredSize, aReflowInput, aStatus, aConfig,
void nsColumnSetFrame::Reflow(nsPresContext* aPresContext,
ReflowOutput& aDesiredSize,
const ReflowInput& aReflowInput,
nsReflowStatus& aStatus) {
// Don't support interruption in columns
nsPresContext::InterruptPreventer noInterrupts(aPresContext);
MOZ_ASSERT(aStatus.IsEmpty(), "Caller should pass a fresh reflow status!");
"The column container should have relevant column styles!");
"The column container should be ColumnSetWrapperFrame!");
"Only the column container can have border and padding!");
GetChildList(FrameChildListID::OverflowContainers).IsEmpty() &&
"ColumnSetFrame should store overflow containers in principal "
"child list!");
//------------ Handle Incremental Reflow -----------------
COLUMN_SET_LOG("%s: Begin Reflow: this=%p, is nested multicol=%d", __func__,
// If inline size is unconstrained, set aForceAuto to true to allow
// the columns to expand in the inline direction. (This typically
// happens in orthogonal flows where the inline direction is the
// container's block direction).
ReflowConfig config = ChooseColumnStrategy(
aReflowInput, aReflowInput.ComputedISize() == NS_UNCONSTRAINEDSIZE);
// If balancing, then we allow the last column to grow to unbounded
// block-size during the first reflow. This gives us a way to estimate
// what the average column block-size should be, because we can measure
// the block-size of all the columns and sum them up. But don't do this
// if we have a next in flow because we don't want to suck all its
// content back here and then have to push it out again!
nsIFrame* nextInFlow = GetNextInFlow();
bool unboundedLastColumn = config.mIsBalancing && !nextInFlow;
const ColumnBalanceData colData = ReflowColumns(
aDesiredSize, aReflowInput, aStatus, config, unboundedLastColumn);
// If we're not balancing, then we're already done, since we should have
// reflown all of our children, and there is no need for a binary search to
// determine proper column block-size.
if (config.mIsBalancing && !aPresContext->HasPendingInterrupt()) {
FindBestBalanceBSize(aReflowInput, aPresContext, config, colData,
aDesiredSize, unboundedLastColumn, aStatus);
if (aPresContext->HasPendingInterrupt() &&
aReflowInput.AvailableBSize() == NS_UNCONSTRAINEDSIZE) {
// In this situation, we might be lying about our reflow status, because
// our last kid (the one that got interrupted) was incomplete. Fix that.
NS_ASSERTION(aStatus.IsFullyComplete() ||
aReflowInput.AvailableBSize() != NS_UNCONSTRAINEDSIZE,
"Column set should be complete if the available block-size is "
"ColumnSetWrapperFrame should be the abs.pos container!");
FinishAndStoreOverflow(&aDesiredSize, aReflowInput.mStyleDisplay);
COLUMN_SET_LOG("%s: End Reflow: this=%p", __func__, this);
void nsColumnSetFrame::BuildDisplayList(nsDisplayListBuilder* aBuilder,
const nsDisplayListSet& aLists) {
DisplayBorderBackgroundOutline(aBuilder, aLists);
if (IsVisibleForPainting()) {
// Our children won't have backgrounds so it doesn't matter where we put them.
for (nsIFrame* f : mFrames) {
BuildDisplayListForChild(aBuilder, f, aLists);
void nsColumnSetFrame::AppendDirectlyOwnedAnonBoxes(
nsTArray<OwnedAnonBox>& aResult) {
// Everything in mFrames is continuations of the first thing in mFrames.
nsIFrame* column = mFrames.FirstChild();
// We might not have any columns, apparently?
if (!column) {
MOZ_ASSERT(column->Style()->GetPseudoType() == PseudoStyleType::columnContent,
"What sort of child is this?");
Maybe<nscoord> nsColumnSetFrame::GetNaturalBaselineBOffset(
WritingMode aWM, BaselineSharingGroup aBaselineGroup,
BaselineExportContext aExportContext) const {
Maybe<nscoord> result;
for (const auto* kid : mFrames) {
auto kidBaseline =
kid->GetNaturalBaselineBOffset(aWM, aBaselineGroup, aExportContext);
if (!kidBaseline) {
// The kid frame may not necessarily be aligned with the columnset frame.
LogicalRect kidRect{aWM, kid->GetLogicalNormalPosition(aWM, GetSize()),
if (aBaselineGroup == BaselineSharingGroup::First) {
*kidBaseline += kidRect.BStart(aWM);
} else {
*kidBaseline += (GetLogicalSize().BSize(aWM) - kidRect.BEnd(aWM));
// Take the smallest of the baselines (i.e. Closest to border-block-start
// for `BaselineSharingGroup::First`, border-block-end for
// `BaselineSharingGroup::Last`)
if (!result || *kidBaseline < *result) {
result = kidBaseline;
return result;
#ifdef DEBUG
void nsColumnSetFrame::SetInitialChildList(ChildListID aListID,
nsFrameList&& aChildList) {
MOZ_ASSERT(aListID != FrameChildListID::Principal || aChildList.OnlyChild(),
"initial principal child list must have exactly one child");
nsContainerFrame::SetInitialChildList(aListID, std::move(aChildList));
void nsColumnSetFrame::AppendFrames(ChildListID aListID,
nsFrameList&& aFrameList) {
MOZ_CRASH("unsupported operation");
void nsColumnSetFrame::InsertFrames(ChildListID aListID, nsIFrame* aPrevFrame,
const nsLineList::iterator* aPrevFrameLine,
nsFrameList&& aFrameList) {
MOZ_CRASH("unsupported operation");
void nsColumnSetFrame::RemoveFrame(DestroyContext&, ChildListID, nsIFrame*) {
MOZ_CRASH("unsupported operation");