Adding upstream version 124.0.1.upstream/124.0.1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 6464 insertions, 0 deletions

diff --git a/layout/generic/nsFlexContainerFrame.cpp b/layout/generic/nsFlexContainerFrame.cpp
new file mode 100644
index 0000000000..e5a719fee2
--- /dev/null
+++ b/layout/generic/nsFlexContainerFrame.cpp
@@ -0,0 +1,6464 @@
+/* -*- 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 http://mozilla.org/MPL/2.0/. */
+
+/* rendering object for CSS "display: flex" */
+
+#include "nsFlexContainerFrame.h"
+
+#include <algorithm>
+
+#include "gfxContext.h"
+#include "mozilla/Baseline.h"
+#include "mozilla/ComputedStyle.h"
+#include "mozilla/CSSOrderAwareFrameIterator.h"
+#include "mozilla/FloatingPoint.h"
+#include "mozilla/Logging.h"
+#include "mozilla/PresShell.h"
+#include "mozilla/StaticPrefs_layout.h"
+#include "mozilla/WritingModes.h"
+#include "nsBlockFrame.h"
+#include "nsContentUtils.h"
+#include "nsCSSAnonBoxes.h"
+#include "nsDebug.h"
+#include "nsDisplayList.h"
+#include "nsFieldSetFrame.h"
+#include "nsIFrameInlines.h"
+#include "nsLayoutUtils.h"
+#include "nsPlaceholderFrame.h"
+#include "nsPresContext.h"
+
+using namespace mozilla;
+using namespace mozilla::layout;
+
+// Convenience typedefs for helper classes that we forward-declare in .h file
+// (so that nsFlexContainerFrame methods can use them as parameters):
+using FlexItem = nsFlexContainerFrame::FlexItem;
+using FlexLine = nsFlexContainerFrame::FlexLine;
+using FlexboxAxisTracker = nsFlexContainerFrame::FlexboxAxisTracker;
+using StrutInfo = nsFlexContainerFrame::StrutInfo;
+using CachedBAxisMeasurement = nsFlexContainerFrame::CachedBAxisMeasurement;
+using CachedFlexItemData = nsFlexContainerFrame::CachedFlexItemData;
+
+static mozilla::LazyLogModule gFlexContainerLog("FlexContainer");
+#define FLEX_LOG(...) \
+  MOZ_LOG(gFlexContainerLog, LogLevel::Debug, (__VA_ARGS__));
+#define FLEX_LOGV(...) \
+  MOZ_LOG(gFlexContainerLog, LogLevel::Verbose, (__VA_ARGS__));
+
+// Returns true if aFlexContainer is a frame for some element that has
+// display:-webkit-{inline-}box (or -moz-{inline-}box). aFlexContainer is
+// expected to be an instance of nsFlexContainerFrame (enforced with an assert);
+// otherwise, this function's state-bit-check here is bogus.
+static bool IsLegacyBox(const nsIFrame* aFlexContainer) {
+  MOZ_ASSERT(aFlexContainer->IsFlexContainerFrame(),
+             "only flex containers may be passed to this function");
+  return aFlexContainer->HasAnyStateBits(
+      NS_STATE_FLEX_IS_EMULATING_LEGACY_WEBKIT_BOX);
+}
+
+// Returns the OrderState enum we should pass to CSSOrderAwareFrameIterator
+// (depending on whether aFlexContainer has
+// NS_STATE_FLEX_NORMAL_FLOW_CHILDREN_IN_CSS_ORDER state bit).
+static CSSOrderAwareFrameIterator::OrderState OrderStateForIter(
+    const nsFlexContainerFrame* aFlexContainer) {
+  return aFlexContainer->HasAnyStateBits(
+             NS_STATE_FLEX_NORMAL_FLOW_CHILDREN_IN_CSS_ORDER)
+             ? CSSOrderAwareFrameIterator::OrderState::Ordered
+             : CSSOrderAwareFrameIterator::OrderState::Unordered;
+}
+
+// Returns the OrderingProperty enum that we should pass to
+// CSSOrderAwareFrameIterator (depending on whether it's a legacy box).
+static CSSOrderAwareFrameIterator::OrderingProperty OrderingPropertyForIter(
+    const nsFlexContainerFrame* aFlexContainer) {
+  return IsLegacyBox(aFlexContainer)
+             ? CSSOrderAwareFrameIterator::OrderingProperty::BoxOrdinalGroup
+             : CSSOrderAwareFrameIterator::OrderingProperty::Order;
+}
+
+// Returns the "align-items" value that's equivalent to the legacy "box-align"
+// value in the given style struct.
+static StyleAlignFlags ConvertLegacyStyleToAlignItems(
+    const nsStyleXUL* aStyleXUL) {
+  // -[moz|webkit]-box-align corresponds to modern "align-items"
+  switch (aStyleXUL->mBoxAlign) {
+    case StyleBoxAlign::Stretch:
+      return StyleAlignFlags::STRETCH;
+    case StyleBoxAlign::Start:
+      return StyleAlignFlags::FLEX_START;
+    case StyleBoxAlign::Center:
+      return StyleAlignFlags::CENTER;
+    case StyleBoxAlign::Baseline:
+      return StyleAlignFlags::BASELINE;
+    case StyleBoxAlign::End:
+      return StyleAlignFlags::FLEX_END;
+  }
+
+  MOZ_ASSERT_UNREACHABLE("Unrecognized mBoxAlign enum value");
+  // Fall back to default value of "align-items" property:
+  return StyleAlignFlags::STRETCH;
+}
+
+// Returns the "justify-content" value that's equivalent to the legacy
+// "box-pack" value in the given style struct.
+static StyleContentDistribution ConvertLegacyStyleToJustifyContent(
+    const nsStyleXUL* aStyleXUL) {
+  // -[moz|webkit]-box-pack corresponds to modern "justify-content"
+  switch (aStyleXUL->mBoxPack) {
+    case StyleBoxPack::Start:
+      return {StyleAlignFlags::FLEX_START};
+    case StyleBoxPack::Center:
+      return {StyleAlignFlags::CENTER};
+    case StyleBoxPack::End:
+      return {StyleAlignFlags::FLEX_END};
+    case StyleBoxPack::Justify:
+      return {StyleAlignFlags::SPACE_BETWEEN};
+  }
+
+  MOZ_ASSERT_UNREACHABLE("Unrecognized mBoxPack enum value");
+  // Fall back to default value of "justify-content" property:
+  return {StyleAlignFlags::FLEX_START};
+}
+
+// Check if the size is auto or it is a keyword in the block axis.
+// |aIsInline| should represent whether aSize is in the inline axis, from the
+// perspective of the writing mode of the flex item that the size comes from.
+//
+// max-content and min-content should behave as property's initial value.
+// Bug 567039: We treat -moz-fit-content and -moz-available as property's
+// initial value for now.
+static inline bool IsAutoOrEnumOnBSize(const StyleSize& aSize, bool aIsInline) {
+  return aSize.IsAuto() || (!aIsInline && !aSize.IsLengthPercentage());
+}
+
+// Encapsulates our flex container's main & cross axes. This class is backed by
+// a FlexboxAxisInfo helper member variable, and it adds some convenience APIs
+// on top of what that struct offers.
+class MOZ_STACK_CLASS nsFlexContainerFrame::FlexboxAxisTracker {
+ public:
+  explicit FlexboxAxisTracker(const nsFlexContainerFrame* aFlexContainer);
+
+  // Accessors:
+  LogicalAxis MainAxis() const {
+    return IsRowOriented() ? eLogicalAxisInline : eLogicalAxisBlock;
+  }
+  LogicalAxis CrossAxis() const {
+    return IsRowOriented() ? eLogicalAxisBlock : eLogicalAxisInline;
+  }
+
+  LogicalSide MainAxisStartSide() const;
+  LogicalSide MainAxisEndSide() const {
+    return GetOppositeSide(MainAxisStartSide());
+  }
+
+  LogicalSide CrossAxisStartSide() const;
+  LogicalSide CrossAxisEndSide() const {
+    return GetOppositeSide(CrossAxisStartSide());
+  }
+
+  mozilla::Side MainAxisPhysicalStartSide() const {
+    return mWM.PhysicalSide(MainAxisStartSide());
+  }
+  mozilla::Side MainAxisPhysicalEndSide() const {
+    return mWM.PhysicalSide(MainAxisEndSide());
+  }
+
+  mozilla::Side CrossAxisPhysicalStartSide() const {
+    return mWM.PhysicalSide(CrossAxisStartSide());
+  }
+  mozilla::Side CrossAxisPhysicalEndSide() const {
+    return mWM.PhysicalSide(CrossAxisEndSide());
+  }
+
+  // Returns the flex container's writing mode.
+  WritingMode GetWritingMode() const { return mWM; }
+
+  // Returns true if our main axis is in the reverse direction of our
+  // writing mode's corresponding axis. (From 'flex-direction: *-reverse')
+  bool IsMainAxisReversed() const { return mAxisInfo.mIsMainAxisReversed; }
+  // Returns true if our cross axis is in the reverse direction of our
+  // writing mode's corresponding axis. (From 'flex-wrap: *-reverse')
+  bool IsCrossAxisReversed() const { return mAxisInfo.mIsCrossAxisReversed; }
+
+  bool IsRowOriented() const { return mAxisInfo.mIsRowOriented; }
+  bool IsColumnOriented() const { return !IsRowOriented(); }
+
+  // aSize is expected to match the flex container's WritingMode.
+  nscoord MainComponent(const LogicalSize& aSize) const {
+    return IsRowOriented() ? aSize.ISize(mWM) : aSize.BSize(mWM);
+  }
+  int32_t MainComponent(const LayoutDeviceIntSize& aIntSize) const {
+    return IsMainAxisHorizontal() ? aIntSize.width : aIntSize.height;
+  }
+
+  // aSize is expected to match the flex container's WritingMode.
+  nscoord CrossComponent(const LogicalSize& aSize) const {
+    return IsRowOriented() ? aSize.BSize(mWM) : aSize.ISize(mWM);
+  }
+  int32_t CrossComponent(const LayoutDeviceIntSize& aIntSize) const {
+    return IsMainAxisHorizontal() ? aIntSize.height : aIntSize.width;
+  }
+
+  // NOTE: aMargin is expected to use the flex container's WritingMode.
+  nscoord MarginSizeInMainAxis(const LogicalMargin& aMargin) const {
+    // If we're row-oriented, our main axis is the inline axis.
+    return IsRowOriented() ? aMargin.IStartEnd(mWM) : aMargin.BStartEnd(mWM);
+  }
+  nscoord MarginSizeInCrossAxis(const LogicalMargin& aMargin) const {
+    // If we're row-oriented, our cross axis is the block axis.
+    return IsRowOriented() ? aMargin.BStartEnd(mWM) : aMargin.IStartEnd(mWM);
+  }
+
+  /**
+   * Converts a "flex-relative" point (a main-axis & cross-axis coordinate)
+   * into a LogicalPoint, using the flex container's writing mode.
+   *
+   *  @arg aMainCoord  The main-axis coordinate -- i.e an offset from the
+   *                   main-start edge of the flex container's content box.
+   *  @arg aCrossCoord The cross-axis coordinate -- i.e an offset from the
+   *                   cross-start edge of the flex container's content box.
+   *  @arg aContainerMainSize  The main size of flex container's content box.
+   *  @arg aContainerCrossSize The cross size of flex container's content box.
+   *  @return A LogicalPoint, with the flex container's writing mode, that
+   *          represents the same position. The logical coordinates are
+   *          relative to the flex container's content box.
+   */
+  LogicalPoint LogicalPointFromFlexRelativePoint(
+      nscoord aMainCoord, nscoord aCrossCoord, nscoord aContainerMainSize,
+      nscoord aContainerCrossSize) const {
+    nscoord logicalCoordInMainAxis =
+        IsMainAxisReversed() ? aContainerMainSize - aMainCoord : aMainCoord;
+    nscoord logicalCoordInCrossAxis =
+        IsCrossAxisReversed() ? aContainerCrossSize - aCrossCoord : aCrossCoord;
+
+    return IsRowOriented() ? LogicalPoint(mWM, logicalCoordInMainAxis,
+                                          logicalCoordInCrossAxis)
+                           : LogicalPoint(mWM, logicalCoordInCrossAxis,
+                                          logicalCoordInMainAxis);
+  }
+
+  /**
+   * Converts a "flex-relative" size (a main-axis & cross-axis size)
+   * into a LogicalSize, using the flex container's writing mode.
+   *
+   *  @arg aMainSize  The main-axis size.
+   *  @arg aCrossSize The cross-axis size.
+   *  @return A LogicalSize, with the flex container's writing mode, that
+   *          represents the same size.
+   */
+  LogicalSize LogicalSizeFromFlexRelativeSizes(nscoord aMainSize,
+                                               nscoord aCrossSize) const {
+    return IsRowOriented() ? LogicalSize(mWM, aMainSize, aCrossSize)
+                           : LogicalSize(mWM, aCrossSize, aMainSize);
+  }
+
+  /**
+   * Converts a "flex-relative" ascent (the distance from the flex container's
+   * content-box cross-start edge to its baseline) into a logical ascent (the
+   * distance from the flex container's content-box block-start edge to its
+   * baseline).
+   */
+  nscoord LogicalAscentFromFlexRelativeAscent(
+      nscoord aFlexRelativeAscent, nscoord aContentBoxCrossSize) const {
+    return (IsCrossAxisReversed() ? aContentBoxCrossSize - aFlexRelativeAscent
+                                  : aFlexRelativeAscent);
+  }
+
+  bool IsMainAxisHorizontal() const {
+    // If we're row-oriented, and our writing mode is NOT vertical,
+    // or we're column-oriented and our writing mode IS vertical,
+    // then our main axis is horizontal. This handles all cases:
+    return IsRowOriented() != mWM.IsVertical();
+  }
+
+  // Returns true if this flex item's inline axis in aItemWM is parallel (or
+  // antiparallel) to the container's main axis. Returns false, otherwise.
+  //
+  // Note: this is a helper used before constructing FlexItem. Inside of flex
+  // reflow code, FlexItem::IsInlineAxisMainAxis() is equivalent & more optimal.
+  bool IsInlineAxisMainAxis(WritingMode aItemWM) const {
+    return IsRowOriented() != GetWritingMode().IsOrthogonalTo(aItemWM);
+  }
+
+  // Maps justify-*: 'left' or 'right' to 'start' or 'end'.
+  StyleAlignFlags ResolveJustifyLeftRight(const StyleAlignFlags& aFlags) const {
+    MOZ_ASSERT(
+        aFlags == StyleAlignFlags::LEFT || aFlags == StyleAlignFlags::RIGHT,
+        "This helper accepts only 'LEFT' or 'RIGHT' flags!");
+
+    const auto wm = GetWritingMode();
+    const bool isJustifyLeft = aFlags == StyleAlignFlags::LEFT;
+    if (IsColumnOriented()) {
+      if (!wm.IsVertical()) {
+        // Container's alignment axis (main axis) is *not* parallel to the
+        // line-left <-> line-right axis or the physical left <-> physical right
+        // axis, so we map both 'left' and 'right' to 'start'.
+        return StyleAlignFlags::START;
+      }
+
+      MOZ_ASSERT(wm.PhysicalAxis(MainAxis()) == eAxisHorizontal,
+                 "Vertical column-oriented flex container's main axis should "
+                 "be parallel to physical left <-> right axis!");
+      // Map 'left' or 'right' to 'start' or 'end', depending on its block flow
+      // direction.
+      return isJustifyLeft == wm.IsVerticalLR() ? StyleAlignFlags::START
+                                                : StyleAlignFlags::END;
+    }
+
+    MOZ_ASSERT(MainAxis() == eLogicalAxisInline,
+               "Row-oriented flex container's main axis should be parallel to "
+               "line-left <-> line-right axis!");
+
+    // If we get here, we're operating on the flex container's inline axis,
+    // so we map 'left' to whichever of 'start' or 'end' corresponds to the
+    // *line-relative* left side; and similar for 'right'.
+    return isJustifyLeft == wm.IsBidiLTR() ? StyleAlignFlags::START
+                                           : StyleAlignFlags::END;
+  }
+
+  // Delete copy-constructor & reassignment operator, to prevent accidental
+  // (unnecessary) copying.
+  FlexboxAxisTracker(const FlexboxAxisTracker&) = delete;
+  FlexboxAxisTracker& operator=(const FlexboxAxisTracker&) = delete;
+
+ private:
+  const WritingMode mWM;  // The flex container's writing mode.
+  const FlexboxAxisInfo mAxisInfo;
+};
+
+/**
+ * Represents a flex item.
+ * Includes the various pieces of input that the Flexbox Layout Algorithm uses
+ * to resolve a flexible width.
+ */
+class nsFlexContainerFrame::FlexItem final {
+ public:
+  // Normal constructor:
+  FlexItem(ReflowInput& aFlexItemReflowInput, float aFlexGrow,
+           float aFlexShrink, nscoord aFlexBaseSize, nscoord aMainMinSize,
+           nscoord aMainMaxSize, nscoord aTentativeCrossSize,
+           nscoord aCrossMinSize, nscoord aCrossMaxSize,
+           const FlexboxAxisTracker& aAxisTracker);
+
+  // Simplified constructor, to be used only for generating "struts":
+  // (NOTE: This "strut" constructor uses the *container's* writing mode, which
+  // we'll use on this FlexItem instead of the child frame's real writing mode.
+  // This is fine - it doesn't matter what writing mode we use for a
+  // strut, since it won't render any content and we already know its size.)
+  FlexItem(nsIFrame* aChildFrame, nscoord aCrossSize, WritingMode aContainerWM,
+           const FlexboxAxisTracker& aAxisTracker);
+
+  // Clone existing FlexItem for its underlying frame's continuation.
+  // @param aContinuation a continuation in our next-in-flow chain.
+  FlexItem CloneFor(nsIFrame* const aContinuation) const {
+    MOZ_ASSERT(Frame() == aContinuation->FirstInFlow(),
+               "aContinuation should be in aItem's continuation chain!");
+    FlexItem item(*this);
+    item.mFrame = aContinuation;
+    item.mHadMeasuringReflow = false;
+    return item;
+  }
+
+  // Accessors
+  nsIFrame* Frame() const { return mFrame; }
+  nscoord FlexBaseSize() const { return mFlexBaseSize; }
+
+  nscoord MainMinSize() const {
+    MOZ_ASSERT(!mNeedsMinSizeAutoResolution,
+               "Someone's using an unresolved 'auto' main min-size");
+    return mMainMinSize;
+  }
+  nscoord MainMaxSize() const { return mMainMaxSize; }
+
+  // Note: These return the main-axis position and size of our *content box*.
+  nscoord MainSize() const { return mMainSize; }
+  nscoord MainPosition() const { return mMainPosn; }
+
+  nscoord CrossMinSize() const { return mCrossMinSize; }
+  nscoord CrossMaxSize() const { return mCrossMaxSize; }
+
+  // Note: These return the cross-axis position and size of our *content box*.
+  nscoord CrossSize() const { return mCrossSize; }
+  nscoord CrossPosition() const { return mCrossPosn; }
+
+  // Lazy getter for mAscent or mAscentForLast.
+  nscoord ResolvedAscent(bool aUseFirstBaseline) const {
+    // XXX We should be using the *container's* writing-mode (mCBWM) here,
+    // instead of the item's (mWM). This is essentially bug 1155322.
+    nscoord& ascent = aUseFirstBaseline ? mAscent : mAscentForLast;
+    if (ascent != ReflowOutput::ASK_FOR_BASELINE) {
+      return ascent;
+    }
+
+    // Use GetFirstLineBaseline() or GetLastLineBaseline() as appropriate:
+    bool found = aUseFirstBaseline
+                     ? nsLayoutUtils::GetFirstLineBaseline(mWM, mFrame, &ascent)
+                     : nsLayoutUtils::GetLastLineBaseline(mWM, mFrame, &ascent);
+    if (found) {
+      return ascent;
+    }
+
+    // If the nsLayoutUtils getter fails, then ask the frame directly:
+    auto baselineGroup = aUseFirstBaseline ? BaselineSharingGroup::First
+                                           : BaselineSharingGroup::Last;
+    if (auto baseline = mFrame->GetNaturalBaselineBOffset(
+            mWM, baselineGroup, BaselineExportContext::Other)) {
+      // Offset for last baseline from `GetNaturalBaselineBOffset` originates
+      // from the frame's block end, so convert it back.
+      ascent = baselineGroup == BaselineSharingGroup::First
+                   ? *baseline
+                   : mFrame->BSize(mWM) - *baseline;
+      return ascent;
+    }
+
+    // We couldn't determine a baseline, so we synthesize one from border box:
+    ascent = Baseline::SynthesizeBOffsetFromBorderBox(
+        mFrame, mWM, BaselineSharingGroup::First);
+    return ascent;
+  }
+
+  // Convenience methods to compute the main & cross size of our *margin-box*.
+  nscoord OuterMainSize() const {
+    return mMainSize + MarginBorderPaddingSizeInMainAxis();
+  }
+
+  nscoord OuterCrossSize() const {
+    return mCrossSize + MarginBorderPaddingSizeInCrossAxis();
+  }
+
+  // Convenience method to return the content-box block-size.
+  nscoord BSize() const {
+    return IsBlockAxisMainAxis() ? MainSize() : CrossSize();
+  }
+
+  // Convenience method to return the measured content-box block-size computed
+  // in nsFlexContainerFrame::MeasureBSizeForFlexItem().
+  Maybe<nscoord> MeasuredBSize() const;
+
+  // Convenience methods to synthesize a style main size or a style cross size
+  // with box-size considered, to provide the size overrides when constructing
+  // ReflowInput for flex items.
+  StyleSize StyleMainSize() const {
+    nscoord mainSize = MainSize();
+    if (Frame()->StylePosition()->mBoxSizing == StyleBoxSizing::Border) {
+      mainSize += BorderPaddingSizeInMainAxis();
+    }
+    return StyleSize::LengthPercentage(
+        LengthPercentage::FromAppUnits(mainSize));
+  }
+
+  StyleSize StyleCrossSize() const {
+    nscoord crossSize = CrossSize();
+    if (Frame()->StylePosition()->mBoxSizing == StyleBoxSizing::Border) {
+      crossSize += BorderPaddingSizeInCrossAxis();
+    }
+    return StyleSize::LengthPercentage(
+        LengthPercentage::FromAppUnits(crossSize));
+  }
+
+  // Returns the distance between this FlexItem's baseline and the cross-start
+  // edge of its margin-box. Used in baseline alignment.
+  //
+  // (This function needs to be told which physical start side we're measuring
+  // the baseline from, so that it can look up the appropriate components from
+  // margin.)
+  nscoord BaselineOffsetFromOuterCrossEdge(mozilla::Side aStartSide,
+                                           bool aUseFirstLineBaseline) const;
+
+  double ShareOfWeightSoFar() const { return mShareOfWeightSoFar; }
+
+  bool IsFrozen() const { return mIsFrozen; }
+
+  bool HadMinViolation() const {
+    MOZ_ASSERT(!mIsFrozen, "min violation has no meaning for frozen items.");
+    return mHadMinViolation;
+  }
+
+  bool HadMaxViolation() const {
+    MOZ_ASSERT(!mIsFrozen, "max violation has no meaning for frozen items.");
+    return mHadMaxViolation;
+  }
+
+  bool WasMinClamped() const {
+    MOZ_ASSERT(mIsFrozen, "min clamping has no meaning for unfrozen items.");
+    return mHadMinViolation;
+  }
+
+  bool WasMaxClamped() const {
+    MOZ_ASSERT(mIsFrozen, "max clamping has no meaning for unfrozen items.");
+    return mHadMaxViolation;
+  }
+
+  // Indicates whether this item received a preliminary "measuring" reflow
+  // before its actual reflow.
+  bool HadMeasuringReflow() const { return mHadMeasuringReflow; }
+
+  // Indicates whether this item's computed cross-size property is 'auto'.
+  bool IsCrossSizeAuto() const;
+
+  // Indicates whether the cross-size property is set to something definite,
+  // for the purpose of preferred aspect ratio calculations.
+  bool IsCrossSizeDefinite(const ReflowInput& aItemReflowInput) const;
+
+  // Indicates whether this item's cross-size has been stretched (from having
+  // "align-self: stretch" with an auto cross-size and no auto margins in the
+  // cross axis).
+  bool IsStretched() const { return mIsStretched; }
+
+  bool IsFlexBaseSizeContentBSize() const {
+    return mIsFlexBaseSizeContentBSize;
+  }
+
+  bool IsMainMinSizeContentBSize() const { return mIsMainMinSizeContentBSize; }
+
+  // Indicates whether we need to resolve an 'auto' value for the main-axis
+  // min-[width|height] property.
+  bool NeedsMinSizeAutoResolution() const {
+    return mNeedsMinSizeAutoResolution;
+  }
+
+  bool HasAnyAutoMargin() const { return mHasAnyAutoMargin; }
+
+  BaselineSharingGroup ItemBaselineSharingGroup() const {
+    MOZ_ASSERT(mAlignSelf._0 == StyleAlignFlags::BASELINE ||
+                   mAlignSelf._0 == StyleAlignFlags::LAST_BASELINE,
+               "mBaselineSharingGroup only gets a meaningful value "
+               "for baseline-aligned items");
+    return mBaselineSharingGroup;
+  }
+
+  // Indicates whether this item is a "strut" left behind by an element with
+  // visibility:collapse.
+  bool IsStrut() const { return mIsStrut; }
+
+  // The main axis and cross axis are relative to mCBWM.
+  LogicalAxis MainAxis() const { return mMainAxis; }
+  LogicalAxis CrossAxis() const { return GetOrthogonalAxis(mMainAxis); }
+
+  // IsInlineAxisMainAxis() returns true if this item's inline axis is parallel
+  // (or antiparallel) to the container's main axis. Otherwise (i.e. if this
+  // item's inline axis is orthogonal to the container's main axis), this
+  // function returns false. The next 3 methods are all other ways of asking
+  // the same question, and only exist for readability at callsites (depending
+  // on which axes those callsites are reasoning about).
+  bool IsInlineAxisMainAxis() const { return mIsInlineAxisMainAxis; }
+  bool IsInlineAxisCrossAxis() const { return !mIsInlineAxisMainAxis; }
+  bool IsBlockAxisMainAxis() const { return !mIsInlineAxisMainAxis; }
+  bool IsBlockAxisCrossAxis() const { return mIsInlineAxisMainAxis; }
+
+  WritingMode GetWritingMode() const { return mWM; }
+  WritingMode ContainingBlockWM() const { return mCBWM; }
+  StyleAlignSelf AlignSelf() const { return mAlignSelf; }
+  StyleAlignFlags AlignSelfFlags() const { return mAlignSelfFlags; }
+
+  // Returns the flex factor (flex-grow or flex-shrink), depending on
+  // 'aIsUsingFlexGrow'.
+  //
+  // Asserts fatally if called on a frozen item (since frozen items are not
+  // flexible).
+  float GetFlexFactor(bool aIsUsingFlexGrow) {
+    MOZ_ASSERT(!IsFrozen(), "shouldn't need flex factor after item is frozen");
+
+    return aIsUsingFlexGrow ? mFlexGrow : mFlexShrink;
+  }
+
+  // Returns the weight that we should use in the "resolving flexible lengths"
+  // algorithm.  If we're using the flex grow factor, we just return that;
+  // otherwise, we return the "scaled flex shrink factor" (scaled by our flex
+  // base size, so that when both large and small items are shrinking, the large
+  // items shrink more).
+  //
+  // I'm calling this a "weight" instead of a "[scaled] flex-[grow|shrink]
+  // factor", to more clearly distinguish it from the actual flex-grow &
+  // flex-shrink factors.
+  //
+  // Asserts fatally if called on a frozen item (since frozen items are not
+  // flexible).
+  float GetWeight(bool aIsUsingFlexGrow) {
+    MOZ_ASSERT(!IsFrozen(), "shouldn't need weight after item is frozen");
+
+    if (aIsUsingFlexGrow) {
+      return mFlexGrow;
+    }
+
+    // We're using flex-shrink --> return mFlexShrink * mFlexBaseSize
+    if (mFlexBaseSize == 0) {
+      // Special-case for mFlexBaseSize == 0 -- we have no room to shrink, so
+      // regardless of mFlexShrink, we should just return 0.
+      // (This is really a special-case for when mFlexShrink is infinity, to
+      // avoid performing mFlexShrink * mFlexBaseSize = inf * 0 = undefined.)
+      return 0.0f;
+    }
+    return mFlexShrink * mFlexBaseSize;
+  }
+
+  bool TreatBSizeAsIndefinite() const { return mTreatBSizeAsIndefinite; }
+
+  const AspectRatio& GetAspectRatio() const { return mAspectRatio; }
+  bool HasAspectRatio() const { return !!mAspectRatio; }
+
+  // Getters for margin:
+  // ===================
+  LogicalMargin Margin() const { return mMargin; }
+  nsMargin PhysicalMargin() const { return mMargin.GetPhysicalMargin(mCBWM); }
+
+  // Returns the margin component for a given LogicalSide in flex container's
+  // writing-mode.
+  nscoord GetMarginComponentForSide(LogicalSide aSide) const {
+    return mMargin.Side(aSide, mCBWM);
+  }
+
+  // Returns the total space occupied by this item's margins in the given axis
+  nscoord MarginSizeInMainAxis() const {
+    return mMargin.StartEnd(MainAxis(), mCBWM);
+  }
+  nscoord MarginSizeInCrossAxis() const {
+    return mMargin.StartEnd(CrossAxis(), mCBWM);
+  }
+
+  // Getters for border/padding
+  // ==========================
+  // Returns the total space occupied by this item's borders and padding in
+  // the given axis
+  LogicalMargin BorderPadding() const { return mBorderPadding; }
+  nscoord BorderPaddingSizeInMainAxis() const {
+    return mBorderPadding.StartEnd(MainAxis(), mCBWM);
+  }
+  nscoord BorderPaddingSizeInCrossAxis() const {
+    return mBorderPadding.StartEnd(CrossAxis(), mCBWM);
+  }
+
+  // Getter for combined margin/border/padding
+  // =========================================
+  // Returns the total space occupied by this item's margins, borders and
+  // padding in the given axis
+  nscoord MarginBorderPaddingSizeInMainAxis() const {
+    return MarginSizeInMainAxis() + BorderPaddingSizeInMainAxis();
+  }
+  nscoord MarginBorderPaddingSizeInCrossAxis() const {
+    return MarginSizeInCrossAxis() + BorderPaddingSizeInCrossAxis();
+  }
+
+  // Setters
+  // =======
+  // Helper to set the resolved value of min-[width|height]:auto for the main
+  // axis. (Should only be used if NeedsMinSizeAutoResolution() returns true.)
+  void UpdateMainMinSize(nscoord aNewMinSize) {
+    NS_ASSERTION(aNewMinSize >= 0,
+                 "How did we end up with a negative min-size?");
+    MOZ_ASSERT(
+        mMainMaxSize == NS_UNCONSTRAINEDSIZE || mMainMaxSize >= aNewMinSize,
+        "Should only use this function for resolving min-size:auto, "
+        "and main max-size should be an upper-bound for resolved val");
+    MOZ_ASSERT(
+        mNeedsMinSizeAutoResolution &&
+            (mMainMinSize == 0 || mFrame->IsThemed(mFrame->StyleDisplay())),
+        "Should only use this function for resolving min-size:auto, "
+        "so we shouldn't already have a nonzero min-size established "
+        "(unless it's a themed-widget-imposed minimum size)");
+
+    if (aNewMinSize > mMainMinSize) {
+      mMainMinSize = aNewMinSize;
+      // Also clamp main-size to be >= new min-size:
+      mMainSize = std::max(mMainSize, aNewMinSize);
+    }
+    mNeedsMinSizeAutoResolution = false;
+  }
+
+  // This sets our flex base size, and then sets our main size to the
+  // resulting "hypothetical main size" (the base size clamped to our
+  // main-axis [min,max] sizing constraints).
+  void SetFlexBaseSizeAndMainSize(nscoord aNewFlexBaseSize) {
+    MOZ_ASSERT(!mIsFrozen || mFlexBaseSize == NS_UNCONSTRAINEDSIZE,
+               "flex base size shouldn't change after we're frozen "
+               "(unless we're just resolving an intrinsic size)");
+    mFlexBaseSize = aNewFlexBaseSize;
+
+    // Before we've resolved flexible lengths, we keep mMainSize set to
+    // the 'hypothetical main size', which is the flex base size, clamped
+    // to the [min,max] range:
+    mMainSize = NS_CSS_MINMAX(mFlexBaseSize, mMainMinSize, mMainMaxSize);
+
+    FLEX_LOGV(
+        "Set flex base size: %d, hypothetical main size: %d for flex item %p",
+        mFlexBaseSize, mMainSize, mFrame);
+  }
+
+  // Setters used while we're resolving flexible lengths
+  // ---------------------------------------------------
+
+  // Sets the main-size of our flex item's content-box.
+  void SetMainSize(nscoord aNewMainSize) {
+    MOZ_ASSERT(!mIsFrozen, "main size shouldn't change after we're frozen");
+    mMainSize = aNewMainSize;
+  }
+
+  void SetShareOfWeightSoFar(double aNewShare) {
+    MOZ_ASSERT(!mIsFrozen || aNewShare == 0.0,
+               "shouldn't be giving this item any share of the weight "
+               "after it's frozen");
+    mShareOfWeightSoFar = aNewShare;
+  }
+
+  void Freeze() {
+    mIsFrozen = true;
+    // Now that we are frozen, the meaning of mHadMinViolation and
+    // mHadMaxViolation changes to indicate min and max clamping. Clear
+    // both of the member variables so that they are ready to be set
+    // as clamping state later, if necessary.
+    mHadMinViolation = false;
+    mHadMaxViolation = false;
+  }
+
+  void SetHadMinViolation() {
+    MOZ_ASSERT(!mIsFrozen,
+               "shouldn't be changing main size & having violations "
+               "after we're frozen");
+    mHadMinViolation = true;
+  }
+  void SetHadMaxViolation() {
+    MOZ_ASSERT(!mIsFrozen,
+               "shouldn't be changing main size & having violations "
+               "after we're frozen");
+    mHadMaxViolation = true;
+  }
+  void ClearViolationFlags() {
+    MOZ_ASSERT(!mIsFrozen,
+               "shouldn't be altering violation flags after we're "
+               "frozen");
+    mHadMinViolation = mHadMaxViolation = false;
+  }
+
+  void SetWasMinClamped() {
+    MOZ_ASSERT(!mHadMinViolation && !mHadMaxViolation, "only clamp once");
+    // This reuses the mHadMinViolation member variable to track clamping
+    // events. This is allowable because mHadMinViolation only reflects
+    // a violation up until the item is frozen.
+    MOZ_ASSERT(mIsFrozen, "shouldn't set clamping state when we are unfrozen");
+    mHadMinViolation = true;
+  }
+  void SetWasMaxClamped() {
+    MOZ_ASSERT(!mHadMinViolation && !mHadMaxViolation, "only clamp once");
+    // This reuses the mHadMaxViolation member variable to track clamping
+    // events. This is allowable because mHadMaxViolation only reflects
+    // a violation up until the item is frozen.
+    MOZ_ASSERT(mIsFrozen, "shouldn't set clamping state when we are unfrozen");
+    mHadMaxViolation = true;
+  }
+
+  // Setters for values that are determined after we've resolved our main size
+  // -------------------------------------------------------------------------
+
+  // Sets the main-axis position of our flex item's content-box.
+  // (This is the distance between the main-start edge of the flex container
+  // and the main-start edge of the flex item's content-box.)
+  void SetMainPosition(nscoord aPosn) {
+    MOZ_ASSERT(mIsFrozen, "main size should be resolved before this");
+    mMainPosn = aPosn;
+  }
+
+  // Sets the cross-size of our flex item's content-box.
+  void SetCrossSize(nscoord aCrossSize) {
+    MOZ_ASSERT(!mIsStretched,
+               "Cross size shouldn't be modified after it's been stretched");
+    mCrossSize = aCrossSize;
+  }
+
+  // Sets the cross-axis position of our flex item's content-box.
+  // (This is the distance between the cross-start edge of the flex container
+  // and the cross-start edge of the flex item.)
+  void SetCrossPosition(nscoord aPosn) {
+    MOZ_ASSERT(mIsFrozen, "main size should be resolved before this");
+    mCrossPosn = aPosn;
+  }
+
+  // After a FlexItem has had a reflow, this method can be used to cache its
+  // (possibly-unresolved) ascent, in case it's needed later for
+  // baseline-alignment or to establish the container's baseline.
+  // (NOTE: This can be marked 'const' even though it's modifying mAscent,
+  // because mAscent is mutable. It's nice for this to be 'const', because it
+  // means our final reflow can iterate over const FlexItem pointers, and we
+  // can be sure it's not modifying those FlexItems, except via this method.)
+  void SetAscent(nscoord aAscent) const {
+    mAscent = aAscent;  // NOTE: this may be ASK_FOR_BASELINE
+  }
+
+  void SetHadMeasuringReflow() { mHadMeasuringReflow = true; }
+
+  void SetIsStretched() {
+    MOZ_ASSERT(mIsFrozen, "main size should be resolved before this");
+    mIsStretched = true;
+  }
+
+  void SetIsFlexBaseSizeContentBSize() { mIsFlexBaseSizeContentBSize = true; }
+
+  void SetIsMainMinSizeContentBSize() { mIsMainMinSizeContentBSize = true; }
+
+  // Setter for margin components (for resolving "auto" margins)
+  void SetMarginComponentForSide(LogicalSide aSide, nscoord aLength) {
+    MOZ_ASSERT(mIsFrozen, "main size should be resolved before this");
+    mMargin.Side(aSide, mCBWM) = aLength;
+  }
+
+  void ResolveStretchedCrossSize(nscoord aLineCrossSize);
+
+  // Resolves flex base size if flex-basis' used value is 'content', using this
+  // item's preferred aspect ratio and cross size.
+  void ResolveFlexBaseSizeFromAspectRatio(const ReflowInput& aItemReflowInput);
+
+  uint32_t NumAutoMarginsInMainAxis() const {
+    return NumAutoMarginsInAxis(MainAxis());
+  };
+
+  uint32_t NumAutoMarginsInCrossAxis() const {
+    return NumAutoMarginsInAxis(CrossAxis());
+  };
+
+  // Once the main size has been resolved, should we bother doing layout to
+  // establish the cross size?
+  bool CanMainSizeInfluenceCrossSize() const;
+
+  // Returns a main size, clamped by any definite min and max cross size
+  // converted through the preferred aspect ratio. The caller is responsible for
+  // ensuring that the flex item's preferred aspect ratio is not zero.
+  nscoord ClampMainSizeViaCrossAxisConstraints(
+      nscoord aMainSize, const ReflowInput& aItemReflowInput) const;
+
+  // Indicates whether we think this flex item needs a "final" reflow
+  // (after its final flexed size & final position have been determined).
+  //
+  // @param aParentReflowInput the flex container's reflow input.
+  // @return true if such a reflow is needed, or false if we believe it can
+  // simply be moved to its final position and skip the reflow.
+  bool NeedsFinalReflow(const ReflowInput& aParentReflowInput) const;
+
+  // Gets the block frame that contains the flex item's content.  This is
+  // Frame() itself or one of its descendants.
+  nsBlockFrame* BlockFrame() const;
+
+ protected:
+  bool IsMinSizeAutoResolutionNeeded() const;
+
+  uint32_t NumAutoMarginsInAxis(LogicalAxis aAxis) const;
+
+  // Values that we already know in constructor, and remain unchanged:
+  // The flex item's frame.
+  nsIFrame* mFrame = nullptr;
+  float mFlexGrow = 0.0f;
+  float mFlexShrink = 0.0f;
+  AspectRatio mAspectRatio;
+
+  // The flex item's writing mode.
+  WritingMode mWM;
+
+  // The flex container's writing mode.
+  WritingMode mCBWM;
+
+  // The flex container's main axis in flex container's writing mode.
+  LogicalAxis mMainAxis;
+
+  // Stored in flex container's writing mode.
+  LogicalMargin mBorderPadding;
+
+  // Stored in flex container's writing mode. Its value can change when we
+  // resolve "auto" marigns.
+  LogicalMargin mMargin;
+
+  // These are non-const so that we can lazily update them with the item's
+  // intrinsic size (obtained via a "measuring" reflow), when necessary.
+  // (e.g. for "flex-basis:auto;height:auto" & "min-height:auto")
+  nscoord mFlexBaseSize = 0;
+  nscoord mMainMinSize = 0;
+  nscoord mMainMaxSize = 0;
+
+  // mCrossMinSize and mCrossMaxSize are not changed after constructor.
+  nscoord mCrossMinSize = 0;
+  nscoord mCrossMaxSize = 0;
+
+  // Values that we compute after constructor:
+  nscoord mMainSize = 0;
+  nscoord mMainPosn = 0;
+  nscoord mCrossSize = 0;
+  nscoord mCrossPosn = 0;
+
+  // Mutable b/c it's set & resolved lazily, sometimes via const pointer. See
+  // comment above SetAscent().
+  // We initialize this to ASK_FOR_BASELINE, and opportunistically fill it in
+  // with a real value if we end up reflowing this flex item. (But if we don't
+  // reflow this flex item, then this sentinel tells us that we don't know it
+  // yet & anyone who cares will need to explicitly request it.)
+  //
+  // Both mAscent and mAscentForLast are distance from the frame's border-box
+  // block-start edge.
+  mutable nscoord mAscent = ReflowOutput::ASK_FOR_BASELINE;
+  mutable nscoord mAscentForLast = ReflowOutput::ASK_FOR_BASELINE;
+
+  // Temporary state, while we're resolving flexible widths (for our main size)
+  // XXXdholbert To save space, we could use a union to make these variables
+  // overlay the same memory as some other member vars that aren't touched
+  // until after main-size has been resolved. In particular, these could share
+  // memory with mMainPosn through mAscent, and mIsStretched.
+  double mShareOfWeightSoFar = 0.0;
+
+  bool mIsFrozen = false;
+  bool mHadMinViolation = false;
+  bool mHadMaxViolation = false;
+
+  // Did this item get a preliminary reflow, to measure its desired height?
+  bool mHadMeasuringReflow = false;
+
+  // See IsStretched() documentation.
+  bool mIsStretched = false;
+
+  // Is this item a "strut" left behind by an element with visibility:collapse?
+  bool mIsStrut = false;
+
+  // See IsInlineAxisMainAxis() documentation. This is not changed after
+  // constructor.
+  bool mIsInlineAxisMainAxis = true;
+
+  // Does this item need to resolve a min-[width|height]:auto (in main-axis)?
+  //
+  // Note: mNeedsMinSizeAutoResolution needs to be declared towards the end of
+  // the member variables since it's initialized in a method that depends on
+  // other members declared above such as mCBWM, mMainAxis, and
+  // mIsInlineAxisMainAxis.
+  bool mNeedsMinSizeAutoResolution = false;
+
+  // Should we take care to treat this item's resolved BSize as indefinite?
+  bool mTreatBSizeAsIndefinite = false;
+
+  // Does this item have an auto margin in either main or cross axis?
+  bool mHasAnyAutoMargin = false;
+
+  // Does this item have a content-based flex base size (and is that a size in
+  // its block-axis)?
+  bool mIsFlexBaseSizeContentBSize = false;
+
+  // Does this item have a content-based resolved auto min size (and is that a
+  // size in its block-axis)?
+  bool mIsMainMinSizeContentBSize = false;
+
+  // If this item is {first,last}-baseline-aligned using 'align-self', which of
+  // its FlexLine's baseline sharing groups does it participate in?
+  BaselineSharingGroup mBaselineSharingGroup = BaselineSharingGroup::First;
+
+  // My "align-self" computed value (with "auto" swapped out for parent"s
+  // "align-items" value, in our constructor).
+  StyleAlignSelf mAlignSelf{StyleAlignFlags::AUTO};
+
+  // Flags for 'align-self' (safe/unsafe/legacy).
+  StyleAlignFlags mAlignSelfFlags{0};
+};
+
+/**
+ * Represents a single flex line in a flex container.
+ * Manages an array of the FlexItems that are in the line.
+ */
+class nsFlexContainerFrame::FlexLine final {
+ public:
+  explicit FlexLine(nscoord aMainGapSize) : mMainGapSize(aMainGapSize) {}
+
+  nscoord SumOfGaps() const {
+    return NumItems() > 0 ? (NumItems() - 1) * mMainGapSize : 0;
+  }
+
+  // Returns the sum of our FlexItems' outer hypothetical main sizes plus the
+  // sum of main axis {row,column}-gaps between items.
+  // ("outer" = margin-box, and "hypothetical" = before flexing)
+  AuCoord64 TotalOuterHypotheticalMainSize() const {
+    return mTotalOuterHypotheticalMainSize;
+  }
+
+  // Accessors for our FlexItems & information about them:
+  //
+  // Note: Callers must use IsEmpty() to ensure that the FlexLine is non-empty
+  // before calling accessors that return FlexItem.
+  FlexItem& FirstItem() { return mItems[0]; }
+  const FlexItem& FirstItem() const { return mItems[0]; }
+
+  FlexItem& LastItem() { return mItems.LastElement(); }
+  const FlexItem& LastItem() const { return mItems.LastElement(); }
+
+  // The "startmost"/"endmost" is from the perspective of the flex container's
+  // writing-mode, not from the perspective of the flex-relative main axis.
+  const FlexItem& StartmostItem(const FlexboxAxisTracker& aAxisTracker) const {
+    return aAxisTracker.IsMainAxisReversed() ? LastItem() : FirstItem();
+  }
+  const FlexItem& EndmostItem(const FlexboxAxisTracker& aAxisTracker) const {
+    return aAxisTracker.IsMainAxisReversed() ? FirstItem() : LastItem();
+  }
+
+  bool IsEmpty() const { return mItems.IsEmpty(); }
+
+  uint32_t NumItems() const { return mItems.Length(); }
+
+  nsTArray<FlexItem>& Items() { return mItems; }
+  const nsTArray<FlexItem>& Items() const { return mItems; }
+
+  // Adds the last flex item's hypothetical outer main-size and
+  // margin/border/padding to our totals. This should be called exactly once for
+  // each flex item, after we've determined that this line is the correct home
+  // for that item.
+  void AddLastItemToMainSizeTotals() {
+    const FlexItem& lastItem = Items().LastElement();
+
+    // Update our various bookkeeping member-vars:
+    if (lastItem.IsFrozen()) {
+      mNumFrozenItems++;
+    }
+
+    mTotalItemMBP += lastItem.MarginBorderPaddingSizeInMainAxis();
+    mTotalOuterHypotheticalMainSize += lastItem.OuterMainSize();
+
+    // If the item added was not the first item in the line, we add in any gap
+    // space as needed.
+    if (NumItems() >= 2) {
+      mTotalOuterHypotheticalMainSize += mMainGapSize;
+    }
+  }
+
+  // Computes the cross-size and baseline position of this FlexLine, based on
+  // its FlexItems.
+  void ComputeCrossSizeAndBaseline(const FlexboxAxisTracker& aAxisTracker);
+
+  // Returns the cross-size of this line.
+  nscoord LineCrossSize() const { return mLineCrossSize; }
+
+  // Setter for line cross-size -- needed for cases where the flex container
+  // imposes a cross-size on the line. (e.g. for single-line flexbox, or for
+  // multi-line flexbox with 'align-content: stretch')
+  void SetLineCrossSize(nscoord aLineCrossSize) {
+    mLineCrossSize = aLineCrossSize;
+  }
+
+  /**
+   * Returns the offset within this line where any baseline-aligned FlexItems
+   * should place their baseline. The return value represents a distance from
+   * the line's cross-start edge.
+   *
+   * If there are no baseline-aligned FlexItems, returns nscoord_MIN.
+   */
+  nscoord FirstBaselineOffset() const { return mFirstBaselineOffset; }
+
+  /**
+   * Returns the offset within this line where any last baseline-aligned
+   * FlexItems should place their baseline. Opposite the case of the first
+   * baseline offset, this represents a distance from the line's cross-end
+   * edge (since last baseline-aligned items are flush to the cross-end edge).
+   *
+   * If there are no last baseline-aligned FlexItems, returns nscoord_MIN.
+   */
+  nscoord LastBaselineOffset() const { return mLastBaselineOffset; }
+
+  // Extract a baseline from this line, which would be suitable for use as the
+  // flex container's 'aBaselineGroup' (i.e. first/last) baseline.
+  // https://drafts.csswg.org/css-flexbox-1/#flex-baselines
+  //
+  // The return value always represents a distance from the line's cross-start
+  // edge, even if we are querying last baseline. If this line has no flex items
+  // in its aBaselineGroup group, this method falls back to trying the opposite
+  // group. If this line has no baseline-aligned items at all, this returns
+  // nscoord_MIN.
+  nscoord ExtractBaselineOffset(BaselineSharingGroup aBaselineGroup) const;
+
+  /**
+   * Returns the gap size in the main axis for this line. Used for gap
+   * calculations.
+   */
+  nscoord MainGapSize() const { return mMainGapSize; }
+
+  // Runs the "Resolving Flexible Lengths" algorithm from section 9.7 of the
+  // CSS flexbox spec to distribute aFlexContainerMainSize among our flex items.
+  // https://drafts.csswg.org/css-flexbox-1/#resolve-flexible-lengths
+  void ResolveFlexibleLengths(nscoord aFlexContainerMainSize,
+                              ComputedFlexLineInfo* aLineInfo);
+
+  void PositionItemsInMainAxis(const StyleContentDistribution& aJustifyContent,
+                               nscoord aContentBoxMainSize,
+                               const FlexboxAxisTracker& aAxisTracker);
+
+  void PositionItemsInCrossAxis(nscoord aLineStartPosition,
+                                const FlexboxAxisTracker& aAxisTracker);
+
+ private:
+  // Helpers for ResolveFlexibleLengths():
+  void FreezeItemsEarly(bool aIsUsingFlexGrow, ComputedFlexLineInfo* aLineInfo);
+
+  void FreezeOrRestoreEachFlexibleSize(const nscoord aTotalViolation,
+                                       bool aIsFinalIteration);
+
+  // Stores this line's flex items.
+  nsTArray<FlexItem> mItems;
+
+  // Number of *frozen* FlexItems in this line, based on FlexItem::IsFrozen().
+  // Mostly used for optimization purposes, e.g. to bail out early from loops
+  // when we can tell they have nothing left to do.
+  uint32_t mNumFrozenItems = 0;
+
+  // Sum of margin/border/padding for the FlexItems in this FlexLine.
+  nscoord mTotalItemMBP = 0;
+
+  // Sum of FlexItems' outer hypothetical main sizes and all main-axis
+  // {row,columnm}-gaps between items.
+  // (i.e. their flex base sizes, clamped via their min/max-size properties,
+  // plus their main-axis margin/border/padding, plus the sum of the gaps.)
+  //
+  // This variable uses a 64-bit coord type to avoid integer overflow in case
+  // several of the individual items have huge hypothetical main sizes, which
+  // can happen with percent-width table-layout:fixed descendants. We have to
+  // avoid integer overflow in order to shrink items properly in that scenario.
+  AuCoord64 mTotalOuterHypotheticalMainSize = 0;
+
+  nscoord mLineCrossSize = 0;
+  nscoord mFirstBaselineOffset = nscoord_MIN;
+  nscoord mLastBaselineOffset = nscoord_MIN;
+
+  // Maintain size of each {row,column}-gap in the main axis
+  const nscoord mMainGapSize;
+};
+
+// The "startmost"/"endmost" is from the perspective of the flex container's
+// writing-mode, not from the perspective of the flex-relative cross axis.
+const FlexLine& StartmostLine(const nsTArray<FlexLine>& aLines,
+                              const FlexboxAxisTracker& aAxisTracker) {
+  return aAxisTracker.IsCrossAxisReversed() ? aLines.LastElement() : aLines[0];
+}
+const FlexLine& EndmostLine(const nsTArray<FlexLine>& aLines,
+                            const FlexboxAxisTracker& aAxisTracker) {
+  return aAxisTracker.IsCrossAxisReversed() ? aLines[0] : aLines.LastElement();
+}
+
+// Information about a strut left behind by a FlexItem that's been collapsed
+// using "visibility:collapse".
+struct nsFlexContainerFrame::StrutInfo {
+  StrutInfo(uint32_t aItemIdx, nscoord aStrutCrossSize)
+      : mItemIdx(aItemIdx), mStrutCrossSize(aStrutCrossSize) {}
+
+  uint32_t mItemIdx;        // Index in the child list.
+  nscoord mStrutCrossSize;  // The cross-size of this strut.
+};
+
+// Flex data shared by the flex container frames in a continuation chain, owned
+// by the first-in-flow. The data is initialized at the end of the
+// first-in-flow's Reflow().
+struct nsFlexContainerFrame::SharedFlexData final {
+  // The flex lines generated in DoFlexLayout() by our first-in-flow.
+  nsTArray<FlexLine> mLines;
+
+  // The final content main/cross size computed by DoFlexLayout.
+  nscoord mContentBoxMainSize = NS_UNCONSTRAINEDSIZE;
+  nscoord mContentBoxCrossSize = NS_UNCONSTRAINEDSIZE;
+
+  // Update this struct. Called by the first-in-flow.
+  void Update(FlexLayoutResult&& aFlr) {
+    mLines = std::move(aFlr.mLines);
+    mContentBoxMainSize = aFlr.mContentBoxMainSize;
+    mContentBoxCrossSize = aFlr.mContentBoxCrossSize;
+  }
+
+  // The frame property under which this struct is stored. Set only on the
+  // first-in-flow.
+  NS_DECLARE_FRAME_PROPERTY_DELETABLE(Prop, SharedFlexData)
+};
+
+// Flex data stored in every flex container's in-flow fragment (continuation).
+//
+// It's intended to prevent quadratic operations resulting from each fragment
+// having to walk its full prev-in-flow chain, and also serves as an argument to
+// the flex container next-in-flow's ReflowChildren(), to compute the position
+// offset for each flex item.
+struct nsFlexContainerFrame::PerFragmentFlexData final {
+  // Suppose D is the distance from a flex container fragment's content-box
+  // block-start edge to whichever is larger of either (a) the block-end edge of
+  // its children, or (b) the available space's block-end edge. (Note: in case
+  // (b), D is conceptually the sum of the block-size of the children, the
+  // packing space before & in between them, and part of the packing space after
+  // them.)
+  //
+  // This variable stores the sum of the D values for the current flex container
+  // fragments and for all its previous fragments
+  nscoord mCumulativeContentBoxBSize = 0;
+
+  // This variable accumulates FirstLineOrFirstItemBAxisMetrics::mBEndEdgeShift,
+  // for the current flex container fragment and for all its previous fragments.
+  // See the comment of mBEndEdgeShift for its computation details. In short,
+  // this value is the net block-end edge shift, accumulated for the children in
+  // all the previous fragments. This number is non-negative.
+  //
+  // This value is also used to grow a flex container's block-size if the
+  // container's computed block-size is unconstrained. For example: a tall item
+  // may be pushed to the next page/column, which leaves some wasted area at the
+  // bottom of the current flex container fragment, and causes the flex
+  // container fragments to be (collectively) larger than the hypothetical
+  // unfragmented size. Another example: a tall flex item may be broken into
+  // multiple fragments, and those fragments may have a larger collective
+  // block-size as compared to the item's original unfragmented size; the
+  // container would need to increase its block-size to account for this.
+  nscoord mCumulativeBEndEdgeShift = 0;
+
+  // The frame property under which this struct is stored. Cached on every
+  // in-flow fragment (continuation) at the end of the flex container's
+  // Reflow().
+  NS_DECLARE_FRAME_PROPERTY_DELETABLE(Prop, PerFragmentFlexData)
+};
+
+static void BuildStrutInfoFromCollapsedItems(const nsTArray<FlexLine>& aLines,
+                                             nsTArray<StrutInfo>& aStruts) {
+  MOZ_ASSERT(aStruts.IsEmpty(),
+             "We should only build up StrutInfo once per reflow, so "
+             "aStruts should be empty when this is called");
+
+  uint32_t itemIdxInContainer = 0;
+  for (const FlexLine& line : aLines) {
+    for (const FlexItem& item : line.Items()) {
+      if (item.Frame()->StyleVisibility()->IsCollapse()) {
+        // Note the cross size of the line as the item's strut size.
+        aStruts.AppendElement(
+            StrutInfo(itemIdxInContainer, line.LineCrossSize()));
+      }
+      itemIdxInContainer++;
+    }
+  }
+}
+
+static mozilla::StyleAlignFlags SimplifyAlignOrJustifyContentForOneItem(
+    const StyleContentDistribution& aAlignmentVal, bool aIsAlign) {
+  // Mask away any explicit fallback, to get the main (non-fallback) part of
+  // the specified value:
+  StyleAlignFlags specified = aAlignmentVal.primary;
+
+  // XXX strip off <overflow-position> bits until we implement it (bug 1311892)
+  specified &= ~StyleAlignFlags::FLAG_BITS;
+
+  // FIRST: handle a special-case for "justify-content:stretch" (or equivalent),
+  // which requires that we ignore any author-provided explicit fallback value.
+  if (specified == StyleAlignFlags::NORMAL) {
+    // In a flex container, *-content: "'normal' behaves as 'stretch'".
+    // Do that conversion early, so it benefits from our 'stretch' special-case.
+    // https://drafts.csswg.org/css-align-3/#distribution-flex
+    specified = StyleAlignFlags::STRETCH;
+  }
+  if (!aIsAlign && specified == StyleAlignFlags::STRETCH) {
+    // In a flex container, in "justify-content Axis: [...] 'stretch' behaves
+    // as 'flex-start' (ignoring the specified fallback alignment, if any)."
+    // https://drafts.csswg.org/css-align-3/#distribution-flex
+    // So, we just directly return 'flex-start', & ignore explicit fallback..
+    return StyleAlignFlags::FLEX_START;
+  }
+
+  // TODO: Check for an explicit fallback value (and if it's present, use it)
+  // here once we parse it, see https://github.com/w3c/csswg-drafts/issues/1002.
+
+  // If there's no explicit fallback, use the implied fallback values for
+  // space-{between,around,evenly} (since those values only make sense with
+  // multiple alignment subjects), and otherwise just use the specified value:
+  if (specified == StyleAlignFlags::SPACE_BETWEEN) {
+    return StyleAlignFlags::FLEX_START;
+  }
+  if (specified == StyleAlignFlags::SPACE_AROUND ||
+      specified == StyleAlignFlags::SPACE_EVENLY) {
+    return StyleAlignFlags::CENTER;
+  }
+  return specified;
+}
+
+bool nsFlexContainerFrame::DrainSelfOverflowList() {
+  return DrainAndMergeSelfOverflowList();
+}
+
+void nsFlexContainerFrame::AppendFrames(ChildListID aListID,
+                                        nsFrameList&& aFrameList) {
+  NoteNewChildren(aListID, aFrameList);
+  nsContainerFrame::AppendFrames(aListID, std::move(aFrameList));
+}
+
+void nsFlexContainerFrame::InsertFrames(
+    ChildListID aListID, nsIFrame* aPrevFrame,
+    const nsLineList::iterator* aPrevFrameLine, nsFrameList&& aFrameList) {
+  NoteNewChildren(aListID, aFrameList);
+  nsContainerFrame::InsertFrames(aListID, aPrevFrame, aPrevFrameLine,
+                                 std::move(aFrameList));
+}
+
+void nsFlexContainerFrame::RemoveFrame(DestroyContext& aContext,
+                                       ChildListID aListID,
+                                       nsIFrame* aOldFrame) {
+  MOZ_ASSERT(aListID == FrameChildListID::Principal, "unexpected child list");
+
+#ifdef DEBUG
+  SetDidPushItemsBitIfNeeded(aListID, aOldFrame);
+#endif
+
+  nsContainerFrame::RemoveFrame(aContext, aListID, aOldFrame);
+}
+
+StyleAlignFlags nsFlexContainerFrame::CSSAlignmentForAbsPosChild(
+    const ReflowInput& aChildRI, LogicalAxis aLogicalAxis) const {
+  const FlexboxAxisTracker axisTracker(this);
+
+  // If we're row-oriented and the caller is asking about our inline axis (or
+  // alternately, if we're column-oriented and the caller is asking about our
+  // block axis), then the caller is really asking about our *main* axis.
+  // Otherwise, the caller is asking about our cross axis.
+  const bool isMainAxis =
+      (axisTracker.IsRowOriented() == (aLogicalAxis == eLogicalAxisInline));
+  const nsStylePosition* containerStylePos = StylePosition();
+  const bool isAxisReversed = isMainAxis ? axisTracker.IsMainAxisReversed()
+                                         : axisTracker.IsCrossAxisReversed();
+
+  StyleAlignFlags alignment{0};
+  StyleAlignFlags alignmentFlags{0};
+  if (isMainAxis) {
+    // We're aligning in the main axis: align according to 'justify-content'.
+    // (We don't care about justify-self; it has no effect on children of flex
+    // containers, unless https://github.com/w3c/csswg-drafts/issues/7644
+    // changes that.)
+    alignment = SimplifyAlignOrJustifyContentForOneItem(
+        containerStylePos->mJustifyContent,
+        /*aIsAlign = */ false);
+  } else {
+    // We're aligning in the cross axis: align according to 'align-self'.
+    // (We don't care about align-content; it has no effect on abspos flex
+    // children, per https://github.com/w3c/csswg-drafts/issues/7596 )
+    alignment = aChildRI.mStylePosition->UsedAlignSelf(Style())._0;
+    // Extract and strip align flag bits
+    alignmentFlags = alignment & StyleAlignFlags::FLAG_BITS;
+    alignment &= ~StyleAlignFlags::FLAG_BITS;
+
+    if (alignment == StyleAlignFlags::NORMAL) {
+      // "the 'normal' keyword behaves as 'start' on replaced
+      // absolutely-positioned boxes, and behaves as 'stretch' on all other
+      // absolutely-positioned boxes."
+      // https://drafts.csswg.org/css-align/#align-abspos
+      alignment = aChildRI.mFrame->IsReplaced() ? StyleAlignFlags::START
+                                                : StyleAlignFlags::STRETCH;
+    }
+  }
+
+  if (alignment == StyleAlignFlags::STRETCH) {
+    // The default fallback alignment for 'stretch' is 'flex-start'.
+    alignment = StyleAlignFlags::FLEX_START;
+  }
+
+  // Resolve flex-start, flex-end, auto, left, right, baseline, last baseline;
+  if (alignment == StyleAlignFlags::FLEX_START) {
+    alignment = isAxisReversed ? StyleAlignFlags::END : StyleAlignFlags::START;
+  } else if (alignment == StyleAlignFlags::FLEX_END) {
+    alignment = isAxisReversed ? StyleAlignFlags::START : StyleAlignFlags::END;
+  } else if (alignment == StyleAlignFlags::LEFT ||
+             alignment == StyleAlignFlags::RIGHT) {
+    MOZ_ASSERT(isMainAxis, "Only justify-* can have 'left' and 'right'!");
+    alignment = axisTracker.ResolveJustifyLeftRight(alignment);
+  } else if (alignment == StyleAlignFlags::BASELINE) {
+    alignment = StyleAlignFlags::START;
+  } else if (alignment == StyleAlignFlags::LAST_BASELINE) {
+    alignment = StyleAlignFlags::END;
+  }
+
+  MOZ_ASSERT(alignment != StyleAlignFlags::STRETCH,
+             "We should've converted 'stretch' to the fallback alignment!");
+  MOZ_ASSERT(alignment != StyleAlignFlags::FLEX_START &&
+                 alignment != StyleAlignFlags::FLEX_END,
+             "nsAbsoluteContainingBlock doesn't know how to handle "
+             "flex-relative axis for flex containers!");
+
+  return (alignment | alignmentFlags);
+}
+
+void nsFlexContainerFrame::GenerateFlexItemForChild(
+    FlexLine& aLine, nsIFrame* aChildFrame,
+    const ReflowInput& aParentReflowInput,
+    const FlexboxAxisTracker& aAxisTracker,
+    const nscoord aTentativeContentBoxCrossSize) {
+  const auto flexWM = aAxisTracker.GetWritingMode();
+  const auto childWM = aChildFrame->GetWritingMode();
+
+  // Note: we use GetStyleFrame() to access the sizing & flex properties here.
+  // This lets us correctly handle table wrapper frames as flex items since
+  // their inline-size and block-size properties are always 'auto'. In order for
+  // 'flex-basis:auto' to actually resolve to the author's specified inline-size
+  // or block-size, we need to dig through to the inner table.
+  const auto* stylePos =
+      nsLayoutUtils::GetStyleFrame(aChildFrame)->StylePosition();
+
+  // Construct a StyleSizeOverrides for this flex item so that its ReflowInput
+  // below will use and resolve its flex base size rather than its corresponding
+  // preferred main size property (only for modern CSS flexbox).
+  StyleSizeOverrides sizeOverrides;
+  if (!IsLegacyBox(this)) {
+    Maybe<StyleSize> styleFlexBaseSize;
+
+    // When resolving flex base size, flex items use their 'flex-basis' property
+    // in place of their preferred main size (e.g. 'width') for sizing purposes,
+    // *unless* they have 'flex-basis:auto' in which case they use their
+    // preferred main size after all.
+    const auto& flexBasis = stylePos->mFlexBasis;
+    const auto& styleMainSize = stylePos->Size(aAxisTracker.MainAxis(), flexWM);
+    if (IsUsedFlexBasisContent(flexBasis, styleMainSize)) {
+      // If we get here, we're resolving the flex base size for a flex item, and
+      // we fall into the flexbox spec section 9.2 step 3, substep C (if we have
+      // a definite cross size) or E (if not).
+      styleFlexBaseSize.emplace(StyleSize::MaxContent());
+    } else if (flexBasis.IsSize() && !flexBasis.IsAuto()) {
+      // For all other non-'auto' flex-basis values, we just swap in the
+      // flex-basis itself for the preferred main-size property.
+      styleFlexBaseSize.emplace(flexBasis.AsSize());
+    } else {
+      // else: flex-basis is 'auto', which is deferring to some explicit value
+      // in the preferred main size.
+      MOZ_ASSERT(flexBasis.IsAuto());
+      styleFlexBaseSize.emplace(styleMainSize);
+    }
+
+    MOZ_ASSERT(styleFlexBaseSize, "We should've emplace styleFlexBaseSize!");
+
+    // Provide the size override for the preferred main size property.
+    if (aAxisTracker.IsInlineAxisMainAxis(childWM)) {
+      sizeOverrides.mStyleISize = std::move(styleFlexBaseSize);
+    } else {
+      sizeOverrides.mStyleBSize = std::move(styleFlexBaseSize);
+    }
+
+    // 'flex-basis' should works on the inner table frame for a table flex item,
+    // just like how 'height' works on a table element.
+    sizeOverrides.mApplyOverridesVerbatim = true;
+  }
+
+  // Create temporary reflow input just for sizing -- to get hypothetical
+  // main-size and the computed values of min / max main-size property.
+  // (This reflow input will _not_ be used for reflow.)
+  ReflowInput childRI(PresContext(), aParentReflowInput, aChildFrame,
+                      aParentReflowInput.ComputedSize(childWM), Nothing(), {},
+                      sizeOverrides, {ComputeSizeFlag::ShrinkWrap});
+
+  // FLEX GROW & SHRINK WEIGHTS
+  // --------------------------
+  float flexGrow, flexShrink;
+  if (IsLegacyBox(this)) {
+    flexGrow = flexShrink = aChildFrame->StyleXUL()->mBoxFlex;
+  } else {
+    flexGrow = stylePos->mFlexGrow;
+    flexShrink = stylePos->mFlexShrink;
+  }
+
+  // MAIN SIZES (flex base size, min/max size)
+  // -----------------------------------------
+  const LogicalSize computedSizeInFlexWM = childRI.ComputedSize(flexWM);
+  const LogicalSize computedMinSizeInFlexWM = childRI.ComputedMinSize(flexWM);
+  const LogicalSize computedMaxSizeInFlexWM = childRI.ComputedMaxSize(flexWM);
+
+  const nscoord flexBaseSize = aAxisTracker.MainComponent(computedSizeInFlexWM);
+  const nscoord mainMinSize =
+      aAxisTracker.MainComponent(computedMinSizeInFlexWM);
+  const nscoord mainMaxSize =
+      aAxisTracker.MainComponent(computedMaxSizeInFlexWM);
+
+  // This is enforced by the ReflowInput where these values come from:
+  MOZ_ASSERT(mainMinSize <= mainMaxSize, "min size is larger than max size");
+
+  // CROSS SIZES (tentative cross size, min/max cross size)
+  // ------------------------------------------------------
+  // Grab the cross size from the reflow input. This might be the right value,
+  // or we might resolve it to something else in SizeItemInCrossAxis(); hence,
+  // it's tentative. See comment under "Cross Size Determination" for more.
+  const nscoord tentativeCrossSize =
+      aAxisTracker.CrossComponent(computedSizeInFlexWM);
+  const nscoord crossMinSize =
+      aAxisTracker.CrossComponent(computedMinSizeInFlexWM);
+  const nscoord crossMaxSize =
+      aAxisTracker.CrossComponent(computedMaxSizeInFlexWM);
+
+  // Construct the flex item!
+  FlexItem& item = *aLine.Items().EmplaceBack(
+      childRI, flexGrow, flexShrink, flexBaseSize, mainMinSize, mainMaxSize,
+      tentativeCrossSize, crossMinSize, crossMaxSize, aAxisTracker);
+
+  // We may be about to do computations based on our item's cross-size
+  // (e.g. using it as a constraint when measuring our content in the
+  // main axis, or using it with the preferred aspect ratio to obtain a main
+  // size). BEFORE WE DO THAT, we need let the item "pre-stretch" its cross size
+  // (if it's got 'align-self:stretch'), for a certain case where the spec says
+  // the stretched cross size is considered "definite". That case is if we
+  // have a single-line (nowrap) flex container which itself has a definite
+  // cross-size.  Otherwise, we'll wait to do stretching, since (in other
+  // cases) we don't know how much the item should stretch yet.
+  const bool isSingleLine =
+      StyleFlexWrap::Nowrap == aParentReflowInput.mStylePosition->mFlexWrap;
+  if (isSingleLine) {
+    // Is container's cross size "definite"?
+    // - If it's column-oriented, then "yes", because its cross size is its
+    // inline-size which is always definite from its descendants' perspective.
+    // - Otherwise (if it's row-oriented), then we check the actual size
+    // and call it definite if it's not NS_UNCONSTRAINEDSIZE.
+    if (aAxisTracker.IsColumnOriented() ||
+        aTentativeContentBoxCrossSize != NS_UNCONSTRAINEDSIZE) {
+      // Container's cross size is "definite", so we can resolve the item's
+      // stretched cross size using that.
+      item.ResolveStretchedCrossSize(aTentativeContentBoxCrossSize);
+    }
+  }
+
+  // Before thinking about freezing the item at its base size, we need to give
+  // it a chance to recalculate the base size from its cross size and aspect
+  // ratio (since its cross size might've *just* now become definite due to
+  // 'stretch' above)
+  item.ResolveFlexBaseSizeFromAspectRatio(childRI);
+
+  // If we're inflexible, we can just freeze to our hypothetical main-size
+  // up-front.
+  if (flexGrow == 0.0f && flexShrink == 0.0f) {
+    item.Freeze();
+    if (flexBaseSize < mainMinSize) {
+      item.SetWasMinClamped();
+    } else if (flexBaseSize > mainMaxSize) {
+      item.SetWasMaxClamped();
+    }
+  }
+
+  // Resolve "flex-basis:auto" and/or "min-[width|height]:auto" (which might
+  // require us to reflow the item to measure content height)
+  ResolveAutoFlexBasisAndMinSize(item, childRI, aAxisTracker);
+}
+
+// Static helper-functions for ResolveAutoFlexBasisAndMinSize():
+// -------------------------------------------------------------
+// Partially resolves "min-[width|height]:auto" and returns the resulting value.
+// By "partially", I mean we don't consider the min-content size (but we do
+// consider the main-size and main max-size properties, and the preferred aspect
+// ratio). The caller is responsible for computing & considering the min-content
+// size in combination with the partially-resolved value that this function
+// returns.
+//
+// Basically, this function gets the specified size suggestion; if not, the
+// transferred size suggestion; if both sizes do not exist, return nscoord_MAX.
+//
+// Spec reference: https://drafts.csswg.org/css-flexbox-1/#min-size-auto
+static nscoord PartiallyResolveAutoMinSize(
+    const FlexItem& aFlexItem, const ReflowInput& aItemReflowInput,
+    const FlexboxAxisTracker& aAxisTracker) {
+  MOZ_ASSERT(aFlexItem.NeedsMinSizeAutoResolution(),
+             "only call for FlexItems that need min-size auto resolution");
+
+  const auto itemWM = aFlexItem.GetWritingMode();
+  const auto cbWM = aAxisTracker.GetWritingMode();
+  const auto& mainStyleSize =
+      aItemReflowInput.mStylePosition->Size(aAxisTracker.MainAxis(), cbWM);
+  const auto& maxMainStyleSize =
+      aItemReflowInput.mStylePosition->MaxSize(aAxisTracker.MainAxis(), cbWM);
+  const auto boxSizingAdjust =
+      aItemReflowInput.mStylePosition->mBoxSizing == StyleBoxSizing::Border
+          ? aFlexItem.BorderPadding().Size(cbWM)
+          : LogicalSize(cbWM);
+
+  // If this flex item is a compressible replaced element list in CSS Sizing 3
+  // §5.2.2, CSS Sizing 3 §5.2.1c requires us to resolve the percentage part of
+  // the preferred main size property against zero, yielding a definite
+  // specified size suggestion. Here we can use a zero percentage basis to
+  // fulfill this requirement.
+  const auto percentBasis =
+      aFlexItem.Frame()->IsPercentageResolvedAgainstZero(mainStyleSize,
+                                                         maxMainStyleSize)
+          ? LogicalSize(cbWM, 0, 0)
+          : aItemReflowInput.mContainingBlockSize.ConvertTo(cbWM, itemWM);
+
+  // Compute the specified size suggestion, which is the main-size property if
+  // it's definite.
+  nscoord specifiedSizeSuggestion = nscoord_MAX;
+
+  if (aAxisTracker.IsRowOriented()) {
+    if (mainStyleSize.IsLengthPercentage()) {
+      // NOTE: We ignore extremum inline-size. This is OK because the caller is
+      // responsible for computing the min-content inline-size and min()'ing it
+      // with the value we return.
+      specifiedSizeSuggestion = aFlexItem.Frame()->ComputeISizeValue(
+          cbWM, percentBasis, boxSizingAdjust,
+          mainStyleSize.AsLengthPercentage());
+    }
+  } else {
+    if (!nsLayoutUtils::IsAutoBSize(mainStyleSize, percentBasis.BSize(cbWM))) {
+      // NOTE: We ignore auto and extremum block-size. This is OK because the
+      // caller is responsible for computing the min-content block-size and
+      // min()'ing it with the value we return.
+      specifiedSizeSuggestion = nsLayoutUtils::ComputeBSizeValue(
+          percentBasis.BSize(cbWM), boxSizingAdjust.BSize(cbWM),
+          mainStyleSize.AsLengthPercentage());
+    }
+  }
+
+  if (specifiedSizeSuggestion != nscoord_MAX) {
+    // We have the specified size suggestion. Return it now since we don't need
+    // to consider transferred size suggestion.
+    FLEX_LOGV(" Specified size suggestion: %d", specifiedSizeSuggestion);
+    return specifiedSizeSuggestion;
+  }
+
+  // Compute the transferred size suggestion, which is the cross size converted
+  // through the aspect ratio (if the item is replaced, and it has an aspect
+  // ratio and a definite cross size).
+  if (const auto& aspectRatio = aFlexItem.GetAspectRatio();
+      aFlexItem.Frame()->IsReplaced() && aspectRatio &&
+      aFlexItem.IsCrossSizeDefinite(aItemReflowInput)) {
+    // We have a usable aspect ratio. (not going to divide by 0)
+    nscoord transferredSizeSuggestion = aspectRatio.ComputeRatioDependentSize(
+        aFlexItem.MainAxis(), cbWM, aFlexItem.CrossSize(), boxSizingAdjust);
+
+    // Clamp the transferred size suggestion by any definite min and max
+    // cross size converted through the aspect ratio.
+    transferredSizeSuggestion = aFlexItem.ClampMainSizeViaCrossAxisConstraints(
+        transferredSizeSuggestion, aItemReflowInput);
+
+    FLEX_LOGV(" Transferred size suggestion: %d", transferredSizeSuggestion);
+    return transferredSizeSuggestion;
+  }
+
+  return nscoord_MAX;
+}
+
+// Note: If & when we handle "min-height: min-content" for flex items,
+// we may want to resolve that in this function, too.
+void nsFlexContainerFrame::ResolveAutoFlexBasisAndMinSize(
+    FlexItem& aFlexItem, const ReflowInput& aItemReflowInput,
+    const FlexboxAxisTracker& aAxisTracker) {
+  // (Note: We can guarantee that the flex-basis will have already been
+  // resolved if the main axis is the same as the item's inline
+  // axis. Inline-axis values should always be resolvable without reflow.)
+  const bool isMainSizeAuto =
+      (!aFlexItem.IsInlineAxisMainAxis() &&
+       NS_UNCONSTRAINEDSIZE == aFlexItem.FlexBaseSize());
+
+  const bool isMainMinSizeAuto = aFlexItem.NeedsMinSizeAutoResolution();
+
+  if (!isMainSizeAuto && !isMainMinSizeAuto) {
+    // Nothing to do; this function is only needed for flex items
+    // with a used flex-basis of "auto" or a min-main-size of "auto".
+    return;
+  }
+
+  FLEX_LOGV("Resolving auto main size or auto min main size for flex item %p",
+            aFlexItem.Frame());
+
+  nscoord resolvedMinSize;  // (only set/used if isMainMinSizeAuto==true)
+  bool minSizeNeedsToMeasureContent = false;  // assume the best
+  if (isMainMinSizeAuto) {
+    // Resolve the min-size, except for considering the min-content size.
+    // (We'll consider that later, if we need to.)
+    resolvedMinSize =
+        PartiallyResolveAutoMinSize(aFlexItem, aItemReflowInput, aAxisTracker);
+    if (resolvedMinSize > 0) {
+      // If resolvedMinSize were already at 0, we could skip calculating content
+      // size suggestion because it can't go any lower.
+      minSizeNeedsToMeasureContent = true;
+    }
+  }
+
+  const bool flexBasisNeedsToMeasureContent = isMainSizeAuto;
+
+  // Measure content, if needed (w/ intrinsic-width method or a reflow)
+  if (minSizeNeedsToMeasureContent || flexBasisNeedsToMeasureContent) {
+    // Compute the content size suggestion, which is the min-content size in the
+    // main axis.
+    nscoord contentSizeSuggestion = nscoord_MAX;
+
+    if (aFlexItem.IsInlineAxisMainAxis()) {
+      if (minSizeNeedsToMeasureContent) {
+        // Compute the flex item's content size suggestion, which is the
+        // 'min-content' size on the main axis.
+        // https://drafts.csswg.org/css-flexbox-1/#content-size-suggestion
+        const auto cbWM = aAxisTracker.GetWritingMode();
+        const auto itemWM = aFlexItem.GetWritingMode();
+        const nscoord availISize = 0;  // for min-content size
+        StyleSizeOverrides sizeOverrides;
+        sizeOverrides.mStyleISize.emplace(StyleSize::Auto());
+        const auto sizeInItemWM = aFlexItem.Frame()->ComputeSize(
+            aItemReflowInput.mRenderingContext, itemWM,
+            aItemReflowInput.mContainingBlockSize, availISize,
+            aItemReflowInput.ComputedLogicalMargin(itemWM).Size(itemWM),
+            aItemReflowInput.ComputedLogicalBorderPadding(itemWM).Size(itemWM),
+            sizeOverrides, {ComputeSizeFlag::ShrinkWrap});
+
+        contentSizeSuggestion = aAxisTracker.MainComponent(
+            sizeInItemWM.mLogicalSize.ConvertTo(cbWM, itemWM));
+      }
+      NS_ASSERTION(!flexBasisNeedsToMeasureContent,
+                   "flex-basis:auto should have been resolved in the "
+                   "reflow input, for horizontal flexbox. It shouldn't need "
+                   "special handling here");
+    } else {
+      // If this item is flexible (in its block axis)...
+      // OR if we're measuring its 'auto' min-BSize, with its main-size (in its
+      // block axis) being something non-"auto"...
+      // THEN: we assume that the computed BSize that we're reflowing with now
+      // could be different from the one we'll use for this flex item's
+      // "actual" reflow later on.  In that case, we need to be sure the flex
+      // item treats this as a block-axis resize (regardless of whether there
+      // are actually any ancestors being resized in that axis).
+      // (Note: We don't have to do this for the inline axis, because
+      // InitResizeFlags will always turn on mIsIResize on when it sees that
+      // the computed ISize is different from current ISize, and that's all we
+      // need.)
+      bool forceBResizeForMeasuringReflow =
+          !aFlexItem.IsFrozen() ||          // Is the item flexible?
+          !flexBasisNeedsToMeasureContent;  // Are we *only* measuring it for
+                                            // 'min-block-size:auto'?
+
+      const ReflowInput& flexContainerRI = *aItemReflowInput.mParentReflowInput;
+      nscoord contentBSize = MeasureFlexItemContentBSize(
+          aFlexItem, forceBResizeForMeasuringReflow, flexContainerRI);
+      if (minSizeNeedsToMeasureContent) {
+        contentSizeSuggestion = contentBSize;
+      }
+      if (flexBasisNeedsToMeasureContent) {
+        aFlexItem.SetFlexBaseSizeAndMainSize(contentBSize);
+        aFlexItem.SetIsFlexBaseSizeContentBSize();
+      }
+    }
+
+    if (minSizeNeedsToMeasureContent) {
+      // Clamp the content size suggestion by any definite min and max cross
+      // size converted through the aspect ratio.
+      if (aFlexItem.HasAspectRatio()) {
+        contentSizeSuggestion = aFlexItem.ClampMainSizeViaCrossAxisConstraints(
+            contentSizeSuggestion, aItemReflowInput);
+      }
+
+      FLEX_LOGV(" Content size suggestion: %d", contentSizeSuggestion);
+      resolvedMinSize = std::min(resolvedMinSize, contentSizeSuggestion);
+
+      // Clamp the resolved min main size by the max main size if it's definite.
+      if (aFlexItem.MainMaxSize() != NS_UNCONSTRAINEDSIZE) {
+        resolvedMinSize = std::min(resolvedMinSize, aFlexItem.MainMaxSize());
+      } else if (MOZ_UNLIKELY(resolvedMinSize > nscoord_MAX)) {
+        NS_WARNING("Bogus resolved auto min main size!");
+        // Our resolved min-size is bogus, probably due to some huge sizes in
+        // the content. Clamp it to the valid nscoord range, so that we can at
+        // least depend on it being <= the max-size (which is also the
+        // nscoord_MAX sentinel value if we reach this point).
+        resolvedMinSize = nscoord_MAX;
+      }
+      FLEX_LOGV(" Resolved auto min main size: %d", resolvedMinSize);
+
+      if (resolvedMinSize == contentSizeSuggestion) {
+        // When we are here, we've measured the item's content-based size, and
+        // we used it as the resolved auto min main size. Record the fact so
+        // that we can use it to determine whether we allow a flex item to grow
+        // its block-size in ReflowFlexItem().
+        aFlexItem.SetIsMainMinSizeContentBSize();
+      }
+    }
+  }
+
+  if (isMainMinSizeAuto) {
+    aFlexItem.UpdateMainMinSize(resolvedMinSize);
+  }
+}
+
+/**
+ * A cached result for a flex item's block-axis measuring reflow. This cache
+ * prevents us from doing exponential reflows in cases of deeply nested flex
+ * and scroll frames.
+ *
+ * We store the cached value in the flex item's frame property table, for
+ * simplicity.
+ *
+ * Right now, we cache the following as a "key", from the item's ReflowInput:
+ *   - its ComputedSize
+ *   - its min/max block size (in case its ComputedBSize is unconstrained)
+ *   - its AvailableBSize
+ * ...and we cache the following as the "value", from the item's ReflowOutput:
+ *   - its final content-box BSize
+ *
+ * The assumption here is that a given flex item measurement from our "value"
+ * won't change unless one of the pieces of the "key" change, or the flex
+ * item's intrinsic size is marked as dirty (due to a style or DOM change).
+ * (The latter will cause the cached value to be discarded, in
+ * nsIFrame::MarkIntrinsicISizesDirty.)
+ *
+ * Note that the components of "Key" (mComputed{MinB,MaxB,}Size and
+ * mAvailableBSize) are sufficient to catch any changes to the flex container's
+ * size that the item may care about for its measuring reflow. Specifically:
+ *  - If the item cares about the container's size (e.g. if it has a percent
+ *    height and the container's height changes, in a horizontal-WM container)
+ *    then that'll be detectable via the item's ReflowInput's "ComputedSize()"
+ *    differing from the value in our Key.  And the same applies for the
+ *    inline axis.
+ *  - If the item is fragmentable (pending bug 939897) and its measured BSize
+ *    depends on where it gets fragmented, then that sort of change can be
+ *    detected due to the item's ReflowInput's "AvailableBSize()" differing
+ *    from the value in our Key.
+ *
+ * One particular case to consider (& need to be sure not to break when
+ * changing this class): the flex item's computed BSize may change between
+ * measuring reflows due to how the mIsFlexContainerMeasuringBSize flag affects
+ * size computation (see bug 1336708). This is one reason we need to use the
+ * computed BSize as part of the key.
+ */
+class nsFlexContainerFrame::CachedBAxisMeasurement {
+  struct Key {
+    const LogicalSize mComputedSize;
+    const nscoord mComputedMinBSize;
+    const nscoord mComputedMaxBSize;
+    const nscoord mAvailableBSize;
+
+    explicit Key(const ReflowInput& aRI)
+        : mComputedSize(aRI.ComputedSize()),
+          mComputedMinBSize(aRI.ComputedMinBSize()),
+          mComputedMaxBSize(aRI.ComputedMaxBSize()),
+          mAvailableBSize(aRI.AvailableBSize()) {}
+
+    bool operator==(const Key& aOther) const {
+      return mComputedSize == aOther.mComputedSize &&
+             mComputedMinBSize == aOther.mComputedMinBSize &&
+             mComputedMaxBSize == aOther.mComputedMaxBSize &&
+             mAvailableBSize == aOther.mAvailableBSize;
+    }
+  };
+
+  const Key mKey;
+
+  // This could/should be const, but it's non-const for now just because it's
+  // assigned via a series of steps in the constructor body:
+  nscoord mBSize;
+
+ public:
+  CachedBAxisMeasurement(const ReflowInput& aReflowInput,
+                         const ReflowOutput& aReflowOutput)
+      : mKey(aReflowInput) {
+    // To get content-box bsize, we have to subtract off border & padding
+    // (and floor at 0 in case the border/padding are too large):
+    WritingMode itemWM = aReflowInput.GetWritingMode();
+    nscoord borderBoxBSize = aReflowOutput.BSize(itemWM);
+    mBSize =
+        borderBoxBSize -
+        aReflowInput.ComputedLogicalBorderPadding(itemWM).BStartEnd(itemWM);
+    mBSize = std::max(0, mBSize);
+  }
+
+  /**
+   * Returns true if this cached flex item measurement is valid for (i.e. can
+   * be expected to match the output of) a measuring reflow whose input
+   * parameters are given via aReflowInput.
+   */
+  bool IsValidFor(const ReflowInput& aReflowInput) const {
+    return mKey == Key(aReflowInput);
+  }
+
+  nscoord BSize() const { return mBSize; }
+};
+
+/**
+ * A cached copy of various metrics from a flex item's most recent final reflow.
+ * It can be used to determine whether we can optimize away the flex item's
+ * final reflow, when we perform an incremental reflow of its flex container.
+ */
+class CachedFinalReflowMetrics final {
+ public:
+  CachedFinalReflowMetrics(const ReflowInput& aReflowInput,
+                           const ReflowOutput& aReflowOutput)
+      : CachedFinalReflowMetrics(aReflowInput.GetWritingMode(), aReflowInput,
+                                 aReflowOutput) {}
+
+  CachedFinalReflowMetrics(const FlexItem& aItem, const LogicalSize& aSize)
+      : mBorderPadding(aItem.BorderPadding().ConvertTo(
+            aItem.GetWritingMode(), aItem.ContainingBlockWM())),
+        mSize(aSize),
+        mTreatBSizeAsIndefinite(aItem.TreatBSizeAsIndefinite()) {}
+
+  const LogicalSize& Size() const { return mSize; }
+  const LogicalMargin& BorderPadding() const { return mBorderPadding; }
+  bool TreatBSizeAsIndefinite() const { return mTreatBSizeAsIndefinite; }
+
+ private:
+  // A convenience constructor with a WritingMode argument.
+  CachedFinalReflowMetrics(WritingMode aWM, const ReflowInput& aReflowInput,
+                           const ReflowOutput& aReflowOutput)
+      : mBorderPadding(aReflowInput.ComputedLogicalBorderPadding(aWM)),
+        mSize(aReflowOutput.Size(aWM) - mBorderPadding.Size(aWM)),
+        mTreatBSizeAsIndefinite(aReflowInput.mFlags.mTreatBSizeAsIndefinite) {}
+
+  // The flex item's border and padding, in its own writing-mode, that it used
+  // used during its most recent "final reflow".
+  LogicalMargin mBorderPadding;
+
+  // The flex item's content-box size, in its own writing-mode, that it used
+  // during its most recent "final reflow".
+  LogicalSize mSize;
+
+  // True if the flex item's BSize was considered "indefinite" in its most
+  // recent "final reflow". (For a flex item "final reflow", this is fully
+  // determined by the mTreatBSizeAsIndefinite flag in ReflowInput. See the
+  // flag's documentation for more information.)
+  bool mTreatBSizeAsIndefinite;
+};
+
+/**
+ * When we instantiate/update a CachedFlexItemData, this enum must be used to
+ * indicate the sort of reflow whose results we're capturing. This impacts
+ * what we cache & how we use the cached information.
+ */
+enum class FlexItemReflowType {
+  // A reflow to measure the block-axis size of a flex item (as an input to the
+  // flex layout algorithm).
+  Measuring,
+
+  // A reflow with the flex item's "final" size at the end of the flex layout
+  // algorithm.
+  Final,
+};
+
+/**
+ * This class stores information about the conditions and results for the most
+ * recent ReflowChild call that we made on a given flex item.  This information
+ * helps us reason about whether we can assume that a subsequent ReflowChild()
+ * invocation is unnecessary & skippable.
+ */
+class nsFlexContainerFrame::CachedFlexItemData {
+ public:
+  CachedFlexItemData(const ReflowInput& aReflowInput,
+                     const ReflowOutput& aReflowOutput,
+                     FlexItemReflowType aType) {
+    Update(aReflowInput, aReflowOutput, aType);
+  }
+
+  // This method is intended to be called after we perform either a "measuring
+  // reflow" or a "final reflow" for a given flex item.
+  void Update(const ReflowInput& aReflowInput,
+              const ReflowOutput& aReflowOutput, FlexItemReflowType aType) {
+    if (aType == FlexItemReflowType::Measuring) {
+      mBAxisMeasurement.reset();
+      mBAxisMeasurement.emplace(aReflowInput, aReflowOutput);
+      // Clear any cached "last final reflow metrics", too, because now the most
+      // recent reflow was *not* a "final reflow".
+      mFinalReflowMetrics.reset();
+      return;
+    }
+
+    MOZ_ASSERT(aType == FlexItemReflowType::Final);
+    mFinalReflowMetrics.reset();
+    mFinalReflowMetrics.emplace(aReflowInput, aReflowOutput);
+  }
+
+  // This method is intended to be called for situations where we decide to
+  // skip a final reflow because we've just done a measuring reflow which left
+  // us (and our descendants) with the correct sizes. In this scenario, we
+  // still want to cache the size as if we did a final reflow (because we've
+  // determined that the recent measuring reflow was sufficient).  That way,
+  // our flex container can still skip a final reflow for this item in the
+  // future as long as conditions are right.
+  void Update(const FlexItem& aItem, const LogicalSize& aSize) {
+    MOZ_ASSERT(!mFinalReflowMetrics,
+               "This version of the method is only intended to be called when "
+               "the most recent reflow was a 'measuring reflow'; and that "
+               "should have cleared out mFinalReflowMetrics");
+
+    mFinalReflowMetrics.reset();  // Just in case this assert^ fails.
+    mFinalReflowMetrics.emplace(aItem, aSize);
+  }
+
+  // If the flex container needs a measuring reflow for the flex item, then the
+  // resulting block-axis measurements can be cached here.  If no measurement
+  // has been needed so far, then this member will be Nothing().
+  Maybe<CachedBAxisMeasurement> mBAxisMeasurement;
+
+  // The metrics that the corresponding flex item used in its most recent
+  // "final reflow". (Note: the assumption here is that this reflow was this
+  // item's most recent reflow of any type.  If the item ends up undergoing a
+  // subsequent measuring reflow, then this value needs to be cleared, because
+  // at that point it's no longer an accurate way of reasoning about the
+  // current state of the frame tree.)
+  Maybe<CachedFinalReflowMetrics> mFinalReflowMetrics;
+
+  // Instances of this class are stored under this frame property, on
+  // frames that are flex items:
+  NS_DECLARE_FRAME_PROPERTY_DELETABLE(Prop, CachedFlexItemData)
+};
+
+void nsFlexContainerFrame::MarkCachedFlexMeasurementsDirty(
+    nsIFrame* aItemFrame) {
+  MOZ_ASSERT(aItemFrame->IsFlexItem());
+  if (auto* cache = aItemFrame->GetProperty(CachedFlexItemData::Prop())) {
+    cache->mBAxisMeasurement.reset();
+    cache->mFinalReflowMetrics.reset();
+  }
+}
+
+const CachedBAxisMeasurement& nsFlexContainerFrame::MeasureBSizeForFlexItem(
+    FlexItem& aItem, ReflowInput& aChildReflowInput) {
+  auto* cachedData = aItem.Frame()->GetProperty(CachedFlexItemData::Prop());
+
+  if (cachedData && cachedData->mBAxisMeasurement) {
+    if (!aItem.Frame()->IsSubtreeDirty() &&
+        cachedData->mBAxisMeasurement->IsValidFor(aChildReflowInput)) {
+      FLEX_LOG("[perf] MeasureBSizeForFlexItem accepted cached value");
+      return *(cachedData->mBAxisMeasurement);
+    }
+    FLEX_LOG("[perf] MeasureBSizeForFlexItem rejected cached value");
+  } else {
+    FLEX_LOG("[perf] MeasureBSizeForFlexItem didn't have a cached value");
+  }
+
+  // CachedFlexItemData is stored in item's writing mode, so we pass
+  // aChildReflowInput into ReflowOutput's constructor.
+  ReflowOutput childReflowOutput(aChildReflowInput);
+  nsReflowStatus childReflowStatus;
+
+  const ReflowChildFlags flags = ReflowChildFlags::NoMoveFrame;
+  const WritingMode outerWM = GetWritingMode();
+  const LogicalPoint dummyPosition(outerWM);
+  const nsSize dummyContainerSize;
+
+  // We use NoMoveFrame, so the position and container size used here are
+  // unimportant.
+  ReflowChild(aItem.Frame(), PresContext(), childReflowOutput,
+              aChildReflowInput, outerWM, dummyPosition, dummyContainerSize,
+              flags, childReflowStatus);
+  aItem.SetHadMeasuringReflow();
+
+  // We always use unconstrained available block-size to measure flex items,
+  // which means they should always complete.
+  MOZ_ASSERT(childReflowStatus.IsComplete(),
+             "We gave flex item unconstrained available block-size, so it "
+             "should be complete");
+
+  // Tell the child we're done with its initial reflow.
+  // (Necessary for e.g. GetBaseline() to work below w/out asserting)
+  FinishReflowChild(aItem.Frame(), PresContext(), childReflowOutput,
+                    &aChildReflowInput, outerWM, dummyPosition,
+                    dummyContainerSize, flags);
+
+  aItem.SetAscent(childReflowOutput.BlockStartAscent());
+
+  // Update (or add) our cached measurement, so that we can hopefully skip this
+  // measuring reflow the next time around:
+  if (cachedData) {
+    cachedData->Update(aChildReflowInput, childReflowOutput,
+                       FlexItemReflowType::Measuring);
+  } else {
+    cachedData = new CachedFlexItemData(aChildReflowInput, childReflowOutput,
+                                        FlexItemReflowType::Measuring);
+    aItem.Frame()->SetProperty(CachedFlexItemData::Prop(), cachedData);
+  }
+  return *(cachedData->mBAxisMeasurement);
+}
+
+/* virtual */
+void nsFlexContainerFrame::MarkIntrinsicISizesDirty() {
+  mCachedMinISize = NS_INTRINSIC_ISIZE_UNKNOWN;
+  mCachedPrefISize = NS_INTRINSIC_ISIZE_UNKNOWN;
+
+  nsContainerFrame::MarkIntrinsicISizesDirty();
+}
+
+nscoord nsFlexContainerFrame::MeasureFlexItemContentBSize(
+    FlexItem& aFlexItem, bool aForceBResizeForMeasuringReflow,
+    const ReflowInput& aParentReflowInput) {
+  FLEX_LOG("Measuring flex item's content block-size");
+
+  // Set up a reflow input for measuring the flex item's content block-size:
+  WritingMode wm = aFlexItem.Frame()->GetWritingMode();
+  LogicalSize availSize = aParentReflowInput.ComputedSize(wm);
+  availSize.BSize(wm) = NS_UNCONSTRAINEDSIZE;
+
+  StyleSizeOverrides sizeOverrides;
+  if (aFlexItem.IsStretched()) {
+    sizeOverrides.mStyleISize.emplace(aFlexItem.StyleCrossSize());
+    // Suppress any AspectRatio that we might have to prevent ComputeSize() from
+    // transferring our inline-size override through the aspect-ratio to set the
+    // block-size, because that would prevent us from measuring the content
+    // block-size.
+    sizeOverrides.mAspectRatio.emplace(AspectRatio());
+    FLEX_LOGV(" Cross size override: %d", aFlexItem.CrossSize());
+  }
+  sizeOverrides.mStyleBSize.emplace(StyleSize::Auto());
+
+  ReflowInput childRIForMeasuringBSize(
+      PresContext(), aParentReflowInput, aFlexItem.Frame(), availSize,
+      Nothing(), {}, sizeOverrides, {ComputeSizeFlag::ShrinkWrap});
+
+  // When measuring flex item's content block-size, disregard the item's
+  // min-block-size and max-block-size by resetting both to to their
+  // unconstraining (extreme) values. The flexbox layout algorithm does still
+  // explicitly clamp both sizes when resolving the target main size.
+  childRIForMeasuringBSize.SetComputedMinBSize(0);
+  childRIForMeasuringBSize.SetComputedMaxBSize(NS_UNCONSTRAINEDSIZE);
+
+  if (aForceBResizeForMeasuringReflow) {
+    childRIForMeasuringBSize.SetBResize(true);
+    // Not 100% sure this is needed, but be conservative for now:
+    childRIForMeasuringBSize.mFlags.mIsBResizeForPercentages = true;
+  }
+
+  const CachedBAxisMeasurement& measurement =
+      MeasureBSizeForFlexItem(aFlexItem, childRIForMeasuringBSize);
+
+  return measurement.BSize();
+}
+
+FlexItem::FlexItem(ReflowInput& aFlexItemReflowInput, float aFlexGrow,
+                   float aFlexShrink, nscoord aFlexBaseSize,
+                   nscoord aMainMinSize, nscoord aMainMaxSize,
+                   nscoord aTentativeCrossSize, nscoord aCrossMinSize,
+                   nscoord aCrossMaxSize,
+                   const FlexboxAxisTracker& aAxisTracker)
+    : mFrame(aFlexItemReflowInput.mFrame),
+      mFlexGrow(aFlexGrow),
+      mFlexShrink(aFlexShrink),
+      mAspectRatio(mFrame->GetAspectRatio()),
+      mWM(aFlexItemReflowInput.GetWritingMode()),
+      mCBWM(aAxisTracker.GetWritingMode()),
+      mMainAxis(aAxisTracker.MainAxis()),
+      mBorderPadding(aFlexItemReflowInput.ComputedLogicalBorderPadding(mCBWM)),
+      mMargin(aFlexItemReflowInput.ComputedLogicalMargin(mCBWM)),
+      mMainMinSize(aMainMinSize),
+      mMainMaxSize(aMainMaxSize),
+      mCrossMinSize(aCrossMinSize),
+      mCrossMaxSize(aCrossMaxSize),
+      mCrossSize(aTentativeCrossSize),
+      mIsInlineAxisMainAxis(aAxisTracker.IsInlineAxisMainAxis(mWM)),
+      mNeedsMinSizeAutoResolution(IsMinSizeAutoResolutionNeeded())
+// mAlignSelf, mHasAnyAutoMargin see below
+{
+  MOZ_ASSERT(mFrame, "expecting a non-null child frame");
+  MOZ_ASSERT(!mFrame->IsPlaceholderFrame(),
+             "placeholder frames should not be treated as flex items");
+  MOZ_ASSERT(!mFrame->HasAnyStateBits(NS_FRAME_OUT_OF_FLOW),
+             "out-of-flow frames should not be treated as flex items");
+  MOZ_ASSERT(mIsInlineAxisMainAxis ==
+                 nsFlexContainerFrame::IsItemInlineAxisMainAxis(mFrame),
+             "public API should be consistent with internal state (about "
+             "whether flex item's inline axis is flex container's main axis)");
+
+  const ReflowInput* containerRS = aFlexItemReflowInput.mParentReflowInput;
+  if (IsLegacyBox(containerRS->mFrame)) {
+    // For -webkit-{inline-}box and -moz-{inline-}box, we need to:
+    // (1) Use prefixed "box-align" instead of "align-items" to determine the
+    //     container's cross-axis alignment behavior.
+    // (2) Suppress the ability for flex items to override that with their own
+    //     cross-axis alignment. (The legacy box model doesn't support this.)
+    // So, each FlexItem simply copies the container's converted "align-items"
+    // value and disregards their own "align-self" property.
+    const nsStyleXUL* containerStyleXUL = containerRS->mFrame->StyleXUL();
+    mAlignSelf = {ConvertLegacyStyleToAlignItems(containerStyleXUL)};
+    mAlignSelfFlags = {0};
+  } else {
+    mAlignSelf = aFlexItemReflowInput.mStylePosition->UsedAlignSelf(
+        containerRS->mFrame->Style());
+    if (MOZ_LIKELY(mAlignSelf._0 == StyleAlignFlags::NORMAL)) {
+      mAlignSelf = {StyleAlignFlags::STRETCH};
+    }
+
+    // Store and strip off the <overflow-position> bits
+    mAlignSelfFlags = mAlignSelf._0 & StyleAlignFlags::FLAG_BITS;
+    mAlignSelf._0 &= ~StyleAlignFlags::FLAG_BITS;
+  }
+
+  // Our main-size is considered definite if any of these are true:
+  // (a) main axis is the item's inline axis.
+  // (b) flex container has definite main size.
+  // (c) flex item has a definite flex basis.
+  //
+  // Hence, we need to take care to treat the final main-size as *indefinite*
+  // if none of these conditions are satisfied.
+  if (mIsInlineAxisMainAxis) {
+    // The item's block-axis is the flex container's cross axis. We don't need
+    // any special handling to treat cross sizes as indefinite, because the
+    // cases where we stomp on the cross size with a definite value are all...
+    // - situations where the spec requires us to treat the cross size as
+    // definite; specifically, `align-self:stretch` whose cross size is
+    // definite.
+    // - situations where definiteness doesn't matter (e.g. for an element with
+    // an aspect ratio, which for now are all leaf nodes and hence
+    // can't have any percent-height descendants that would care about the
+    // definiteness of its size. (Once bug 1528375 is fixed, we might need to
+    // be more careful about definite vs. indefinite sizing on flex items with
+    // aspect ratios.)
+    mTreatBSizeAsIndefinite = false;
+  } else {
+    // The item's block-axis is the flex container's main axis. So, the flex
+    // item's main size is its BSize, and is considered definite under certain
+    // conditions laid out for definite flex-item main-sizes in the spec.
+    if (aAxisTracker.IsRowOriented() ||
+        (containerRS->ComputedBSize() != NS_UNCONSTRAINEDSIZE &&
+         !containerRS->mFlags.mTreatBSizeAsIndefinite)) {
+      // The flex *container* has a definite main-size (either by being
+      // row-oriented [and using its own inline size which is by definition
+      // definite, or by being column-oriented and having a definite
+      // block-size).  The spec says this means all of the flex items'
+      // post-flexing main sizes should *also* be treated as definite.
+      mTreatBSizeAsIndefinite = false;
+    } else if (aFlexBaseSize != NS_UNCONSTRAINEDSIZE) {
+      // The flex item has a definite flex basis, which we'll treat as making
+      // its main-size definite.
+      mTreatBSizeAsIndefinite = false;
+    } else {
+      // Otherwise, we have to treat the item's BSize as indefinite.
+      mTreatBSizeAsIndefinite = true;
+    }
+  }
+
+  SetFlexBaseSizeAndMainSize(aFlexBaseSize);
+
+  const nsStyleMargin* styleMargin = aFlexItemReflowInput.mStyleMargin;
+  mHasAnyAutoMargin = styleMargin->HasInlineAxisAuto(mCBWM) ||
+                      styleMargin->HasBlockAxisAuto(mCBWM);
+
+  // Assert that any "auto" margin components are set to 0.
+  // (We'll resolve them later; until then, we want to treat them as 0-sized.)
+#ifdef DEBUG
+  {
+    for (const auto side : AllLogicalSides()) {
+      if (styleMargin->mMargin.Get(mCBWM, side).IsAuto()) {
+        MOZ_ASSERT(GetMarginComponentForSide(side) == 0,
+                   "Someone else tried to resolve our auto margin");
+      }
+    }
+  }
+#endif  // DEBUG
+
+  if (mAlignSelf._0 == StyleAlignFlags::BASELINE ||
+      mAlignSelf._0 == StyleAlignFlags::LAST_BASELINE) {
+    // Check which of the item's baselines we're meant to use (first vs. last)
+    const bool usingItemFirstBaseline =
+        (mAlignSelf._0 == StyleAlignFlags::BASELINE);
+    if (IsBlockAxisCrossAxis()) {
+      // The flex item wants to be aligned in the cross axis using one of its
+      // baselines; and the cross axis is the item's block axis, so
+      // baseline-alignment in that axis makes sense.
+
+      // To determine the item's baseline sharing group, we check whether the
+      // item's block axis has the same vs. opposite flow direction as the
+      // corresponding LogicalAxis on the flex container.  We do this by
+      // getting the physical side that corresponds to these axes' "logical
+      // start" sides, and we compare those physical sides to find out if
+      // they're the same vs. opposite.
+      mozilla::Side itemBlockStartSide = mWM.PhysicalSide(eLogicalSideBStart);
+
+      // (Note: this is *not* the "flex-start" side; rather, it's the *logical*
+      // i.e. WM-relative block-start or inline-start side.)
+      mozilla::Side containerStartSideInCrossAxis = mCBWM.PhysicalSide(
+          MakeLogicalSide(aAxisTracker.CrossAxis(), eLogicalEdgeStart));
+
+      // We already know these two Sides (the item's block-start and the
+      // container's 'logical start' side for its cross axis) are in the same
+      // physical axis, since we're inside of a check for
+      // FlexItem::IsBlockAxisCrossAxis().  So these two Sides must be either
+      // the same physical side or opposite from each other.  If the Sides are
+      // the same, then the flow direction is the same, which means the item's
+      // {first,last} baseline participates in the {first,last}
+      // baseline-sharing group in its FlexLine.  Otherwise, the flow direction
+      // is opposite, and so the item's {first,last} baseline participates in
+      // the opposite i.e. {last,first} baseline-sharing group.  This is
+      // roughly per css-align-3 section 9.2, specifically the definition of
+      // what makes baseline alignment preferences "compatible".
+      bool itemBlockAxisFlowDirMatchesContainer =
+          (itemBlockStartSide == containerStartSideInCrossAxis);
+      mBaselineSharingGroup =
+          (itemBlockAxisFlowDirMatchesContainer == usingItemFirstBaseline)
+              ? BaselineSharingGroup::First
+              : BaselineSharingGroup::Last;
+    } else {
+      // The flex item wants to be aligned in the cross axis using one of its
+      // baselines, but we cannot get its baseline because the FlexItem's block
+      // axis is *orthogonal* to the container's cross axis. To handle this, we
+      // are supposed to synthesize a baseline from the item's border box and
+      // using that for baseline alignment.
+      mBaselineSharingGroup = usingItemFirstBaseline
+                                  ? BaselineSharingGroup::First
+                                  : BaselineSharingGroup::Last;
+    }
+  }
+}
+
+// Simplified constructor for creating a special "strut" FlexItem, for a child
+// with visibility:collapse. The strut has 0 main-size, and it only exists to
+// impose a minimum cross size on whichever FlexLine it ends up in.
+FlexItem::FlexItem(nsIFrame* aChildFrame, nscoord aCrossSize,
+                   WritingMode aContainerWM,
+                   const FlexboxAxisTracker& aAxisTracker)
+    : mFrame(aChildFrame),
+      mWM(aChildFrame->GetWritingMode()),
+      mCBWM(aContainerWM),
+      mMainAxis(aAxisTracker.MainAxis()),
+      mBorderPadding(mCBWM),
+      mMargin(mCBWM),
+      mCrossSize(aCrossSize),
+      // Struts don't do layout, so its WM doesn't matter at this point. So, we
+      // just share container's WM for simplicity:
+      mIsFrozen(true),
+      mIsStrut(true),  // (this is the constructor for making struts, after all)
+      mAlignSelf({StyleAlignFlags::FLEX_START}) {
+  MOZ_ASSERT(mFrame, "expecting a non-null child frame");
+  MOZ_ASSERT(mFrame->StyleVisibility()->IsCollapse(),
+             "Should only make struts for children with 'visibility:collapse'");
+  MOZ_ASSERT(!mFrame->IsPlaceholderFrame(),
+             "placeholder frames should not be treated as flex items");
+  MOZ_ASSERT(!mFrame->HasAnyStateBits(NS_FRAME_OUT_OF_FLOW),
+             "out-of-flow frames should not be treated as flex items");
+}
+
+bool FlexItem::IsMinSizeAutoResolutionNeeded() const {
+  // We'll need special behavior for "min-[width|height]:auto" (whichever is in
+  // the flex container's main axis) iff:
+  // (a) its computed value is "auto", and
+  // (b) the item is *not* a scroll container. (A scroll container's automatic
+  //     minimum size is zero.)
+  // https://drafts.csswg.org/css-flexbox-1/#min-size-auto
+  //
+  // Note that the scroll container case is redefined to be looking at the
+  // computed value instead, see https://github.com/w3c/csswg-drafts/issues/7714
+  const auto& mainMinSize =
+      Frame()->StylePosition()->MinSize(MainAxis(), ContainingBlockWM());
+
+  return IsAutoOrEnumOnBSize(mainMinSize, IsInlineAxisMainAxis()) &&
+         !Frame()->StyleDisplay()->IsScrollableOverflow();
+}
+
+Maybe<nscoord> FlexItem::MeasuredBSize() const {
+  auto* cachedData =
+      Frame()->FirstInFlow()->GetProperty(CachedFlexItemData::Prop());
+  if (!cachedData || !cachedData->mBAxisMeasurement) {
+    return Nothing();
+  }
+  return Some(cachedData->mBAxisMeasurement->BSize());
+}
+
+nscoord FlexItem::BaselineOffsetFromOuterCrossEdge(
+    mozilla::Side aStartSide, bool aUseFirstLineBaseline) const {
+  // NOTE:
+  //  * We only use baselines for aligning in the flex container's cross axis.
+  //  * Baselines are a measurement in the item's block axis.
+  if (IsBlockAxisMainAxis()) {
+    // We get here if the item's block axis is *orthogonal* the container's
+    // cross axis. For example, a flex item with writing-mode:horizontal-tb in a
+    // column-oriented flex container. We need to synthesize the item's baseline
+    // from its border-box edge.
+    const bool isMainAxisHorizontal =
+        mCBWM.PhysicalAxis(MainAxis()) == mozilla::eAxisHorizontal;
+
+    // When the main axis is horizontal, the synthesized baseline is the bottom
+    // edge of the item's border-box. Otherwise, when the main axis is vertical,
+    // the left edge. This is for compatibility with Google Chrome.
+    nscoord marginTopOrLeftToBaseline =
+        isMainAxisHorizontal ? PhysicalMargin().top : PhysicalMargin().left;
+    if (mCBWM.IsAlphabeticalBaseline()) {
+      marginTopOrLeftToBaseline += (isMainAxisHorizontal ? CrossSize() : 0);
+    } else {
+      MOZ_ASSERT(mCBWM.IsCentralBaseline());
+      marginTopOrLeftToBaseline += CrossSize() / 2;
+    }
+
+    return aStartSide == mozilla::eSideTop || aStartSide == mozilla::eSideLeft
+               ? marginTopOrLeftToBaseline
+               : OuterCrossSize() - marginTopOrLeftToBaseline;
+  }
+
+  // We get here if the item's block axis is parallel (or antiparallel) to the
+  // container's cross axis. We call ResolvedAscent() to get the item's
+  // baseline. If the item has no baseline, the method will synthesize one from
+  // the border-box edge.
+  MOZ_ASSERT(IsBlockAxisCrossAxis(),
+             "Only expecting to be doing baseline computations when the "
+             "cross axis is the block axis");
+
+  mozilla::Side itemBlockStartSide = mWM.PhysicalSide(eLogicalSideBStart);
+
+  nscoord marginBStartToBaseline = ResolvedAscent(aUseFirstLineBaseline) +
+                                   PhysicalMargin().Side(itemBlockStartSide);
+
+  return (aStartSide == itemBlockStartSide)
+             ? marginBStartToBaseline
+             : OuterCrossSize() - marginBStartToBaseline;
+}
+
+bool FlexItem::IsCrossSizeAuto() const {
+  const nsStylePosition* stylePos =
+      nsLayoutUtils::GetStyleFrame(mFrame)->StylePosition();
+  // Check whichever component is in the flex container's cross axis.
+  // (IsInlineAxisCrossAxis() tells us whether that's our ISize or BSize, in
+  // terms of our own WritingMode, mWM.)
+  return IsInlineAxisCrossAxis() ? stylePos->ISize(mWM).IsAuto()
+                                 : stylePos->BSize(mWM).IsAuto();
+}
+
+bool FlexItem::IsCrossSizeDefinite(const ReflowInput& aItemReflowInput) const {
+  if (IsStretched()) {
+    // Definite cross-size, imposed via 'align-self:stretch' & flex container.
+    return true;
+  }
+
+  const nsStylePosition* pos = aItemReflowInput.mStylePosition;
+  const auto itemWM = GetWritingMode();
+
+  // The logic here should be similar to the logic for isAutoISize/isAutoBSize
+  // in nsContainerFrame::ComputeSizeWithIntrinsicDimensions().
+  if (IsInlineAxisCrossAxis()) {
+    return !pos->ISize(itemWM).IsAuto();
+  }
+
+  nscoord cbBSize = aItemReflowInput.mContainingBlockSize.BSize(itemWM);
+  return !nsLayoutUtils::IsAutoBSize(pos->BSize(itemWM), cbBSize);
+}
+
+void FlexItem::ResolveFlexBaseSizeFromAspectRatio(
+    const ReflowInput& aItemReflowInput) {
+  // This implements the Flex Layout Algorithm Step 3B:
+  // https://drafts.csswg.org/css-flexbox-1/#algo-main-item
+  // If the flex item has ...
+  //  - an aspect ratio,
+  //  - a [used] flex-basis of 'content', and
+  //  - a definite cross size
+  // then the flex base size is calculated from its inner cross size and the
+  // flex item's preferred aspect ratio.
+  if (HasAspectRatio() &&
+      nsFlexContainerFrame::IsUsedFlexBasisContent(
+          aItemReflowInput.mStylePosition->mFlexBasis,
+          aItemReflowInput.mStylePosition->Size(MainAxis(), mCBWM)) &&
+      IsCrossSizeDefinite(aItemReflowInput)) {
+    const LogicalSize contentBoxSizeToBoxSizingAdjust =
+        aItemReflowInput.mStylePosition->mBoxSizing == StyleBoxSizing::Border
+            ? BorderPadding().Size(mCBWM)
+            : LogicalSize(mCBWM);
+    const nscoord mainSizeFromRatio = mAspectRatio.ComputeRatioDependentSize(
+        MainAxis(), mCBWM, CrossSize(), contentBoxSizeToBoxSizingAdjust);
+    SetFlexBaseSizeAndMainSize(mainSizeFromRatio);
+  }
+}
+
+uint32_t FlexItem::NumAutoMarginsInAxis(LogicalAxis aAxis) const {
+  uint32_t numAutoMargins = 0;
+  const auto& styleMargin = mFrame->StyleMargin()->mMargin;
+  for (const auto edge : {eLogicalEdgeStart, eLogicalEdgeEnd}) {
+    const auto side = MakeLogicalSide(aAxis, edge);
+    if (styleMargin.Get(mCBWM, side).IsAuto()) {
+      numAutoMargins++;
+    }
+  }
+
+  // Mostly for clarity:
+  MOZ_ASSERT(numAutoMargins <= 2,
+             "We're just looking at one item along one dimension, so we "
+             "should only have examined 2 margins");
+
+  return numAutoMargins;
+}
+
+bool FlexItem::CanMainSizeInfluenceCrossSize() const {
+  if (mIsStretched) {
+    // We've already had our cross-size stretched for "align-self:stretch").
+    // The container is imposing its cross size on us.
+    return false;
+  }
+
+  if (mIsStrut) {
+    // Struts (for visibility:collapse items) have a predetermined size;
+    // no need to measure anything.
+    return false;
+  }
+
+  if (HasAspectRatio()) {
+    // For flex items that have an aspect ratio (and maintain it, i.e. are
+    // not stretched, which we already checked above): changes to main-size
+    // *do* influence the cross size.
+    return true;
+  }
+
+  if (IsInlineAxisCrossAxis()) {
+    // If we get here, this function is really asking: "can changes to this
+    // item's block size have an influence on its inline size"?  For blocks and
+    // tables, the answer is "no".
+    if (mFrame->IsBlockFrame() || mFrame->IsTableWrapperFrame()) {
+      // XXXdholbert (Maybe use an IsFrameOfType query or something more
+      // general to test this across all frame types? For now, I'm just
+      // optimizing for block and table, since those are common containers that
+      // can contain arbitrarily-large subtrees (and that reliably have ISize
+      // being unaffected by BSize, per CSS2).  So optimizing away needless
+      // relayout is possible & especially valuable for these containers.)
+      return false;
+    }
+    // Other opt-outs can go here, as they're identified as being useful
+    // (particularly for containers where an extra reflow is expensive). But in
+    // general, we have to assume that a flexed BSize *could* influence the
+    // ISize. Some examples where this can definitely happen:
+    // * Intrinsically-sized multicol with fixed-ISize columns, which adds
+    // columns (i.e. grows in inline axis) depending on its block size.
+    // * Intrinsically-sized multi-line column-oriented flex container, which
+    // adds flex lines (i.e. grows in inline axis) depending on its block size.
+  }
+
+  // Default assumption, if we haven't proven otherwise: the resolved main size
+  // *can* change the cross size.
+  return true;
+}
+
+nscoord FlexItem::ClampMainSizeViaCrossAxisConstraints(
+    nscoord aMainSize, const ReflowInput& aItemReflowInput) const {
+  MOZ_ASSERT(HasAspectRatio(), "Caller should've checked the ratio is valid!");
+
+  const LogicalSize contentBoxSizeToBoxSizingAdjust =
+      aItemReflowInput.mStylePosition->mBoxSizing == StyleBoxSizing::Border
+          ? BorderPadding().Size(mCBWM)
+          : LogicalSize(mCBWM);
+
+  const nscoord mainMinSizeFromRatio = mAspectRatio.ComputeRatioDependentSize(
+      MainAxis(), mCBWM, CrossMinSize(), contentBoxSizeToBoxSizingAdjust);
+  nscoord clampedMainSize = std::max(aMainSize, mainMinSizeFromRatio);
+
+  if (CrossMaxSize() != NS_UNCONSTRAINEDSIZE) {
+    const nscoord mainMaxSizeFromRatio = mAspectRatio.ComputeRatioDependentSize(
+        MainAxis(), mCBWM, CrossMaxSize(), contentBoxSizeToBoxSizingAdjust);
+    clampedMainSize = std::min(clampedMainSize, mainMaxSizeFromRatio);
+  }
+
+  return clampedMainSize;
+}
+
+/**
+ * Returns true if aFrame or any of its children have the
+ * NS_FRAME_CONTAINS_RELATIVE_BSIZE flag set -- i.e. if any of these frames (or
+ * their descendants) might have a relative-BSize dependency on aFrame (or its
+ * ancestors).
+ */
+static bool FrameHasRelativeBSizeDependency(nsIFrame* aFrame) {
+  if (aFrame->HasAnyStateBits(NS_FRAME_CONTAINS_RELATIVE_BSIZE)) {
+    return true;
+  }
+  for (const auto& childList : aFrame->ChildLists()) {
+    for (nsIFrame* childFrame : childList.mList) {
+      if (childFrame->HasAnyStateBits(NS_FRAME_CONTAINS_RELATIVE_BSIZE)) {
+        return true;
+      }
+    }
+  }
+  return false;
+}
+
+bool FlexItem::NeedsFinalReflow(const ReflowInput& aParentReflowInput) const {
+  if (!StaticPrefs::layout_flexbox_item_final_reflow_optimization_enabled()) {
+    FLEX_LOG(
+        "[perf] Flex item %p needed a final reflow due to optimization being "
+        "disabled via the preference",
+        mFrame);
+    return true;
+  }
+
+  // NOTE: We can have continuations from an earlier constrained reflow.
+  if (mFrame->GetPrevInFlow() || mFrame->GetNextInFlow()) {
+    // This is an item has continuation(s). Reflow it.
+    FLEX_LOG("[frag] Flex item %p needed a final reflow due to continuation(s)",
+             mFrame);
+    return true;
+  }
+
+  // A flex item can grow its block-size in a fragmented context if there's any
+  // force break within it (bug 1663079), or if it has a repeated table header
+  // or footer (bug 1744363). We currently always reflow it.
+  //
+  // Bug 1815294: investigate if we can design a more specific condition to
+  // prevent triggering O(n^2) behavior when printing a deeply-nested flex
+  // container.
+  if (aParentReflowInput.IsInFragmentedContext()) {
+    FLEX_LOG(
+        "[frag] Flex item %p needed both a measuring reflow and a final "
+        "reflow due to being in a fragmented context.",
+        mFrame);
+    return true;
+  }
+
+  // Flex item's final content-box size (in terms of its own writing-mode):
+  const LogicalSize finalSize = mIsInlineAxisMainAxis
+                                    ? LogicalSize(mWM, mMainSize, mCrossSize)
+                                    : LogicalSize(mWM, mCrossSize, mMainSize);
+
+  if (HadMeasuringReflow()) {
+    // We've already reflowed this flex item once, to measure it. In that
+    // reflow, did its frame happen to end up with the correct final size
+    // that the flex container would like it to have?
+    if (finalSize != mFrame->ContentSize(mWM)) {
+      // The measuring reflow left the item with a different size than its
+      // final flexed size. So, we need to reflow to give it the correct size.
+      FLEX_LOG(
+          "[perf] Flex item %p needed both a measuring reflow and a final "
+          "reflow due to measured size disagreeing with final size",
+          mFrame);
+      return true;
+    }
+
+    if (FrameHasRelativeBSizeDependency(mFrame)) {
+      // This item has descendants with relative BSizes who may care that its
+      // size may now be considered "definite" in the final reflow (whereas it
+      // was indefinite during the measuring reflow).
+      FLEX_LOG(
+          "[perf] Flex item %p needed both a measuring reflow and a final "
+          "reflow due to BSize potentially becoming definite",
+          mFrame);
+      return true;
+    }
+
+    // If we get here, then this flex item had a measuring reflow, it left us
+    // with the correct size, none of its descendants care that its BSize may
+    // now be considered definite, and it can fit into the available block-size.
+    // So it doesn't need a final reflow.
+    //
+    // We now cache this size as if we had done a final reflow (because we've
+    // determined that the measuring reflow was effectively equivalent).  This
+    // way, in our next time through flex layout, we may be able to skip both
+    // the measuring reflow *and* the final reflow (if conditions are the same
+    // as they are now).
+    if (auto* cache = mFrame->GetProperty(CachedFlexItemData::Prop())) {
+      cache->Update(*this, finalSize);
+    }
+
+    return false;
+  }
+
+  // This item didn't receive a measuring reflow (at least, not during this
+  // reflow of our flex container).  We may still be able to skip reflowing it
+  // (i.e. return false from this function), if its subtree is clean & its most
+  // recent "final reflow" had it at the correct content-box size &
+  // definiteness.
+  // Let's check for each condition that would still require us to reflow:
+  if (mFrame->IsSubtreeDirty()) {
+    FLEX_LOG(
+        "[perf] Flex item %p needed a final reflow due to its subtree "
+        "being dirty",
+        mFrame);
+    return true;
+  }
+
+  // Cool; this item & its subtree haven't experienced any style/content
+  // changes that would automatically require a reflow.
+
+  // Did we cache the metrics from its most recent "final reflow"?
+  auto* cache = mFrame->GetProperty(CachedFlexItemData::Prop());
+  if (!cache || !cache->mFinalReflowMetrics) {
+    FLEX_LOG(
+        "[perf] Flex item %p needed a final reflow due to lacking a "
+        "cached mFinalReflowMetrics (maybe cache was cleared)",
+        mFrame);
+    return true;
+  }
+
+  // Does the cached size match our current size?
+  if (cache->mFinalReflowMetrics->Size() != finalSize) {
+    FLEX_LOG(
+        "[perf] Flex item %p needed a final reflow due to having a "
+        "different content box size vs. its most recent final reflow",
+        mFrame);
+    return true;
+  }
+
+  // Does the cached border and padding match our current ones?
+  //
+  // Note: this is just to detect cases where we have a percent padding whose
+  // basis has changed. Any other sort of change to BorderPadding() (e.g. a new
+  // specified value) should result in the frame being marked dirty via proper
+  // change hint (see nsStylePadding::CalcDifference()), which will force it to
+  // reflow.
+  if (cache->mFinalReflowMetrics->BorderPadding() !=
+      BorderPadding().ConvertTo(mWM, mCBWM)) {
+    FLEX_LOG(
+        "[perf] Flex item %p needed a final reflow due to having a "
+        "different border and padding vs. its most recent final reflow",
+        mFrame);
+    return true;
+  }
+
+  // The flex container is giving this flex item the same size that the item
+  // had on its most recent "final reflow". But if its definiteness changed and
+  // one of the descendants cares, then it would still need a reflow.
+  if (cache->mFinalReflowMetrics->TreatBSizeAsIndefinite() !=
+          mTreatBSizeAsIndefinite &&
+      FrameHasRelativeBSizeDependency(mFrame)) {
+    FLEX_LOG(
+        "[perf] Flex item %p needed a final reflow due to having "
+        "its BSize change definiteness & having a rel-BSize child",
+        mFrame);
+    return true;
+  }
+
+  // If we get here, we can skip the final reflow! (The item's subtree isn't
+  // dirty, and our current conditions are sufficiently similar to the most
+  // recent "final reflow" that it should have left our subtree in the correct
+  // state.)
+  FLEX_LOG("[perf] Flex item %p didn't need a final reflow", mFrame);
+  return false;
+}
+
+// Keeps track of our position along a particular axis (where a '0' position
+// corresponds to the 'start' edge of that axis).
+// This class shouldn't be instantiated directly -- rather, it should only be
+// instantiated via its subclasses defined below.
+class MOZ_STACK_CLASS PositionTracker {
+ public:
+  // Accessor for the current value of the position that we're tracking.
+  inline nscoord Position() const { return mPosition; }
+  inline LogicalAxis Axis() const { return mAxis; }
+
+  inline LogicalSide StartSide() {
+    return MakeLogicalSide(
+        mAxis, mIsAxisReversed ? eLogicalEdgeEnd : eLogicalEdgeStart);
+  }
+
+  inline LogicalSide EndSide() {
+    return MakeLogicalSide(
+        mAxis, mIsAxisReversed ? eLogicalEdgeStart : eLogicalEdgeEnd);
+  }
+
+  // Advances our position across the start edge of the given margin, in the
+  // axis we're tracking.
+  void EnterMargin(const LogicalMargin& aMargin) {
+    mPosition += aMargin.Side(StartSide(), mWM);
+  }
+
+  // Advances our position across the end edge of the given margin, in the axis
+  // we're tracking.
+  void ExitMargin(const LogicalMargin& aMargin) {
+    mPosition += aMargin.Side(EndSide(), mWM);
+  }
+
+  // Advances our current position from the start side of a child frame's
+  // border-box to the frame's upper or left edge (depending on our axis).
+  // (Note that this is a no-op if our axis grows in the same direction as
+  // the corresponding logical axis.)
+  void EnterChildFrame(nscoord aChildFrameSize) {
+    if (mIsAxisReversed) {
+      mPosition += aChildFrameSize;
+    }
+  }
+
+  // Advances our current position from a frame's upper or left border-box edge
+  // (whichever is in the axis we're tracking) to the 'end' side of the frame
+  // in the axis that we're tracking. (Note that this is a no-op if our axis
+  // is reversed with respect to the corresponding logical axis.)
+  void ExitChildFrame(nscoord aChildFrameSize) {
+    if (!mIsAxisReversed) {
+      mPosition += aChildFrameSize;
+    }
+  }
+
+  // Delete copy-constructor & reassignment operator, to prevent accidental
+  // (unnecessary) copying.
+  PositionTracker(const PositionTracker&) = delete;
+  PositionTracker& operator=(const PositionTracker&) = delete;
+
+ protected:
+  // Protected constructor, to be sure we're only instantiated via a subclass.
+  PositionTracker(WritingMode aWM, LogicalAxis aAxis, bool aIsAxisReversed)
+      : mWM(aWM), mAxis(aAxis), mIsAxisReversed(aIsAxisReversed) {}
+
+  // Member data:
+  // The position we're tracking.
+  nscoord mPosition = 0;
+
+  // The flex container's writing mode.
+  const WritingMode mWM;
+
+  // The axis along which we're moving.
+  const LogicalAxis mAxis = eLogicalAxisInline;
+
+  // Is the axis along which we're moving reversed (e.g. LTR vs RTL) with
+  // respect to the corresponding axis on the flex container's WM?
+  const bool mIsAxisReversed = false;
+};
+
+// Tracks our position in the main axis, when we're laying out flex items.
+// The "0" position represents the main-start edge of the flex container's
+// content-box.
+class MOZ_STACK_CLASS MainAxisPositionTracker : public PositionTracker {
+ public:
+  MainAxisPositionTracker(const FlexboxAxisTracker& aAxisTracker,
+                          const FlexLine* aLine,
+                          const StyleContentDistribution& aJustifyContent,
+                          nscoord aContentBoxMainSize);
+
+  ~MainAxisPositionTracker() {
+    MOZ_ASSERT(mNumPackingSpacesRemaining == 0,
+               "miscounted the number of packing spaces");
+    MOZ_ASSERT(mNumAutoMarginsInMainAxis == 0,
+               "miscounted the number of auto margins");
+  }
+
+  // Advances past the gap space (if any) between two flex items
+  void TraverseGap(nscoord aGapSize) { mPosition += aGapSize; }
+
+  // Advances past the packing space (if any) between two flex items
+  void TraversePackingSpace();
+
+  // If aItem has any 'auto' margins in the main axis, this method updates the
+  // corresponding values in its margin.
+  void ResolveAutoMarginsInMainAxis(FlexItem& aItem);
+
+ private:
+  nscoord mPackingSpaceRemaining = 0;
+  uint32_t mNumAutoMarginsInMainAxis = 0;
+  uint32_t mNumPackingSpacesRemaining = 0;
+  StyleContentDistribution mJustifyContent = {StyleAlignFlags::AUTO};
+};
+
+// Utility class for managing our position along the cross axis along
+// the whole flex container (at a higher level than a single line).
+// The "0" position represents the cross-start edge of the flex container's
+// content-box.
+class MOZ_STACK_CLASS CrossAxisPositionTracker : public PositionTracker {
+ public:
+  CrossAxisPositionTracker(nsTArray<FlexLine>& aLines,
+                           const ReflowInput& aReflowInput,
+                           nscoord aContentBoxCrossSize,
+                           bool aIsCrossSizeDefinite,
+                           const FlexboxAxisTracker& aAxisTracker,
+                           const nscoord aCrossGapSize);
+
+  // Advances past the gap (if any) between two flex lines
+  void TraverseGap() { mPosition += mCrossGapSize; }
+
+  // Advances past the packing space (if any) between two flex lines
+  void TraversePackingSpace();
+
+  // Advances past the given FlexLine
+  void TraverseLine(FlexLine& aLine) { mPosition += aLine.LineCrossSize(); }
+
+  // Redeclare the frame-related methods from PositionTracker with
+  // = delete, to be sure (at compile time) that no client code can invoke
+  // them. (Unlike the other PositionTracker derived classes, this class here
+  // deals with FlexLines, not with individual FlexItems or frames.)
+  void EnterMargin(const LogicalMargin& aMargin) = delete;
+  void ExitMargin(const LogicalMargin& aMargin) = delete;
+  void EnterChildFrame(nscoord aChildFrameSize) = delete;
+  void ExitChildFrame(nscoord aChildFrameSize) = delete;
+
+ private:
+  nscoord mPackingSpaceRemaining = 0;
+  uint32_t mNumPackingSpacesRemaining = 0;
+  StyleContentDistribution mAlignContent = {StyleAlignFlags::AUTO};
+
+  const nscoord mCrossGapSize;
+};
+
+// Utility class for managing our position along the cross axis, *within* a
+// single flex line.
+class MOZ_STACK_CLASS SingleLineCrossAxisPositionTracker
+    : public PositionTracker {
+ public:
+  explicit SingleLineCrossAxisPositionTracker(
+      const FlexboxAxisTracker& aAxisTracker);
+
+  void ResolveAutoMarginsInCrossAxis(const FlexLine& aLine, FlexItem& aItem);
+
+  void EnterAlignPackingSpace(const FlexLine& aLine, const FlexItem& aItem,
+                              const FlexboxAxisTracker& aAxisTracker);
+
+  // Resets our position to the cross-start edge of this line.
+  inline void ResetPosition() { mPosition = 0; }
+};
+
+//----------------------------------------------------------------------
+
+// Frame class boilerplate
+// =======================
+
+NS_QUERYFRAME_HEAD(nsFlexContainerFrame)
+  NS_QUERYFRAME_ENTRY(nsFlexContainerFrame)
+NS_QUERYFRAME_TAIL_INHERITING(nsContainerFrame)
+
+NS_IMPL_FRAMEARENA_HELPERS(nsFlexContainerFrame)
+
+nsContainerFrame* NS_NewFlexContainerFrame(PresShell* aPresShell,
+                                           ComputedStyle* aStyle) {
+  return new (aPresShell)
+      nsFlexContainerFrame(aStyle, aPresShell->GetPresContext());
+}
+
+//----------------------------------------------------------------------
+
+// nsFlexContainerFrame Method Implementations
+// ===========================================
+
+/* virtual */
+nsFlexContainerFrame::~nsFlexContainerFrame() = default;
+
+/* virtual */
+void nsFlexContainerFrame::Init(nsIContent* aContent, nsContainerFrame* aParent,
+                                nsIFrame* aPrevInFlow) {
+  nsContainerFrame::Init(aContent, aParent, aPrevInFlow);
+
+  if (HasAnyStateBits(NS_FRAME_FONT_INFLATION_CONTAINER)) {
+    AddStateBits(NS_FRAME_FONT_INFLATION_FLOW_ROOT);
+  }
+
+  auto displayInside = StyleDisplay()->DisplayInside();
+  // If this frame is for a scrollable element, then it will actually have
+  // "display:block", and its *parent frame* will have the real
+  // flex-flavored display value. So in that case, check the parent frame to
+  // find out if we're legacy.
+  //
+  // TODO(emilio): Maybe ::-moz-scrolled-content and co should inherit `display`
+  // (or a blockified version thereof, to not hit bug 456484).
+  if (displayInside == StyleDisplayInside::Flow) {
+    MOZ_ASSERT(StyleDisplay()->mDisplay == StyleDisplay::Block);
+    MOZ_ASSERT(Style()->GetPseudoType() == PseudoStyleType::buttonContent ||
+                   Style()->GetPseudoType() == PseudoStyleType::scrolledContent,
+               "The only way a nsFlexContainerFrame can have 'display:block' "
+               "should be if it's the inner part of a scrollable or button "
+               "element");
+    displayInside = GetParent()->StyleDisplay()->DisplayInside();
+  }
+
+  // Figure out if we should set a frame state bit to indicate that this frame
+  // represents a legacy -moz-{inline-}box or -webkit-{inline-}box container.
+  if (displayInside == StyleDisplayInside::WebkitBox) {
+    AddStateBits(NS_STATE_FLEX_IS_EMULATING_LEGACY_WEBKIT_BOX);
+  }
+}
+
+#ifdef DEBUG_FRAME_DUMP
+nsresult nsFlexContainerFrame::GetFrameName(nsAString& aResult) const {
+  return MakeFrameName(u"FlexContainer"_ns, aResult);
+}
+#endif
+
+void nsFlexContainerFrame::BuildDisplayList(nsDisplayListBuilder* aBuilder,
+                                            const nsDisplayListSet& aLists) {
+  nsDisplayListCollection tempLists(aBuilder);
+
+  DisplayBorderBackgroundOutline(aBuilder, tempLists);
+  if (GetPrevInFlow()) {
+    DisplayOverflowContainers(aBuilder, tempLists);
+  }
+
+  // Our children are all block-level, so their borders/backgrounds all go on
+  // the BlockBorderBackgrounds list.
+  nsDisplayListSet childLists(tempLists, tempLists.BlockBorderBackgrounds());
+
+  CSSOrderAwareFrameIterator iter(
+      this, FrameChildListID::Principal,
+      CSSOrderAwareFrameIterator::ChildFilter::IncludeAll,
+      OrderStateForIter(this), OrderingPropertyForIter(this));
+
+  const auto flags = DisplayFlagsForFlexOrGridItem();
+  for (; !iter.AtEnd(); iter.Next()) {
+    nsIFrame* childFrame = *iter;
+    BuildDisplayListForChild(aBuilder, childFrame, childLists, flags);
+  }
+
+  tempLists.MoveTo(aLists);
+}
+
+void FlexLine::FreezeItemsEarly(bool aIsUsingFlexGrow,
+                                ComputedFlexLineInfo* aLineInfo) {
+  // After we've established the type of flexing we're doing (growing vs.
+  // shrinking), and before we try to flex any items, we freeze items that
+  // obviously *can't* flex.
+  //
+  // Quoting the spec:
+  //  # Freeze, setting its target main size to its hypothetical main size...
+  //  #  - any item that has a flex factor of zero
+  //  #  - if using the flex grow factor: any item that has a flex base size
+  //  #    greater than its hypothetical main size
+  //  #  - if using the flex shrink factor: any item that has a flex base size
+  //  #    smaller than its hypothetical main size
+  //  https://drafts.csswg.org/css-flexbox/#resolve-flexible-lengths
+  //
+  // (NOTE: At this point, item->MainSize() *is* the item's hypothetical
+  // main size, since SetFlexBaseSizeAndMainSize() sets it up that way, and the
+  // item hasn't had a chance to flex away from that yet.)
+
+  // Since this loop only operates on unfrozen flex items, we can break as
+  // soon as we have seen all of them.
+  uint32_t numUnfrozenItemsToBeSeen = NumItems() - mNumFrozenItems;
+  for (FlexItem& item : Items()) {
+    if (numUnfrozenItemsToBeSeen == 0) {
+      break;
+    }
+
+    if (!item.IsFrozen()) {
+      numUnfrozenItemsToBeSeen--;
+      bool shouldFreeze = (0.0f == item.GetFlexFactor(aIsUsingFlexGrow));
+      if (!shouldFreeze) {
+        if (aIsUsingFlexGrow) {
+          if (item.FlexBaseSize() > item.MainSize()) {
+            shouldFreeze = true;
+          }
+        } else {  // using flex-shrink
+          if (item.FlexBaseSize() < item.MainSize()) {
+            shouldFreeze = true;
+          }
+        }
+      }
+      if (shouldFreeze) {
+        // Freeze item! (at its hypothetical main size)
+        item.Freeze();
+        if (item.FlexBaseSize() < item.MainSize()) {
+          item.SetWasMinClamped();
+        } else if (item.FlexBaseSize() > item.MainSize()) {
+          item.SetWasMaxClamped();
+        }
+        mNumFrozenItems++;
+      }
+    }
+  }
+
+  MOZ_ASSERT(numUnfrozenItemsToBeSeen == 0, "miscounted frozen items?");
+}
+
+// Based on the sign of aTotalViolation, this function freezes a subset of our
+// flexible sizes, and restores the remaining ones to their initial pref sizes.
+void FlexLine::FreezeOrRestoreEachFlexibleSize(const nscoord aTotalViolation,
+                                               bool aIsFinalIteration) {
+  enum FreezeType {
+    eFreezeEverything,
+    eFreezeMinViolations,
+    eFreezeMaxViolations
+  };
+
+  FreezeType freezeType;
+  if (aTotalViolation == 0) {
+    freezeType = eFreezeEverything;
+  } else if (aTotalViolation > 0) {
+    freezeType = eFreezeMinViolations;
+  } else {  // aTotalViolation < 0
+    freezeType = eFreezeMaxViolations;
+  }
+
+  // Since this loop only operates on unfrozen flex items, we can break as
+  // soon as we have seen all of them.
+  uint32_t numUnfrozenItemsToBeSeen = NumItems() - mNumFrozenItems;
+  for (FlexItem& item : Items()) {
+    if (numUnfrozenItemsToBeSeen == 0) {
+      break;
+    }
+
+    if (!item.IsFrozen()) {
+      numUnfrozenItemsToBeSeen--;
+
+      MOZ_ASSERT(!item.HadMinViolation() || !item.HadMaxViolation(),
+                 "Can have either min or max violation, but not both");
+
+      bool hadMinViolation = item.HadMinViolation();
+      bool hadMaxViolation = item.HadMaxViolation();
+      if (eFreezeEverything == freezeType ||
+          (eFreezeMinViolations == freezeType && hadMinViolation) ||
+          (eFreezeMaxViolations == freezeType && hadMaxViolation)) {
+        MOZ_ASSERT(item.MainSize() >= item.MainMinSize(),
+                   "Freezing item at a size below its minimum");
+        MOZ_ASSERT(item.MainSize() <= item.MainMaxSize(),
+                   "Freezing item at a size above its maximum");
+
+        item.Freeze();
+        if (hadMinViolation) {
+          item.SetWasMinClamped();
+        } else if (hadMaxViolation) {
+          item.SetWasMaxClamped();
+        }
+        mNumFrozenItems++;
+      } else if (MOZ_UNLIKELY(aIsFinalIteration)) {
+        // XXXdholbert If & when bug 765861 is fixed, we should upgrade this
+        // assertion to be fatal except in documents with enormous lengths.
+        NS_ERROR(
+            "Final iteration still has unfrozen items, this shouldn't"
+            " happen unless there was nscoord under/overflow.");
+        item.Freeze();
+        mNumFrozenItems++;
+      }  // else, we'll reset this item's main size to its flex base size on the
+         // next iteration of this algorithm.
+
+      if (!item.IsFrozen()) {
+        // Clear this item's violation(s), now that we've dealt with them
+        item.ClearViolationFlags();
+      }
+    }
+  }
+
+  MOZ_ASSERT(numUnfrozenItemsToBeSeen == 0, "miscounted frozen items?");
+}
+
+void FlexLine::ResolveFlexibleLengths(nscoord aFlexContainerMainSize,
+                                      ComputedFlexLineInfo* aLineInfo) {
+  // In this function, we use 64-bit coord type to avoid integer overflow in
+  // case several of the individual items have huge hypothetical main sizes,
+  // which can happen with percent-width table-layout:fixed descendants. Here we
+  // promote the container's main size to 64-bit to make the arithmetic
+  // convenient.
+  AuCoord64 flexContainerMainSize(aFlexContainerMainSize);
+
+  // Before we start resolving sizes: if we have an aLineInfo structure to fill
+  // out, we inform it of each item's base size, and we initialize the "delta"
+  // for each item to 0. (And if the flex algorithm wants to grow or shrink the
+  // item, we'll update this delta further down.)
+  if (aLineInfo) {
+    uint32_t itemIndex = 0;
+    for (FlexItem& item : Items()) {
+      aLineInfo->mItems[itemIndex].mMainBaseSize = item.FlexBaseSize();
+      aLineInfo->mItems[itemIndex].mMainDeltaSize = 0;
+      ++itemIndex;
+    }
+  }
+
+  // Determine whether we're going to be growing or shrinking items.
+  const bool isUsingFlexGrow =
+      (mTotalOuterHypotheticalMainSize < flexContainerMainSize);
+
+  if (aLineInfo) {
+    aLineInfo->mGrowthState =
+        isUsingFlexGrow ? mozilla::dom::FlexLineGrowthState::Growing
+                        : mozilla::dom::FlexLineGrowthState::Shrinking;
+  }
+
+  // Do an "early freeze" for flex items that obviously can't flex in the
+  // direction we've chosen:
+  FreezeItemsEarly(isUsingFlexGrow, aLineInfo);
+
+  if ((mNumFrozenItems == NumItems()) && !aLineInfo) {
+    // All our items are frozen, so we have no flexible lengths to resolve,
+    // and we aren't being asked to generate computed line info.
+    FLEX_LOG("No flexible length to resolve");
+    return;
+  }
+  MOZ_ASSERT(!IsEmpty() || aLineInfo,
+             "empty lines should take the early-return above");
+
+  FLEX_LOG("Resolving flexible lengths for items");
+
+  // Subtract space occupied by our items' margins/borders/padding/gaps, so
+  // we can just be dealing with the space available for our flex items' content
+  // boxes.
+  const AuCoord64 totalItemMBPAndGaps = mTotalItemMBP + SumOfGaps();
+  const AuCoord64 spaceAvailableForFlexItemsContentBoxes =
+      flexContainerMainSize - totalItemMBPAndGaps;
+
+  Maybe<AuCoord64> origAvailableFreeSpace;
+
+  // NOTE: I claim that this chunk of the algorithm (the looping part) needs to
+  // run the loop at MOST NumItems() times.  This claim should hold up
+  // because we'll freeze at least one item on each loop iteration, and once
+  // we've run out of items to freeze, there's nothing left to do.  However,
+  // in most cases, we'll break out of this loop long before we hit that many
+  // iterations.
+  for (uint32_t iterationCounter = 0; iterationCounter < NumItems();
+       iterationCounter++) {
+    // Set every not-yet-frozen item's used main size to its
+    // flex base size, and subtract all the used main sizes from our
+    // total amount of space to determine the 'available free space'
+    // (positive or negative) to be distributed among our flexible items.
+    AuCoord64 availableFreeSpace = spaceAvailableForFlexItemsContentBoxes;
+    for (FlexItem& item : Items()) {
+      if (!item.IsFrozen()) {
+        item.SetMainSize(item.FlexBaseSize());
+      }
+      availableFreeSpace -= item.MainSize();
+    }
+
+    FLEX_LOG(" available free space: %" PRId64 "; flex items should \"%s\"",
+             availableFreeSpace.value, isUsingFlexGrow ? "grow" : "shrink");
+
+    // The sign of our free space should agree with the type of flexing
+    // (grow/shrink) that we're doing. Any disagreement should've made us use
+    // the other type of flexing, or should've been resolved in
+    // FreezeItemsEarly.
+    //
+    // Note: it's possible that an individual flex item has huge
+    // margin/border/padding that makes either its
+    // MarginBorderPaddingSizeInMainAxis() or OuterMainSize() negative due to
+    // integer overflow. If that happens, the accumulated
+    // mTotalOuterHypotheticalMainSize or mTotalItemMBP could be negative due to
+    // that one item's negative (overflowed) size. Likewise, a huge main gap
+    // size between flex items can also make our accumulated SumOfGaps()
+    // negative. In these case, we throw up our hands and don't require
+    // isUsingFlexGrow to agree with availableFreeSpace. Luckily, we won't get
+    // stuck in the algorithm below, and just distribute the wrong
+    // availableFreeSpace with the wrong grow/shrink factors.
+    MOZ_ASSERT(!(mTotalOuterHypotheticalMainSize >= 0 && mTotalItemMBP >= 0 &&
+                 totalItemMBPAndGaps >= 0) ||
+                   (isUsingFlexGrow && availableFreeSpace >= 0) ||
+                   (!isUsingFlexGrow && availableFreeSpace <= 0),
+               "availableFreeSpace's sign should match isUsingFlexGrow");
+
+    // If we have any free space available, give each flexible item a portion
+    // of availableFreeSpace.
+    if (availableFreeSpace != AuCoord64(0)) {
+      // The first time we do this, we initialize origAvailableFreeSpace.
+      if (!origAvailableFreeSpace) {
+        origAvailableFreeSpace.emplace(availableFreeSpace);
+      }
+
+      // STRATEGY: On each item, we compute & store its "share" of the total
+      // weight that we've seen so far:
+      //   curWeight / weightSum
+      //
+      // Then, when we go to actually distribute the space (in the next loop),
+      // we can simply walk backwards through the elements and give each item
+      // its "share" multiplied by the remaining available space.
+      //
+      // SPECIAL CASE: If the sum of the weights is larger than the
+      // maximum representable double (overflowing to infinity), then we can't
+      // sensibly divide out proportional shares anymore. In that case, we
+      // simply treat the flex item(s) with the largest weights as if
+      // their weights were infinite (dwarfing all the others), and we
+      // distribute all of the available space among them.
+      double weightSum = 0.0;
+      double flexFactorSum = 0.0;
+      double largestWeight = 0.0;
+      uint32_t numItemsWithLargestWeight = 0;
+
+      // Since this loop only operates on unfrozen flex items, we can break as
+      // soon as we have seen all of them.
+      uint32_t numUnfrozenItemsToBeSeen = NumItems() - mNumFrozenItems;
+      for (FlexItem& item : Items()) {
+        if (numUnfrozenItemsToBeSeen == 0) {
+          break;
+        }
+
+        if (!item.IsFrozen()) {
+          numUnfrozenItemsToBeSeen--;
+
+          const double curWeight = item.GetWeight(isUsingFlexGrow);
+          const double curFlexFactor = item.GetFlexFactor(isUsingFlexGrow);
+          MOZ_ASSERT(curWeight >= 0.0, "weights are non-negative");
+          MOZ_ASSERT(curFlexFactor >= 0.0, "flex factors are non-negative");
+
+          weightSum += curWeight;
+          flexFactorSum += curFlexFactor;
+
+          if (std::isfinite(weightSum)) {
+            if (curWeight == 0.0) {
+              item.SetShareOfWeightSoFar(0.0);
+            } else {
+              item.SetShareOfWeightSoFar(curWeight / weightSum);
+            }
+          }  // else, the sum of weights overflows to infinity, in which
+             // case we don't bother with "SetShareOfWeightSoFar" since
+             // we know we won't use it. (instead, we'll just give every
+             // item with the largest weight an equal share of space.)
+
+          // Update our largest-weight tracking vars
+          if (curWeight > largestWeight) {
+            largestWeight = curWeight;
+            numItemsWithLargestWeight = 1;
+          } else if (curWeight == largestWeight) {
+            numItemsWithLargestWeight++;
+          }
+        }
+      }
+
+      MOZ_ASSERT(numUnfrozenItemsToBeSeen == 0, "miscounted frozen items?");
+
+      if (weightSum != 0.0) {
+        MOZ_ASSERT(flexFactorSum != 0.0,
+                   "flex factor sum can't be 0, if a weighted sum "
+                   "of its components (weightSum) is nonzero");
+        if (flexFactorSum < 1.0) {
+          // Our unfrozen flex items don't want all of the original free space!
+          // (Their flex factors add up to something less than 1.)
+          // Hence, make sure we don't distribute any more than the portion of
+          // our original free space that these items actually want.
+          auto totalDesiredPortionOfOrigFreeSpace =
+              AuCoord64::FromRound(*origAvailableFreeSpace * flexFactorSum);
+
+          // Clamp availableFreeSpace to be no larger than that ^^.
+          // (using min or max, depending on sign).
+          // This should not change the sign of availableFreeSpace (except
+          // possibly by setting it to 0), as enforced by this assertion:
+          NS_ASSERTION(totalDesiredPortionOfOrigFreeSpace == AuCoord64(0) ||
+                           ((totalDesiredPortionOfOrigFreeSpace > 0) ==
+                            (availableFreeSpace > 0)),
+                       "When we reduce available free space for flex "
+                       "factors < 1, we shouldn't change the sign of the "
+                       "free space...");
+
+          if (availableFreeSpace > 0) {
+            availableFreeSpace = std::min(availableFreeSpace,
+                                          totalDesiredPortionOfOrigFreeSpace);
+          } else {
+            availableFreeSpace = std::max(availableFreeSpace,
+                                          totalDesiredPortionOfOrigFreeSpace);
+          }
+        }
+
+        FLEX_LOG(" Distributing available space:");
+        // Since this loop only operates on unfrozen flex items, we can break as
+        // soon as we have seen all of them.
+        numUnfrozenItemsToBeSeen = NumItems() - mNumFrozenItems;
+
+        // NOTE: It's important that we traverse our items in *reverse* order
+        // here, for correct width distribution according to the items'
+        // "ShareOfWeightSoFar" progressively-calculated values.
+        for (FlexItem& item : Reversed(Items())) {
+          if (numUnfrozenItemsToBeSeen == 0) {
+            break;
+          }
+
+          if (!item.IsFrozen()) {
+            numUnfrozenItemsToBeSeen--;
+
+            // To avoid rounding issues, we compute the change in size for this
+            // item, and then subtract it from the remaining available space.
+            AuCoord64 sizeDelta = 0;
+            if (std::isfinite(weightSum)) {
+              double myShareOfRemainingSpace = item.ShareOfWeightSoFar();
+
+              MOZ_ASSERT(myShareOfRemainingSpace >= 0.0 &&
+                             myShareOfRemainingSpace <= 1.0,
+                         "my share should be nonnegative fractional amount");
+
+              if (myShareOfRemainingSpace == 1.0) {
+                // (We special-case 1.0 to avoid float error from converting
+                // availableFreeSpace from integer*1.0 --> double --> integer)
+                sizeDelta = availableFreeSpace;
+              } else if (myShareOfRemainingSpace > 0.0) {
+                sizeDelta = AuCoord64::FromRound(availableFreeSpace *
+                                                 myShareOfRemainingSpace);
+              }
+            } else if (item.GetWeight(isUsingFlexGrow) == largestWeight) {
+              // Total flexibility is infinite, so we're just distributing
+              // the available space equally among the items that are tied for
+              // having the largest weight (and this is one of those items).
+              sizeDelta = AuCoord64::FromRound(
+                  availableFreeSpace / double(numItemsWithLargestWeight));
+              numItemsWithLargestWeight--;
+            }
+
+            availableFreeSpace -= sizeDelta;
+
+            item.SetMainSize(item.MainSize() +
+                             nscoord(sizeDelta.ToMinMaxClamped()));
+            FLEX_LOG("  flex item %p receives %" PRId64 ", for a total of %d",
+                     item.Frame(), sizeDelta.value, item.MainSize());
+          }
+        }
+
+        MOZ_ASSERT(numUnfrozenItemsToBeSeen == 0, "miscounted frozen items?");
+
+        // If we have an aLineInfo structure to fill out, capture any
+        // size changes that may have occurred in the previous loop.
+        // We don't do this inside the previous loop, because we don't
+        // want to burden layout when aLineInfo is null.
+        if (aLineInfo) {
+          uint32_t itemIndex = 0;
+          for (FlexItem& item : Items()) {
+            if (!item.IsFrozen()) {
+              // Calculate a deltaSize that represents how much the flex sizing
+              // algorithm "wants" to stretch or shrink this item during this
+              // pass through the algorithm. Later passes through the algorithm
+              // may overwrite this, until this item is frozen. Note that this
+              // value may not reflect how much the size of the item is
+              // actually changed, since the size of the item will be clamped
+              // to min and max values later in this pass. That's intentional,
+              // since we want to report the value that the sizing algorithm
+              // tried to stretch or shrink the item.
+              nscoord deltaSize =
+                  item.MainSize() - aLineInfo->mItems[itemIndex].mMainBaseSize;
+
+              aLineInfo->mItems[itemIndex].mMainDeltaSize = deltaSize;
+            }
+            ++itemIndex;
+          }
+        }
+      }
+    }
+
+    // Fix min/max violations:
+    nscoord totalViolation = 0;  // keeps track of adjustments for min/max
+    FLEX_LOG(" Checking for violations:");
+
+    // Since this loop only operates on unfrozen flex items, we can break as
+    // soon as we have seen all of them.
+    uint32_t numUnfrozenItemsToBeSeen = NumItems() - mNumFrozenItems;
+    for (FlexItem& item : Items()) {
+      if (numUnfrozenItemsToBeSeen == 0) {
+        break;
+      }
+
+      if (!item.IsFrozen()) {
+        numUnfrozenItemsToBeSeen--;
+
+        if (item.MainSize() < item.MainMinSize()) {
+          // min violation
+          totalViolation += item.MainMinSize() - item.MainSize();
+          item.SetMainSize(item.MainMinSize());
+          item.SetHadMinViolation();
+        } else if (item.MainSize() > item.MainMaxSize()) {
+          // max violation
+          totalViolation += item.MainMaxSize() - item.MainSize();
+          item.SetMainSize(item.MainMaxSize());
+          item.SetHadMaxViolation();
+        }
+      }
+    }
+
+    MOZ_ASSERT(numUnfrozenItemsToBeSeen == 0, "miscounted frozen items?");
+
+    FreezeOrRestoreEachFlexibleSize(totalViolation,
+                                    iterationCounter + 1 == NumItems());
+
+    FLEX_LOG(" Total violation: %d", totalViolation);
+
+    if (mNumFrozenItems == NumItems()) {
+      break;
+    }
+
+    MOZ_ASSERT(totalViolation != 0,
+               "Zero violation should've made us freeze all items & break");
+  }
+
+#ifdef DEBUG
+  // Post-condition: all items should've been frozen.
+  // Make sure the counts match:
+  MOZ_ASSERT(mNumFrozenItems == NumItems(), "All items should be frozen");
+
+  // For good measure, check each item directly, in case our counts are busted:
+  for (const FlexItem& item : Items()) {
+    MOZ_ASSERT(item.IsFrozen(), "All items should be frozen");
+  }
+#endif  // DEBUG
+}
+
+MainAxisPositionTracker::MainAxisPositionTracker(
+    const FlexboxAxisTracker& aAxisTracker, const FlexLine* aLine,
+    const StyleContentDistribution& aJustifyContent,
+    nscoord aContentBoxMainSize)
+    : PositionTracker(aAxisTracker.GetWritingMode(), aAxisTracker.MainAxis(),
+                      aAxisTracker.IsMainAxisReversed()),
+      // we chip away at this below
+      mPackingSpaceRemaining(aContentBoxMainSize),
+      mJustifyContent(aJustifyContent) {
+  // Extract the flag portion of mJustifyContent and strip off the flag bits
+  // NOTE: This must happen before any assignment to mJustifyContent to
+  // avoid overwriting the flag bits.
+  StyleAlignFlags justifyContentFlags =
+      mJustifyContent.primary & StyleAlignFlags::FLAG_BITS;
+  mJustifyContent.primary &= ~StyleAlignFlags::FLAG_BITS;
+
+  // 'normal' behaves as 'stretch', and 'stretch' behaves as 'flex-start',
+  // in the main axis
+  // https://drafts.csswg.org/css-align-3/#propdef-justify-content
+  if (mJustifyContent.primary == StyleAlignFlags::NORMAL ||
+      mJustifyContent.primary == StyleAlignFlags::STRETCH) {
+    mJustifyContent.primary = StyleAlignFlags::FLEX_START;
+  }
+
+  // mPackingSpaceRemaining is initialized to the container's main size.  Now
+  // we'll subtract out the main sizes of our flex items, so that it ends up
+  // with the *actual* amount of packing space.
+  for (const FlexItem& item : aLine->Items()) {
+    mPackingSpaceRemaining -= item.OuterMainSize();
+    mNumAutoMarginsInMainAxis += item.NumAutoMarginsInMainAxis();
+  }
+
+  // Subtract space required for row/col gap from the remaining packing space
+  mPackingSpaceRemaining -= aLine->SumOfGaps();
+
+  if (mPackingSpaceRemaining <= 0) {
+    // No available packing space to use for resolving auto margins.
+    mNumAutoMarginsInMainAxis = 0;
+    // If packing space is negative and <overflow-position> is set to 'safe'
+    // all justify options fall back to 'start'
+    if (justifyContentFlags & StyleAlignFlags::SAFE) {
+      mJustifyContent.primary = StyleAlignFlags::START;
+    }
+  }
+
+  // If packing space is negative or we only have one item, 'space-between'
+  // falls back to 'flex-start', and 'space-around' & 'space-evenly' fall back
+  // to 'center'. In those cases, it's simplest to just pretend we have a
+  // different 'justify-content' value and share code.
+  if (mPackingSpaceRemaining < 0 || aLine->NumItems() == 1) {
+    if (mJustifyContent.primary == StyleAlignFlags::SPACE_BETWEEN) {
+      mJustifyContent.primary = StyleAlignFlags::FLEX_START;
+    } else if (mJustifyContent.primary == StyleAlignFlags::SPACE_AROUND ||
+               mJustifyContent.primary == StyleAlignFlags::SPACE_EVENLY) {
+      mJustifyContent.primary = StyleAlignFlags::CENTER;
+    }
+  }
+
+  // Map 'left'/'right' to 'start'/'end'
+  if (mJustifyContent.primary == StyleAlignFlags::LEFT ||
+      mJustifyContent.primary == StyleAlignFlags::RIGHT) {
+    mJustifyContent.primary =
+        aAxisTracker.ResolveJustifyLeftRight(mJustifyContent.primary);
+  }
+
+  // Map 'start'/'end' to 'flex-start'/'flex-end'.
+  if (mJustifyContent.primary == StyleAlignFlags::START) {
+    mJustifyContent.primary = aAxisTracker.IsMainAxisReversed()
+                                  ? StyleAlignFlags::FLEX_END
+                                  : StyleAlignFlags::FLEX_START;
+  } else if (mJustifyContent.primary == StyleAlignFlags::END) {
+    mJustifyContent.primary = aAxisTracker.IsMainAxisReversed()
+                                  ? StyleAlignFlags::FLEX_START
+                                  : StyleAlignFlags::FLEX_END;
+  }
+
+  // Figure out how much space we'll set aside for auto margins or
+  // packing spaces, and advance past any leading packing-space.
+  if (mNumAutoMarginsInMainAxis == 0 && mPackingSpaceRemaining != 0 &&
+      !aLine->IsEmpty()) {
+    if (mJustifyContent.primary == StyleAlignFlags::FLEX_START) {
+      // All packing space should go at the end --> nothing to do here.
+    } else if (mJustifyContent.primary == StyleAlignFlags::FLEX_END) {
+      // All packing space goes at the beginning
+      mPosition += mPackingSpaceRemaining;
+    } else if (mJustifyContent.primary == StyleAlignFlags::CENTER) {
+      // Half the packing space goes at the beginning
+      mPosition += mPackingSpaceRemaining / 2;
+    } else if (mJustifyContent.primary == StyleAlignFlags::SPACE_BETWEEN ||
+               mJustifyContent.primary == StyleAlignFlags::SPACE_AROUND ||
+               mJustifyContent.primary == StyleAlignFlags::SPACE_EVENLY) {
+      nsFlexContainerFrame::CalculatePackingSpace(
+          aLine->NumItems(), mJustifyContent, &mPosition,
+          &mNumPackingSpacesRemaining, &mPackingSpaceRemaining);
+    } else {
+      MOZ_ASSERT_UNREACHABLE("Unexpected justify-content value");
+    }
+  }
+
+  MOZ_ASSERT(mNumPackingSpacesRemaining == 0 || mNumAutoMarginsInMainAxis == 0,
+             "extra space should either go to packing space or to "
+             "auto margins, but not to both");
+}
+
+void MainAxisPositionTracker::ResolveAutoMarginsInMainAxis(FlexItem& aItem) {
+  if (mNumAutoMarginsInMainAxis) {
+    const auto& styleMargin = aItem.Frame()->StyleMargin()->mMargin;
+    for (const auto side : {StartSide(), EndSide()}) {
+      if (styleMargin.Get(mWM, side).IsAuto()) {
+        // NOTE: This integer math will skew the distribution of remainder
+        // app-units towards the end, which is fine.
+        nscoord curAutoMarginSize =
+            mPackingSpaceRemaining / mNumAutoMarginsInMainAxis;
+
+        MOZ_ASSERT(aItem.GetMarginComponentForSide(side) == 0,
+                   "Expecting auto margins to have value '0' before we "
+                   "resolve them");
+        aItem.SetMarginComponentForSide(side, curAutoMarginSize);
+
+        mNumAutoMarginsInMainAxis--;
+        mPackingSpaceRemaining -= curAutoMarginSize;
+      }
+    }
+  }
+}
+
+void MainAxisPositionTracker::TraversePackingSpace() {
+  if (mNumPackingSpacesRemaining) {
+    MOZ_ASSERT(mJustifyContent.primary == StyleAlignFlags::SPACE_BETWEEN ||
+                   mJustifyContent.primary == StyleAlignFlags::SPACE_AROUND ||
+                   mJustifyContent.primary == StyleAlignFlags::SPACE_EVENLY,
+               "mNumPackingSpacesRemaining only applies for "
+               "space-between/space-around/space-evenly");
+
+    MOZ_ASSERT(mPackingSpaceRemaining >= 0,
+               "ran out of packing space earlier than we expected");
+
+    // NOTE: This integer math will skew the distribution of remainder
+    // app-units towards the end, which is fine.
+    nscoord curPackingSpace =
+        mPackingSpaceRemaining / mNumPackingSpacesRemaining;
+
+    mPosition += curPackingSpace;
+    mNumPackingSpacesRemaining--;
+    mPackingSpaceRemaining -= curPackingSpace;
+  }
+}
+
+CrossAxisPositionTracker::CrossAxisPositionTracker(
+    nsTArray<FlexLine>& aLines, const ReflowInput& aReflowInput,
+    nscoord aContentBoxCrossSize, bool aIsCrossSizeDefinite,
+    const FlexboxAxisTracker& aAxisTracker, const nscoord aCrossGapSize)
+    : PositionTracker(aAxisTracker.GetWritingMode(), aAxisTracker.CrossAxis(),
+                      aAxisTracker.IsCrossAxisReversed()),
+      mAlignContent(aReflowInput.mStylePosition->mAlignContent),
+      mCrossGapSize(aCrossGapSize) {
+  // Extract and strip the flag bits from alignContent
+  StyleAlignFlags alignContentFlags =
+      mAlignContent.primary & StyleAlignFlags::FLAG_BITS;
+  mAlignContent.primary &= ~StyleAlignFlags::FLAG_BITS;
+
+  // 'normal' behaves as 'stretch'
+  if (mAlignContent.primary == StyleAlignFlags::NORMAL) {
+    mAlignContent.primary = StyleAlignFlags::STRETCH;
+  }
+
+  const bool isSingleLine =
+      StyleFlexWrap::Nowrap == aReflowInput.mStylePosition->mFlexWrap;
+  if (isSingleLine) {
+    MOZ_ASSERT(aLines.Length() == 1,
+               "If we're styled as single-line, we should only have 1 line");
+    // "If the flex container is single-line and has a definite cross size, the
+    // cross size of the flex line is the flex container's inner cross size."
+    //
+    // SOURCE: https://drafts.csswg.org/css-flexbox/#algo-cross-line
+    // NOTE: This means (by definition) that there's no packing space, which
+    // means we don't need to be concerned with "align-content" at all and we
+    // can return early. This is handy, because this is the usual case (for
+    // single-line flexbox).
+    if (aIsCrossSizeDefinite) {
+      aLines[0].SetLineCrossSize(aContentBoxCrossSize);
+      return;
+    }
+
+    // "If the flex container is single-line, then clamp the line's
+    // cross-size to be within the container's computed min and max cross-size
+    // properties."
+    aLines[0].SetLineCrossSize(
+        aReflowInput.ApplyMinMaxBSize(aLines[0].LineCrossSize()));
+  }
+
+  // NOTE: The rest of this function should essentially match
+  // MainAxisPositionTracker's constructor, though with FlexLines instead of
+  // FlexItems, and with the additional value "stretch" (and of course with
+  // cross sizes instead of main sizes.)
+
+  // Figure out how much packing space we have (container's cross size minus
+  // all the lines' cross sizes).  Also, share this loop to count how many
+  // lines we have. (We need that count in some cases below.)
+  mPackingSpaceRemaining = aContentBoxCrossSize;
+  uint32_t numLines = 0;
+  for (FlexLine& line : aLines) {
+    mPackingSpaceRemaining -= line.LineCrossSize();
+    numLines++;
+  }
+
+  // Subtract space required for row/col gap from the remaining packing space
+  MOZ_ASSERT(numLines >= 1,
+             "GenerateFlexLines should've produced at least 1 line");
+  mPackingSpaceRemaining -= aCrossGapSize * (numLines - 1);
+
+  // If <overflow-position> is 'safe' and packing space is negative
+  // all align options fall back to 'start'
+  if ((alignContentFlags & StyleAlignFlags::SAFE) &&
+      mPackingSpaceRemaining < 0) {
+    mAlignContent.primary = StyleAlignFlags::START;
+  }
+
+  // If packing space is negative, 'space-between' and 'stretch' behave like
+  // 'flex-start', and 'space-around' and 'space-evenly' behave like 'center'.
+  // In those cases, it's simplest to just pretend we have a different
+  // 'align-content' value and share code. (If we only have one line, all of
+  // the 'space-*' keywords fall back as well, but 'stretch' doesn't because
+  // even a single line can still stretch.)
+  if (mPackingSpaceRemaining < 0 &&
+      mAlignContent.primary == StyleAlignFlags::STRETCH) {
+    mAlignContent.primary = StyleAlignFlags::FLEX_START;
+  } else if (mPackingSpaceRemaining < 0 || numLines == 1) {
+    if (mAlignContent.primary == StyleAlignFlags::SPACE_BETWEEN) {
+      mAlignContent.primary = StyleAlignFlags::FLEX_START;
+    } else if (mAlignContent.primary == StyleAlignFlags::SPACE_AROUND ||
+               mAlignContent.primary == StyleAlignFlags::SPACE_EVENLY) {
+      mAlignContent.primary = StyleAlignFlags::CENTER;
+    }
+  }
+
+  // Map 'start'/'end' to 'flex-start'/'flex-end'.
+  if (mAlignContent.primary == StyleAlignFlags::START) {
+    mAlignContent.primary = aAxisTracker.IsCrossAxisReversed()
+                                ? StyleAlignFlags::FLEX_END
+                                : StyleAlignFlags::FLEX_START;
+  } else if (mAlignContent.primary == StyleAlignFlags::END) {
+    mAlignContent.primary = aAxisTracker.IsCrossAxisReversed()
+                                ? StyleAlignFlags::FLEX_START
+                                : StyleAlignFlags::FLEX_END;
+  }
+
+  // Figure out how much space we'll set aside for packing spaces, and advance
+  // past any leading packing-space.
+  if (mPackingSpaceRemaining != 0) {
+    if (mAlignContent.primary == StyleAlignFlags::BASELINE ||
+        mAlignContent.primary == StyleAlignFlags::LAST_BASELINE) {
+      // TODO: Bug 1480850 will implement 'align-content: [first/last] baseline'
+      // for flexbox. Until then, behaves as if align-content is 'flex-start' by
+      // doing nothing.
+    } else if (mAlignContent.primary == StyleAlignFlags::FLEX_START) {
+      // All packing space should go at the end --> nothing to do here.
+    } else if (mAlignContent.primary == StyleAlignFlags::FLEX_END) {
+      // All packing space goes at the beginning
+      mPosition += mPackingSpaceRemaining;
+    } else if (mAlignContent.primary == StyleAlignFlags::CENTER) {
+      // Half the packing space goes at the beginning
+      mPosition += mPackingSpaceRemaining / 2;
+    } else if (mAlignContent.primary == StyleAlignFlags::SPACE_BETWEEN ||
+               mAlignContent.primary == StyleAlignFlags::SPACE_AROUND ||
+               mAlignContent.primary == StyleAlignFlags::SPACE_EVENLY) {
+      nsFlexContainerFrame::CalculatePackingSpace(
+          numLines, mAlignContent, &mPosition, &mNumPackingSpacesRemaining,
+          &mPackingSpaceRemaining);
+    } else if (mAlignContent.primary == StyleAlignFlags::STRETCH) {
+      // Split space equally between the lines:
+      MOZ_ASSERT(mPackingSpaceRemaining > 0,
+                 "negative packing space should make us use 'flex-start' "
+                 "instead of 'stretch' (and we shouldn't bother with this "
+                 "code if we have 0 packing space)");
+
+      uint32_t numLinesLeft = numLines;
+      for (FlexLine& line : aLines) {
+        // Our share is the amount of space remaining, divided by the number
+        // of lines remainig.
+        MOZ_ASSERT(numLinesLeft > 0, "miscalculated num lines");
+        nscoord shareOfExtraSpace = mPackingSpaceRemaining / numLinesLeft;
+        nscoord newSize = line.LineCrossSize() + shareOfExtraSpace;
+        line.SetLineCrossSize(newSize);
+
+        mPackingSpaceRemaining -= shareOfExtraSpace;
+        numLinesLeft--;
+      }
+      MOZ_ASSERT(numLinesLeft == 0, "miscalculated num lines");
+    } else {
+      MOZ_ASSERT_UNREACHABLE("Unexpected align-content value");
+    }
+  }
+}
+
+void CrossAxisPositionTracker::TraversePackingSpace() {
+  if (mNumPackingSpacesRemaining) {
+    MOZ_ASSERT(mAlignContent.primary == StyleAlignFlags::SPACE_BETWEEN ||
+                   mAlignContent.primary == StyleAlignFlags::SPACE_AROUND ||
+                   mAlignContent.primary == StyleAlignFlags::SPACE_EVENLY,
+               "mNumPackingSpacesRemaining only applies for "
+               "space-between/space-around/space-evenly");
+
+    MOZ_ASSERT(mPackingSpaceRemaining >= 0,
+               "ran out of packing space earlier than we expected");
+
+    // NOTE: This integer math will skew the distribution of remainder
+    // app-units towards the end, which is fine.
+    nscoord curPackingSpace =
+        mPackingSpaceRemaining / mNumPackingSpacesRemaining;
+
+    mPosition += curPackingSpace;
+    mNumPackingSpacesRemaining--;
+    mPackingSpaceRemaining -= curPackingSpace;
+  }
+}
+
+SingleLineCrossAxisPositionTracker::SingleLineCrossAxisPositionTracker(
+    const FlexboxAxisTracker& aAxisTracker)
+    : PositionTracker(aAxisTracker.GetWritingMode(), aAxisTracker.CrossAxis(),
+                      aAxisTracker.IsCrossAxisReversed()) {}
+
+void FlexLine::ComputeCrossSizeAndBaseline(
+    const FlexboxAxisTracker& aAxisTracker) {
+  // NOTE: in these "cross{Start,End}ToFurthest{First,Last}Baseline" variables,
+  // the "first/last" term is referring to the flex *line's* baseline-sharing
+  // groups, which may or may not match any flex *item's* exact align-self
+  // value. See the code that sets FlexItem::mBaselineSharingGroup for more
+  // details.
+  nscoord crossStartToFurthestFirstBaseline = nscoord_MIN;
+  nscoord crossEndToFurthestFirstBaseline = nscoord_MIN;
+  nscoord crossStartToFurthestLastBaseline = nscoord_MIN;
+  nscoord crossEndToFurthestLastBaseline = nscoord_MIN;
+
+  nscoord largestOuterCrossSize = 0;
+  for (const FlexItem& item : Items()) {
+    nscoord curOuterCrossSize = item.OuterCrossSize();
+
+    if ((item.AlignSelf()._0 == StyleAlignFlags::BASELINE ||
+         item.AlignSelf()._0 == StyleAlignFlags::LAST_BASELINE) &&
+        item.NumAutoMarginsInCrossAxis() == 0) {
+      const bool usingItemFirstBaseline =
+          (item.AlignSelf()._0 == StyleAlignFlags::BASELINE);
+
+      // Find distance from our item's cross-start and cross-end margin-box
+      // edges to its baseline.
+      //
+      // Here's a diagram of a flex-item that we might be doing this on.
+      // "mmm" is the margin-box, "bbb" is the border-box. The bottom of
+      // the text "BASE" is the baseline.
+      //
+      // ---(cross-start)---
+      //                ___              ___            ___
+      //   mmmmmmmmmmmm  |                |margin-start  |
+      //   m          m  |               _|_   ___       |
+      //   m bbbbbbbb m  |curOuterCrossSize     |        |crossStartToBaseline
+      //   m b      b m  |                      |ascent  |
+      //   m b BASE b m  |                     _|_      _|_
+      //   m b      b m  |                               |
+      //   m bbbbbbbb m  |                               |crossEndToBaseline
+      //   m          m  |                               |
+      //   mmmmmmmmmmmm _|_                             _|_
+      //
+      // ---(cross-end)---
+      //
+      // We already have the curOuterCrossSize, margin-start, and the ascent.
+      // * We can get crossStartToBaseline by adding margin-start + ascent.
+      // * If we subtract that from the curOuterCrossSize, we get
+      //   crossEndToBaseline.
+
+      nscoord crossStartToBaseline = item.BaselineOffsetFromOuterCrossEdge(
+          aAxisTracker.CrossAxisPhysicalStartSide(), usingItemFirstBaseline);
+      nscoord crossEndToBaseline = curOuterCrossSize - crossStartToBaseline;
+
+      // Now, update our "largest" values for these (across all the flex items
+      // in this flex line), so we can use them in computing the line's cross
+      // size below:
+      if (item.ItemBaselineSharingGroup() == BaselineSharingGroup::First) {
+        crossStartToFurthestFirstBaseline =
+            std::max(crossStartToFurthestFirstBaseline, crossStartToBaseline);
+        crossEndToFurthestFirstBaseline =
+            std::max(crossEndToFurthestFirstBaseline, crossEndToBaseline);
+      } else {
+        crossStartToFurthestLastBaseline =
+            std::max(crossStartToFurthestLastBaseline, crossStartToBaseline);
+        crossEndToFurthestLastBaseline =
+            std::max(crossEndToFurthestLastBaseline, crossEndToBaseline);
+      }
+    } else {
+      largestOuterCrossSize =
+          std::max(largestOuterCrossSize, curOuterCrossSize);
+    }
+  }
+
+  // The line's baseline offset is the distance from the line's edge to the
+  // furthest item-baseline. The item(s) with that baseline will be exactly
+  // aligned with the line's edge.
+  mFirstBaselineOffset = crossStartToFurthestFirstBaseline;
+  mLastBaselineOffset = crossEndToFurthestLastBaseline;
+
+  // The line's cross-size is the larger of:
+  //  (a) [largest cross-start-to-baseline + largest baseline-to-cross-end] of
+  //      all baseline-aligned items with no cross-axis auto margins...
+  // and
+  //  (b) [largest cross-start-to-baseline + largest baseline-to-cross-end] of
+  //      all last baseline-aligned items with no cross-axis auto margins...
+  // and
+  //  (c) largest cross-size of all other children.
+  mLineCrossSize = std::max(
+      std::max(
+          crossStartToFurthestFirstBaseline + crossEndToFurthestFirstBaseline,
+          crossStartToFurthestLastBaseline + crossEndToFurthestLastBaseline),
+      largestOuterCrossSize);
+}
+
+nscoord FlexLine::ExtractBaselineOffset(
+    BaselineSharingGroup aBaselineGroup) const {
+  auto LastBaselineOffsetFromStartEdge = [this]() {
+    // Convert the distance to be relative from the line's cross-start edge.
+    const nscoord offset = LastBaselineOffset();
+    return offset != nscoord_MIN ? LineCrossSize() - offset : offset;
+  };
+
+  auto PrimaryBaseline = [=]() {
+    return aBaselineGroup == BaselineSharingGroup::First
+               ? FirstBaselineOffset()
+               : LastBaselineOffsetFromStartEdge();
+  };
+  auto SecondaryBaseline = [=]() {
+    return aBaselineGroup == BaselineSharingGroup::First
+               ? LastBaselineOffsetFromStartEdge()
+               : FirstBaselineOffset();
+  };
+
+  const nscoord primaryBaseline = PrimaryBaseline();
+  if (primaryBaseline != nscoord_MIN) {
+    return primaryBaseline;
+  }
+  return SecondaryBaseline();
+}
+
+void FlexItem::ResolveStretchedCrossSize(nscoord aLineCrossSize) {
+  // We stretch IFF we are align-self:stretch, have no auto margins in
+  // cross axis, and have cross-axis size property == "auto". If any of those
+  // conditions don't hold up, we won't stretch.
+  if (mAlignSelf._0 != StyleAlignFlags::STRETCH ||
+      NumAutoMarginsInCrossAxis() != 0 || !IsCrossSizeAuto()) {
+    return;
+  }
+
+  // If we've already been stretched, we can bail out early, too.
+  // No need to redo the calculation.
+  if (mIsStretched) {
+    return;
+  }
+
+  // Reserve space for margins & border & padding, and then use whatever
+  // remains as our item's cross-size (clamped to its min/max range).
+  nscoord stretchedSize = aLineCrossSize - MarginBorderPaddingSizeInCrossAxis();
+
+  stretchedSize = NS_CSS_MINMAX(stretchedSize, mCrossMinSize, mCrossMaxSize);
+
+  // Update the cross-size & make a note that it's stretched, so we know to
+  // override the reflow input's computed cross-size in our final reflow.
+  SetCrossSize(stretchedSize);
+  mIsStretched = true;
+}
+
+static nsBlockFrame* FindFlexItemBlockFrame(nsIFrame* aFrame) {
+  if (nsBlockFrame* block = do_QueryFrame(aFrame)) {
+    return block;
+  }
+  for (nsIFrame* f : aFrame->PrincipalChildList()) {
+    if (nsBlockFrame* block = FindFlexItemBlockFrame(f)) {
+      return block;
+    }
+  }
+  return nullptr;
+}
+
+nsBlockFrame* FlexItem::BlockFrame() const {
+  return FindFlexItemBlockFrame(Frame());
+}
+
+void SingleLineCrossAxisPositionTracker::ResolveAutoMarginsInCrossAxis(
+    const FlexLine& aLine, FlexItem& aItem) {
+  // Subtract the space that our item is already occupying, to see how much
+  // space (if any) is available for its auto margins.
+  nscoord spaceForAutoMargins = aLine.LineCrossSize() - aItem.OuterCrossSize();
+
+  if (spaceForAutoMargins <= 0) {
+    return;  // No available space  --> nothing to do
+  }
+
+  uint32_t numAutoMargins = aItem.NumAutoMarginsInCrossAxis();
+  if (numAutoMargins == 0) {
+    return;  // No auto margins --> nothing to do.
+  }
+
+  // OK, we have at least one auto margin and we have some available space.
+  // Give each auto margin a share of the space.
+  const auto& styleMargin = aItem.Frame()->StyleMargin()->mMargin;
+  for (const auto side : {StartSide(), EndSide()}) {
+    if (styleMargin.Get(mWM, side).IsAuto()) {
+      MOZ_ASSERT(aItem.GetMarginComponentForSide(side) == 0,
+                 "Expecting auto margins to have value '0' before we "
+                 "update them");
+
+      // NOTE: integer divison is fine here; numAutoMargins is either 1 or 2.
+      // If it's 2 & spaceForAutoMargins is odd, 1st margin gets smaller half.
+      nscoord curAutoMarginSize = spaceForAutoMargins / numAutoMargins;
+      aItem.SetMarginComponentForSide(side, curAutoMarginSize);
+      numAutoMargins--;
+      spaceForAutoMargins -= curAutoMarginSize;
+    }
+  }
+}
+
+void SingleLineCrossAxisPositionTracker::EnterAlignPackingSpace(
+    const FlexLine& aLine, const FlexItem& aItem,
+    const FlexboxAxisTracker& aAxisTracker) {
+  // We don't do align-self alignment on items that have auto margins
+  // in the cross axis.
+  if (aItem.NumAutoMarginsInCrossAxis()) {
+    return;
+  }
+
+  StyleAlignFlags alignSelf = aItem.AlignSelf()._0;
+  // NOTE: 'stretch' behaves like 'flex-start' once we've stretched any
+  // auto-sized items (which we've already done).
+  if (alignSelf == StyleAlignFlags::STRETCH) {
+    alignSelf = StyleAlignFlags::FLEX_START;
+  }
+
+  // Map 'self-start'/'self-end' to 'start'/'end'
+  if (alignSelf == StyleAlignFlags::SELF_START ||
+      alignSelf == StyleAlignFlags::SELF_END) {
+    const LogicalAxis logCrossAxis =
+        aAxisTracker.IsRowOriented() ? eLogicalAxisBlock : eLogicalAxisInline;
+    const WritingMode cWM = aAxisTracker.GetWritingMode();
+    const bool sameStart =
+        cWM.ParallelAxisStartsOnSameSide(logCrossAxis, aItem.GetWritingMode());
+    alignSelf = sameStart == (alignSelf == StyleAlignFlags::SELF_START)
+                    ? StyleAlignFlags::START
+                    : StyleAlignFlags::END;
+  }
+
+  // Map 'start'/'end' to 'flex-start'/'flex-end'.
+  if (alignSelf == StyleAlignFlags::START) {
+    alignSelf = aAxisTracker.IsCrossAxisReversed()
+                    ? StyleAlignFlags::FLEX_END
+                    : StyleAlignFlags::FLEX_START;
+  } else if (alignSelf == StyleAlignFlags::END) {
+    alignSelf = aAxisTracker.IsCrossAxisReversed() ? StyleAlignFlags::FLEX_START
+                                                   : StyleAlignFlags::FLEX_END;
+  }
+
+  // 'align-self' falls back to 'flex-start' if it is 'center'/'flex-end' and we
+  // have cross axis overflow
+  // XXX we should really be falling back to 'start' as of bug 1472843
+  if (aLine.LineCrossSize() < aItem.OuterCrossSize() &&
+      (aItem.AlignSelfFlags() & StyleAlignFlags::SAFE)) {
+    alignSelf = StyleAlignFlags::FLEX_START;
+  }
+
+  if (alignSelf == StyleAlignFlags::FLEX_START) {
+    // No space to skip over -- we're done.
+  } else if (alignSelf == StyleAlignFlags::FLEX_END) {
+    mPosition += aLine.LineCrossSize() - aItem.OuterCrossSize();
+  } else if (alignSelf == StyleAlignFlags::CENTER) {
+    // Note: If cross-size is odd, the "after" space will get the extra unit.
+    mPosition += (aLine.LineCrossSize() - aItem.OuterCrossSize()) / 2;
+  } else if (alignSelf == StyleAlignFlags::BASELINE ||
+             alignSelf == StyleAlignFlags::LAST_BASELINE) {
+    const bool usingItemFirstBaseline =
+        (alignSelf == StyleAlignFlags::BASELINE);
+
+    // The first-baseline sharing group gets (collectively) aligned to the
+    // FlexLine's cross-start side, and similarly the last-baseline sharing
+    // group gets snapped to the cross-end side.
+    const bool isFirstBaselineSharingGroup =
+        aItem.ItemBaselineSharingGroup() == BaselineSharingGroup::First;
+    const mozilla::Side alignSide =
+        isFirstBaselineSharingGroup ? aAxisTracker.CrossAxisPhysicalStartSide()
+                                    : aAxisTracker.CrossAxisPhysicalEndSide();
+
+    // To compute the aligned position for our flex item, we determine:
+    // (1) The distance from the item's alignSide edge to the item's relevant
+    //     baseline.
+    nscoord itemBaselineOffset = aItem.BaselineOffsetFromOuterCrossEdge(
+        alignSide, usingItemFirstBaseline);
+
+    // (2) The distance between the FlexLine's alignSide edge and the relevant
+    //     baseline-sharing-group's baseline position.
+    nscoord lineBaselineOffset = isFirstBaselineSharingGroup
+                                     ? aLine.FirstBaselineOffset()
+                                     : aLine.LastBaselineOffset();
+
+    NS_ASSERTION(lineBaselineOffset >= itemBaselineOffset,
+                 "failed at finding largest baseline offset");
+
+    // (3) The difference between the above offsets, which tells us how far we
+    //     need to shift the item away from the FlexLine's alignSide edge so
+    //     that its baseline is at the proper position for its group.
+    nscoord itemOffsetFromLineEdge = lineBaselineOffset - itemBaselineOffset;
+
+    if (isFirstBaselineSharingGroup) {
+      // alignSide is the line's cross-start edge. mPosition is already there.
+      // From there, we step *forward* by the baseline adjustment:
+      mPosition += itemOffsetFromLineEdge;
+    } else {
+      // alignSide is the line's cross-end edge. Advance mPosition to align
+      // item with that edge (as in FLEX_END case)...
+      mPosition += aLine.LineCrossSize() - aItem.OuterCrossSize();
+      // ...and step *back* by the baseline adjustment:
+      mPosition -= itemOffsetFromLineEdge;
+    }
+  } else {
+    MOZ_ASSERT_UNREACHABLE("Unexpected align-self value");
+  }
+}
+
+FlexboxAxisInfo::FlexboxAxisInfo(const nsIFrame* aFlexContainer) {
+  MOZ_ASSERT(aFlexContainer && aFlexContainer->IsFlexContainerFrame(),
+             "Only flex containers may be passed to this constructor!");
+  if (IsLegacyBox(aFlexContainer)) {
+    InitAxesFromLegacyProps(aFlexContainer);
+  } else {
+    InitAxesFromModernProps(aFlexContainer);
+  }
+}
+
+void FlexboxAxisInfo::InitAxesFromLegacyProps(const nsIFrame* aFlexContainer) {
+  const nsStyleXUL* styleXUL = aFlexContainer->StyleXUL();
+
+  const bool boxOrientIsVertical =
+      styleXUL->mBoxOrient == StyleBoxOrient::Vertical;
+  const bool wmIsVertical = aFlexContainer->GetWritingMode().IsVertical();
+
+  // If box-orient agrees with our writing-mode, then we're "row-oriented"
+  // (i.e. the flexbox main axis is the same as our writing mode's inline
+  // direction).  Otherwise, we're column-oriented (i.e. the flexbox's main
+  // axis is perpendicular to the writing-mode's inline direction).
+  mIsRowOriented = (boxOrientIsVertical == wmIsVertical);
+
+  // Legacy flexbox can use "-webkit-box-direction: reverse" to reverse the
+  // main axis (so it runs in the reverse direction of the inline axis):
+  mIsMainAxisReversed = styleXUL->mBoxDirection == StyleBoxDirection::Reverse;
+
+  // Legacy flexbox does not support reversing the cross axis -- it has no
+  // equivalent of modern flexbox's "flex-wrap: wrap-reverse".
+  mIsCrossAxisReversed = false;
+}
+
+void FlexboxAxisInfo::InitAxesFromModernProps(const nsIFrame* aFlexContainer) {
+  const nsStylePosition* stylePos = aFlexContainer->StylePosition();
+  StyleFlexDirection flexDirection = stylePos->mFlexDirection;
+
+  // Determine main axis:
+  switch (flexDirection) {
+    case StyleFlexDirection::Row:
+      mIsRowOriented = true;
+      mIsMainAxisReversed = false;
+      break;
+    case StyleFlexDirection::RowReverse:
+      mIsRowOriented = true;
+      mIsMainAxisReversed = true;
+      break;
+    case StyleFlexDirection::Column:
+      mIsRowOriented = false;
+      mIsMainAxisReversed = false;
+      break;
+    case StyleFlexDirection::ColumnReverse:
+      mIsRowOriented = false;
+      mIsMainAxisReversed = true;
+      break;
+  }
+
+  // "flex-wrap: wrap-reverse" reverses our cross axis.
+  mIsCrossAxisReversed = stylePos->mFlexWrap == StyleFlexWrap::WrapReverse;
+}
+
+FlexboxAxisTracker::FlexboxAxisTracker(
+    const nsFlexContainerFrame* aFlexContainer)
+    : mWM(aFlexContainer->GetWritingMode()), mAxisInfo(aFlexContainer) {}
+
+LogicalSide FlexboxAxisTracker::MainAxisStartSide() const {
+  return MakeLogicalSide(
+      MainAxis(), IsMainAxisReversed() ? eLogicalEdgeEnd : eLogicalEdgeStart);
+}
+
+LogicalSide FlexboxAxisTracker::CrossAxisStartSide() const {
+  return MakeLogicalSide(
+      CrossAxis(), IsCrossAxisReversed() ? eLogicalEdgeEnd : eLogicalEdgeStart);
+}
+
+void nsFlexContainerFrame::GenerateFlexLines(
+    const ReflowInput& aReflowInput, const nscoord aTentativeContentBoxMainSize,
+    const nscoord aTentativeContentBoxCrossSize,
+    const nsTArray<StrutInfo>& aStruts, const FlexboxAxisTracker& aAxisTracker,
+    nscoord aMainGapSize, nsTArray<nsIFrame*>& aPlaceholders,
+    nsTArray<FlexLine>& aLines, bool& aHasCollapsedItems) {
+  MOZ_ASSERT(aLines.IsEmpty(), "Expecting outparam to start out empty");
+
+  auto ConstructNewFlexLine = [&aLines, aMainGapSize]() {
+    return aLines.EmplaceBack(aMainGapSize);
+  };
+
+  const bool isSingleLine =
+      StyleFlexWrap::Nowrap == aReflowInput.mStylePosition->mFlexWrap;
+
+  // We have at least one FlexLine. Even an empty flex container has a single
+  // (empty) flex line.
+  FlexLine* curLine = ConstructNewFlexLine();
+
+  nscoord wrapThreshold;
+  if (isSingleLine) {
+    // Not wrapping. Set threshold to sentinel value that tells us not to wrap.
+    wrapThreshold = NS_UNCONSTRAINEDSIZE;
+  } else {
+    // Wrapping! Set wrap threshold to flex container's content-box main-size.
+    wrapThreshold = aTentativeContentBoxMainSize;
+
+    // If the flex container doesn't have a definite content-box main-size
+    // (e.g. if main axis is vertical & 'height' is 'auto'), make sure we at
+    // least wrap when we hit its max main-size.
+    if (wrapThreshold == NS_UNCONSTRAINEDSIZE) {
+      const nscoord flexContainerMaxMainSize =
+          aAxisTracker.MainComponent(aReflowInput.ComputedMaxSize());
+      wrapThreshold = flexContainerMaxMainSize;
+    }
+  }
+
+  // Tracks the index of the next strut, in aStruts (and when this hits
+  // aStruts.Length(), that means there are no more struts):
+  uint32_t nextStrutIdx = 0;
+
+  // Overall index of the current flex item in the flex container. (This gets
+  // checked against entries in aStruts.)
+  uint32_t itemIdxInContainer = 0;
+
+  CSSOrderAwareFrameIterator iter(
+      this, FrameChildListID::Principal,
+      CSSOrderAwareFrameIterator::ChildFilter::IncludeAll,
+      CSSOrderAwareFrameIterator::OrderState::Unknown,
+      OrderingPropertyForIter(this));
+
+  AddOrRemoveStateBits(NS_STATE_FLEX_NORMAL_FLOW_CHILDREN_IN_CSS_ORDER,
+                       iter.ItemsAreAlreadyInOrder());
+
+  const bool useMozBoxCollapseBehavior =
+      StyleVisibility()->UseLegacyCollapseBehavior();
+
+  for (; !iter.AtEnd(); iter.Next()) {
+    nsIFrame* childFrame = *iter;
+    // Don't create flex items / lines for placeholder frames:
+    if (childFrame->IsPlaceholderFrame()) {
+      aPlaceholders.AppendElement(childFrame);
+      continue;
+    }
+
+    const bool collapsed = childFrame->StyleVisibility()->IsCollapse();
+    aHasCollapsedItems = aHasCollapsedItems || collapsed;
+
+    if (useMozBoxCollapseBehavior && collapsed) {
+      // Legacy visibility:collapse behavior: make a 0-sized strut. (No need to
+      // bother with aStruts and remembering cross size.)
+      curLine->Items().EmplaceBack(childFrame, 0, aReflowInput.GetWritingMode(),
+                                   aAxisTracker);
+    } else if (nextStrutIdx < aStruts.Length() &&
+               aStruts[nextStrutIdx].mItemIdx == itemIdxInContainer) {
+      // Use the simplified "strut" FlexItem constructor:
+      curLine->Items().EmplaceBack(childFrame,
+                                   aStruts[nextStrutIdx].mStrutCrossSize,
+                                   aReflowInput.GetWritingMode(), aAxisTracker);
+      nextStrutIdx++;
+    } else {
+      GenerateFlexItemForChild(*curLine, childFrame, aReflowInput, aAxisTracker,
+                               aTentativeContentBoxCrossSize);
+    }
+
+    // Check if we need to wrap the newly appended item to a new line, i.e. if
+    // its outer hypothetical main size pushes our line over the threshold.
+    // But we don't wrap if the line-length is unconstrained, nor do we wrap if
+    // this was the first item on the line.
+    if (wrapThreshold != NS_UNCONSTRAINEDSIZE &&
+        curLine->Items().Length() > 1) {
+      // If the line will be longer than wrapThreshold or at least as long as
+      // nscoord_MAX because of the newly appended item, then wrap and move the
+      // item to a new line.
+      auto newOuterSize = curLine->TotalOuterHypotheticalMainSize();
+      newOuterSize += curLine->Items().LastElement().OuterMainSize();
+
+      // Account for gap between this line's previous item and this item.
+      newOuterSize += aMainGapSize;
+
+      if (newOuterSize >= nscoord_MAX || newOuterSize > wrapThreshold) {
+        curLine = ConstructNewFlexLine();
+
+        // Get the previous line after adding a new line because the address can
+        // change if nsTArray needs to reallocate a new space for the new line.
+        FlexLine& prevLine = aLines[aLines.Length() - 2];
+
+        // Move the item from the end of prevLine to the end of curLine.
+        curLine->Items().AppendElement(prevLine.Items().PopLastElement());
+      }
+    }
+
+    // Update the line's bookkeeping about how large its items collectively are.
+    curLine->AddLastItemToMainSizeTotals();
+    itemIdxInContainer++;
+  }
+}
+
+nsFlexContainerFrame::FlexLayoutResult
+nsFlexContainerFrame::GenerateFlexLayoutResult() {
+  MOZ_ASSERT(GetPrevInFlow(), "This should be called by non-first-in-flows!");
+
+  auto* data = FirstInFlow()->GetProperty(SharedFlexData::Prop());
+  MOZ_ASSERT(data, "SharedFlexData should be set by our first-in-flow!");
+
+  FlexLayoutResult flr;
+
+  // The order state of the children is consistent across entire continuation
+  // chain due to calling nsContainerFrame::NormalizeChildLists() at the
+  // beginning of Reflow(), so we can align our state bit with our
+  // prev-in-flow's state. Setup here before calling OrderStateForIter() below.
+  AddOrRemoveStateBits(NS_STATE_FLEX_NORMAL_FLOW_CHILDREN_IN_CSS_ORDER,
+                       GetPrevInFlow()->HasAnyStateBits(
+                           NS_STATE_FLEX_NORMAL_FLOW_CHILDREN_IN_CSS_ORDER));
+
+  // Construct flex items for this flex container fragment from existing flex
+  // items in SharedFlexData.
+  CSSOrderAwareFrameIterator iter(
+      this, FrameChildListID::Principal,
+      CSSOrderAwareFrameIterator::ChildFilter::SkipPlaceholders,
+      OrderStateForIter(this), OrderingPropertyForIter(this));
+
+  auto ConstructNewFlexLine = [&flr]() {
+    // Use zero main gap size since it doesn't matter in flex container's
+    // next-in-flows. We've computed flex items' positions in first-in-flow.
+    return flr.mLines.EmplaceBack(0);
+  };
+
+  // We have at least one FlexLine. Even an empty flex container has a single
+  // (empty) flex line.
+  FlexLine* currentLine = ConstructNewFlexLine();
+
+  if (!iter.AtEnd()) {
+    nsIFrame* child = *iter;
+    nsIFrame* childFirstInFlow = child->FirstInFlow();
+
+    // We are iterating nested for-loops over the FlexLines and FlexItems
+    // generated by GenerateFlexLines() and cached in flex container's
+    // first-in-flow. For each flex item, check if its frame (must be a
+    // first-in-flow) is the first-in-flow of the first child frame in this flex
+    // container continuation. If so, clone the data from that FlexItem into a
+    // FlexLine. When we find a match for the item, we know that the next child
+    // frame might have its first-in-flow as the next item in the same original
+    // line. In this case, we'll put the cloned data in the same line here as
+    // well.
+    for (const FlexLine& line : data->mLines) {
+      // If currentLine is empty, either it is the first line, or all the items
+      // in the previous line have been placed in our prev-in-flows. No need to
+      // construct a new line.
+      if (!currentLine->IsEmpty()) {
+        currentLine = ConstructNewFlexLine();
+      }
+      for (const FlexItem& item : line.Items()) {
+        if (item.Frame() == childFirstInFlow) {
+          currentLine->Items().AppendElement(item.CloneFor(child));
+          iter.Next();
+          if (iter.AtEnd()) {
+            // We've constructed flex items for all children. No need to check
+            // rest of the items.
+            child = childFirstInFlow = nullptr;
+            break;
+          }
+          child = *iter;
+          childFirstInFlow = child->FirstInFlow();
+        }
+      }
+      if (iter.AtEnd()) {
+        // We've constructed flex items for all children. No need to check
+        // rest of the lines.
+        break;
+      }
+    }
+  }
+
+  flr.mContentBoxMainSize = data->mContentBoxMainSize;
+  flr.mContentBoxCrossSize = data->mContentBoxCrossSize;
+
+  return flr;
+}
+
+// Returns the largest outer hypothetical main-size of any line in |aLines|.
+// (i.e. the hypothetical main-size of the largest line)
+static AuCoord64 GetLargestLineMainSize(nsTArray<FlexLine>& aLines) {
+  AuCoord64 largestLineOuterSize = 0;
+  for (const FlexLine& line : aLines) {
+    largestLineOuterSize =
+        std::max(largestLineOuterSize, line.TotalOuterHypotheticalMainSize());
+  }
+  return largestLineOuterSize;
+}
+
+nscoord nsFlexContainerFrame::ComputeMainSize(
+    const ReflowInput& aReflowInput, const FlexboxAxisTracker& aAxisTracker,
+    const nscoord aTentativeContentBoxMainSize,
+    nsTArray<FlexLine>& aLines) const {
+  if (aAxisTracker.IsRowOriented()) {
+    // Row-oriented --> our main axis is the inline axis, so our main size
+    // is our inline size (which should already be resolved).
+    return aTentativeContentBoxMainSize;
+  }
+
+  const bool shouldApplyAutomaticMinimumOnBlockAxis =
+      aReflowInput.ShouldApplyAutomaticMinimumOnBlockAxis();
+  if (aTentativeContentBoxMainSize != NS_UNCONSTRAINEDSIZE &&
+      !shouldApplyAutomaticMinimumOnBlockAxis) {
+    // Column-oriented case, with fixed BSize:
+    // Just use our fixed block-size because we always assume the available
+    // block-size is unconstrained, and the reflow input has already done the
+    // appropriate min/max-BSize clamping.
+    return aTentativeContentBoxMainSize;
+  }
+
+  // Column-oriented case, with size-containment in block axis:
+  // Behave as if we had no content and just use our MinBSize.
+  if (Maybe<nscoord> containBSize =
+          aReflowInput.mFrame->ContainIntrinsicBSize()) {
+    return aReflowInput.ApplyMinMaxBSize(*containBSize);
+  }
+
+  const AuCoord64 largestLineMainSize = GetLargestLineMainSize(aLines);
+  const nscoord contentBSize = aReflowInput.ApplyMinMaxBSize(
+      nscoord(largestLineMainSize.ToMinMaxClamped()));
+
+  // If the clamped largest FlexLine length is larger than the tentative main
+  // size (which is resolved by aspect-ratio), we extend it to contain the
+  // entire FlexLine.
+  // https://drafts.csswg.org/css-sizing-4/#aspect-ratio-minimum
+  if (shouldApplyAutomaticMinimumOnBlockAxis) {
+    // Column-oriented case, with auto BSize which is resolved by
+    // aspect-ratio.
+    return std::max(contentBSize, aTentativeContentBoxMainSize);
+  }
+
+  // Column-oriented case, with auto BSize:
+  // Resolve auto BSize to the largest FlexLine length, clamped to our
+  // computed min/max main-size properties.
+  return contentBSize;
+}
+
+nscoord nsFlexContainerFrame::ComputeCrossSize(
+    const ReflowInput& aReflowInput, const FlexboxAxisTracker& aAxisTracker,
+    const nscoord aTentativeContentBoxCrossSize, nscoord aSumLineCrossSizes,
+    bool* aIsDefinite) const {
+  MOZ_ASSERT(aIsDefinite, "outparam pointer must be non-null");
+
+  if (aAxisTracker.IsColumnOriented()) {
+    // Column-oriented --> our cross axis is the inline axis, so our cross size
+    // is our inline size (which should already be resolved).
+    *aIsDefinite = true;
+    // FIXME: Bug 1661847 - there are cases where aTentativeContentBoxCrossSize
+    // (i.e. aReflowInput.ComputedISize()) might not be the right thing to
+    // return here. Specifically: if our cross size is an intrinsic size, and we
+    // have flex items that are flexible and have aspect ratios, then we may
+    // need to take their post-flexing main sizes into account (multiplied
+    // through their aspect ratios to get their cross sizes), in order to
+    // determine their flex line's size & the flex container's cross size (e.g.
+    // as `aSumLineCrossSizes`).
+    return aTentativeContentBoxCrossSize;
+  }
+
+  const bool shouldApplyAutomaticMinimumOnBlockAxis =
+      aReflowInput.ShouldApplyAutomaticMinimumOnBlockAxis();
+  const nscoord computedBSize = aReflowInput.ComputedBSize();
+  if (computedBSize != NS_UNCONSTRAINEDSIZE &&
+      !shouldApplyAutomaticMinimumOnBlockAxis) {
+    // Row-oriented case (cross axis is block-axis), with fixed BSize:
+    *aIsDefinite = true;
+
+    // Just use our fixed block-size because we always assume the available
+    // block-size is unconstrained, and the reflow input has already done the
+    // appropriate min/max-BSize clamping.
+    return computedBSize;
+  }
+
+  // Row-oriented case, with size-containment in block axis:
+  // Behave as if we had no content and just use our MinBSize.
+  if (Maybe<nscoord> containBSize =
+          aReflowInput.mFrame->ContainIntrinsicBSize()) {
+    *aIsDefinite = true;
+    return aReflowInput.ApplyMinMaxBSize(*containBSize);
+  }
+
+  // The cross size must not be definite in the following cases.
+  *aIsDefinite = false;
+
+  const nscoord contentBSize =
+      aReflowInput.ApplyMinMaxBSize(aSumLineCrossSizes);
+  // If the content block-size is larger than the effective computed
+  // block-size, we extend the block-size to contain all the content.
+  // https://drafts.csswg.org/css-sizing-4/#aspect-ratio-minimum
+  if (shouldApplyAutomaticMinimumOnBlockAxis) {
+    // Row-oriented case (cross axis is block-axis), with auto BSize which is
+    // resolved by aspect-ratio or content size.
+    return std::max(contentBSize, computedBSize);
+  }
+
+  // Row-oriented case (cross axis is block axis), with auto BSize:
+  // Shrink-wrap our line(s), subject to our min-size / max-size
+  // constraints in that (block) axis.
+  return contentBSize;
+}
+
+LogicalSize nsFlexContainerFrame::ComputeAvailableSizeForItems(
+    const ReflowInput& aReflowInput,
+    const mozilla::LogicalMargin& aBorderPadding) const {
+  const WritingMode wm = GetWritingMode();
+  nscoord availableBSize = aReflowInput.AvailableBSize();
+
+  if (availableBSize != NS_UNCONSTRAINEDSIZE) {
+    // Available block-size is constrained. Subtract block-start border and
+    // padding from it.
+    availableBSize -= aBorderPadding.BStart(wm);
+
+    if (aReflowInput.mStyleBorder->mBoxDecorationBreak ==
+        StyleBoxDecorationBreak::Clone) {
+      // We have box-decoration-break:clone. Subtract block-end border and
+      // padding from the available block-size as well.
+      availableBSize -= aBorderPadding.BEnd(wm);
+    }
+
+    // Available block-size can became negative after subtracting block-axis
+    // border and padding. Per spec, to guarantee progress, fragmentainers are
+    // assumed to have a minimum block size of 1px regardless of their used
+    // size. https://drafts.csswg.org/css-break/#breaking-rules
+    availableBSize =
+        std::max(nsPresContext::CSSPixelsToAppUnits(1), availableBSize);
+  }
+
+  return LogicalSize(wm, aReflowInput.ComputedISize(), availableBSize);
+}
+
+void FlexLine::PositionItemsInMainAxis(
+    const StyleContentDistribution& aJustifyContent,
+    nscoord aContentBoxMainSize, const FlexboxAxisTracker& aAxisTracker) {
+  MainAxisPositionTracker mainAxisPosnTracker(
+      aAxisTracker, this, aJustifyContent, aContentBoxMainSize);
+  for (FlexItem& item : Items()) {
+    nscoord itemMainBorderBoxSize =
+        item.MainSize() + item.BorderPaddingSizeInMainAxis();
+
+    // Resolve any main-axis 'auto' margins on aChild to an actual value.
+    mainAxisPosnTracker.ResolveAutoMarginsInMainAxis(item);
+
+    // Advance our position tracker to child's upper-left content-box corner,
+    // and use that as its position in the main axis.
+    mainAxisPosnTracker.EnterMargin(item.Margin());
+    mainAxisPosnTracker.EnterChildFrame(itemMainBorderBoxSize);
+
+    item.SetMainPosition(mainAxisPosnTracker.Position());
+
+    mainAxisPosnTracker.ExitChildFrame(itemMainBorderBoxSize);
+    mainAxisPosnTracker.ExitMargin(item.Margin());
+    mainAxisPosnTracker.TraversePackingSpace();
+    if (&item != &Items().LastElement()) {
+      mainAxisPosnTracker.TraverseGap(mMainGapSize);
+    }
+  }
+}
+
+void nsFlexContainerFrame::SizeItemInCrossAxis(ReflowInput& aChildReflowInput,
+                                               FlexItem& aItem) {
+  // If cross axis is the item's inline axis, just use ISize from reflow input,
+  // and don't bother with a full reflow.
+  if (aItem.IsInlineAxisCrossAxis()) {
+    aItem.SetCrossSize(aChildReflowInput.ComputedISize());
+    return;
+  }
+
+  MOZ_ASSERT(!aItem.HadMeasuringReflow(),
+             "We shouldn't need more than one measuring reflow");
+
+  if (aItem.AlignSelf()._0 == StyleAlignFlags::STRETCH) {
+    // This item's got "align-self: stretch", so we probably imposed a
+    // stretched computed cross-size on it during its previous
+    // reflow. We're not imposing that BSize for *this* "measuring" reflow, so
+    // we need to tell it to treat this reflow as a resize in its block axis
+    // (regardless of whether any of its ancestors are actually being resized).
+    // (Note: we know that the cross axis is the item's *block* axis -- if it
+    // weren't, then we would've taken the early-return above.)
+    aChildReflowInput.SetBResize(true);
+    // Not 100% sure this is needed, but be conservative for now:
+    aChildReflowInput.mFlags.mIsBResizeForPercentages = true;
+  }
+
+  // Potentially reflow the item, and get the sizing info.
+  const CachedBAxisMeasurement& measurement =
+      MeasureBSizeForFlexItem(aItem, aChildReflowInput);
+
+  // Save the sizing info that we learned from this reflow
+  // -----------------------------------------------------
+
+  // Tentatively store the child's desired content-box cross-size.
+  aItem.SetCrossSize(measurement.BSize());
+}
+
+void FlexLine::PositionItemsInCrossAxis(
+    nscoord aLineStartPosition, const FlexboxAxisTracker& aAxisTracker) {
+  SingleLineCrossAxisPositionTracker lineCrossAxisPosnTracker(aAxisTracker);
+
+  for (FlexItem& item : Items()) {
+    // First, stretch the item's cross size (if appropriate), and resolve any
+    // auto margins in this axis.
+    item.ResolveStretchedCrossSize(mLineCrossSize);
+    lineCrossAxisPosnTracker.ResolveAutoMarginsInCrossAxis(*this, item);
+
+    // Compute the cross-axis position of this item
+    nscoord itemCrossBorderBoxSize =
+        item.CrossSize() + item.BorderPaddingSizeInCrossAxis();
+    lineCrossAxisPosnTracker.EnterAlignPackingSpace(*this, item, aAxisTracker);
+    lineCrossAxisPosnTracker.EnterMargin(item.Margin());
+    lineCrossAxisPosnTracker.EnterChildFrame(itemCrossBorderBoxSize);
+
+    item.SetCrossPosition(aLineStartPosition +
+                          lineCrossAxisPosnTracker.Position());
+
+    // Back out to cross-axis edge of the line.
+    lineCrossAxisPosnTracker.ResetPosition();
+  }
+}
+
+void nsFlexContainerFrame::Reflow(nsPresContext* aPresContext,
+                                  ReflowOutput& aReflowOutput,
+                                  const ReflowInput& aReflowInput,
+                                  nsReflowStatus& aStatus) {
+  if (IsHiddenByContentVisibilityOfInFlowParentForLayout()) {
+    return;
+  }
+
+  MarkInReflow();
+  DO_GLOBAL_REFLOW_COUNT("nsFlexContainerFrame");
+  DISPLAY_REFLOW(aPresContext, this, aReflowInput, aReflowOutput, aStatus);
+  MOZ_ASSERT(aStatus.IsEmpty(), "Caller should pass a fresh reflow status!");
+  MOZ_ASSERT(aPresContext == PresContext());
+  NS_WARNING_ASSERTION(
+      aReflowInput.ComputedISize() != NS_UNCONSTRAINEDSIZE,
+      "Unconstrained inline size; this should only result from huge sizes "
+      "(not intrinsic sizing w/ orthogonal flows)");
+
+  FLEX_LOG("Reflow() for nsFlexContainerFrame %p", this);
+
+  if (IsFrameTreeTooDeep(aReflowInput, aReflowOutput, aStatus)) {
+    return;
+  }
+
+  NormalizeChildLists();
+
+#ifdef DEBUG
+  mDidPushItemsBitMayLie = false;
+  SanityCheckChildListsBeforeReflow();
+#endif  // DEBUG
+
+  // We (and our children) can only depend on our ancestor's bsize if we have
+  // a percent-bsize, or if we're positioned and we have "block-start" and
+  // "block-end" set and have block-size:auto.  (There are actually other cases,
+  // too -- e.g. if our parent is itself a block-dir flex container and we're
+  // flexible -- but we'll let our ancestors handle those sorts of cases.)
+  //
+  // TODO(emilio): the !bsize.IsLengthPercentage() preserves behavior, but it's
+  // too conservative. min/max-content don't really depend on the container.
+  WritingMode wm = aReflowInput.GetWritingMode();
+  const nsStylePosition* stylePos = StylePosition();
+  const auto& bsize = stylePos->BSize(wm);
+  if (bsize.HasPercent() || (StyleDisplay()->IsAbsolutelyPositionedStyle() &&
+                             (bsize.IsAuto() || !bsize.IsLengthPercentage()) &&
+                             !stylePos->mOffset.GetBStart(wm).IsAuto() &&
+                             !stylePos->mOffset.GetBEnd(wm).IsAuto())) {
+    AddStateBits(NS_FRAME_CONTAINS_RELATIVE_BSIZE);
+  }
+
+  const FlexboxAxisTracker axisTracker(this);
+
+  // Check to see if we need to create a computed info structure, to
+  // be filled out for use by devtools.
+  ComputedFlexContainerInfo* containerInfo = CreateOrClearFlexContainerInfo();
+
+  FlexLayoutResult flr;
+  PerFragmentFlexData fragmentData;
+  const nsIFrame* prevInFlow = GetPrevInFlow();
+  if (!prevInFlow) {
+    const LogicalSize tentativeContentBoxSize = aReflowInput.ComputedSize();
+    const nscoord tentativeContentBoxMainSize =
+        axisTracker.MainComponent(tentativeContentBoxSize);
+    const nscoord tentativeContentBoxCrossSize =
+        axisTracker.CrossComponent(tentativeContentBoxSize);
+
+    // Calculate gap sizes for main and cross axis. We only need them in
+    // DoFlexLayout in the first-in-flow, so no need to worry about consumed
+    // block-size.
+    const auto& mainGapStyle =
+        axisTracker.IsRowOriented() ? stylePos->mColumnGap : stylePos->mRowGap;
+    const auto& crossGapStyle =
+        axisTracker.IsRowOriented() ? stylePos->mRowGap : stylePos->mColumnGap;
+    const nscoord mainGapSize = nsLayoutUtils::ResolveGapToLength(
+        mainGapStyle, tentativeContentBoxMainSize);
+    const nscoord crossGapSize = nsLayoutUtils::ResolveGapToLength(
+        crossGapStyle, tentativeContentBoxCrossSize);
+
+    // When fragmenting a flex container, we run the flex algorithm without
+    // regards to pagination in order to compute the flex container's desired
+    // content-box size. https://drafts.csswg.org/css-flexbox-1/#pagination-algo
+    //
+    // Note: For a multi-line column-oriented flex container, the sample
+    // algorithm suggests we wrap the flex line at the block-end edge of a
+    // column/page, but we do not implement it intentionally. This brings the
+    // layout result closer to the one as if there's no fragmentation.
+    AutoTArray<StrutInfo, 1> struts;
+    flr = DoFlexLayout(aReflowInput, tentativeContentBoxMainSize,
+                       tentativeContentBoxCrossSize, axisTracker, mainGapSize,
+                       crossGapSize, struts, containerInfo);
+
+    if (!struts.IsEmpty()) {
+      // We're restarting flex layout, with new knowledge of collapsed items.
+      flr.mLines.Clear();
+      flr.mPlaceholders.Clear();
+      flr = DoFlexLayout(aReflowInput, tentativeContentBoxMainSize,
+                         tentativeContentBoxCrossSize, axisTracker, mainGapSize,
+                         crossGapSize, struts, containerInfo);
+    }
+  } else {
+    flr = GenerateFlexLayoutResult();
+    auto* fragmentDataProp =
+        prevInFlow->GetProperty(PerFragmentFlexData::Prop());
+    MOZ_ASSERT(fragmentDataProp,
+               "PerFragmentFlexData should be set in our prev-in-flow!");
+    fragmentData = *fragmentDataProp;
+  }
+
+  LogicalSize contentBoxSize = axisTracker.LogicalSizeFromFlexRelativeSizes(
+      flr.mContentBoxMainSize, flr.mContentBoxCrossSize);
+
+  const nscoord consumedBSize = CalcAndCacheConsumedBSize();
+  const nscoord effectiveContentBSize =
+      contentBoxSize.BSize(wm) - consumedBSize;
+  LogicalMargin borderPadding = aReflowInput.ComputedLogicalBorderPadding(wm);
+  if (MOZ_UNLIKELY(aReflowInput.AvailableBSize() != NS_UNCONSTRAINEDSIZE)) {
+    // We assume we are the last fragment by using
+    // PreReflowBlockLevelLogicalSkipSides(), and skip block-end border and
+    // padding if needed.
+    borderPadding.ApplySkipSides(PreReflowBlockLevelLogicalSkipSides());
+  }
+
+  // Determine this frame's tentative border-box size. This is used for logical
+  // to physical coordinate conversion when positioning children.
+  //
+  // Note that vertical-rl writing-mode is the only case where the block flow
+  // direction progresses in a negative physical direction, and therefore block
+  // direction coordinate conversion depends on knowing the width of the
+  // coordinate space in order to translate between the logical and physical
+  // origins. As a result, if our final border-box block-size is different from
+  // this tentative one, and we are in vertical-rl writing mode, we need to
+  // adjust our children's position after reflowing them.
+  const LogicalSize tentativeBorderBoxSize(
+      wm, contentBoxSize.ISize(wm) + borderPadding.IStartEnd(wm),
+      std::min(effectiveContentBSize + borderPadding.BStartEnd(wm),
+               aReflowInput.AvailableBSize()));
+  const nsSize containerSize = tentativeBorderBoxSize.GetPhysicalSize(wm);
+
+  OverflowAreas ocBounds;
+  nsReflowStatus ocStatus;
+  if (prevInFlow) {
+    ReflowOverflowContainerChildren(
+        aPresContext, aReflowInput, ocBounds, ReflowChildFlags::Default,
+        ocStatus, MergeSortedFrameListsFor, Some(containerSize));
+  }
+
+  const LogicalSize availableSizeForItems =
+      ComputeAvailableSizeForItems(aReflowInput, borderPadding);
+  const auto [childrenBEndEdge, childrenStatus] =
+      ReflowChildren(aReflowInput, containerSize, availableSizeForItems,
+                     borderPadding, axisTracker, flr, fragmentData);
+
+  bool mayNeedNextInFlow = false;
+  if (aReflowInput.IsInFragmentedContext()) {
+    // This fragment's contribution to the flex container's cumulative
+    // content-box block-size, if it turns out that this is the final vs.
+    // non-final fragment:
+    //
+    // * If it turns out we *are* the final fragment, then this fragment's
+    // content-box contribution is the distance from the start of our content
+    // box to the block-end edge of our children (note the borderPadding
+    // subtraction is just to get us to a content-box-relative offset here):
+    const nscoord bSizeContributionIfFinalFragment =
+        childrenBEndEdge - borderPadding.BStart(wm);
+
+    // * If it turns out we're *not* the final fragment, then this fragment's
+    // content-box extends to the edge of the availableSizeForItems (at least),
+    // regardless of whether we actually have items at that location:
+    const nscoord bSizeContributionIfNotFinalFragment = std::max(
+        bSizeContributionIfFinalFragment, availableSizeForItems.BSize(wm));
+
+    // mCumulativeBEndEdgeShift was updated in ReflowChildren(), and our
+    // children's block-size may grow in fragmented context. If our block-size
+    // and max-block-size are unconstrained, then we allow the flex container to
+    // grow to accommodate any children whose sizes grew as a result of
+    // fragmentation.
+    if (aReflowInput.ComputedBSize() == NS_UNCONSTRAINEDSIZE) {
+      contentBoxSize.BSize(wm) = aReflowInput.ApplyMinMaxBSize(
+          contentBoxSize.BSize(wm) + fragmentData.mCumulativeBEndEdgeShift);
+
+      if (childrenStatus.IsComplete()) {
+        // All of the children fit! We know that we're using a content-based
+        // block-size, and we know our children's block-size may have grown due
+        // to fragmentation. So we allow ourselves to grow our block-size here
+        // to contain the block-end edge of our last child (subject to our
+        // min/max constraints).
+        contentBoxSize.BSize(wm) = aReflowInput.ApplyMinMaxBSize(std::max(
+            contentBoxSize.BSize(wm), fragmentData.mCumulativeContentBoxBSize +
+                                          bSizeContributionIfFinalFragment));
+      } else {
+        // As in the if-branch above, we extend our block-size, but in this case
+        // we know that a child didn't fit and might overshot our available
+        // size, so we assume this fragment won't be the final fragment, and
+        // hence it should contribute bSizeContributionIfNotFinalFragment
+        // (subject to our min/max constraints).
+        contentBoxSize.BSize(wm) = aReflowInput.ApplyMinMaxBSize(std::max(
+            contentBoxSize.BSize(wm), fragmentData.mCumulativeContentBoxBSize +
+                                          bSizeContributionIfNotFinalFragment));
+
+        if (aReflowInput.ComputedMaxBSize() == NS_UNCONSTRAINEDSIZE) {
+          mayNeedNextInFlow = true;
+        } else {
+          // The definite max-block-size can be the upper bound of our
+          // content-box block-size. We should check whether we need a
+          // next-in-flow.
+          mayNeedNextInFlow = contentBoxSize.BSize(wm) - consumedBSize >
+                              availableSizeForItems.BSize(wm);
+        }
+      }
+    } else {
+      mayNeedNextInFlow = contentBoxSize.BSize(wm) - consumedBSize >
+                          availableSizeForItems.BSize(wm);
+    }
+    fragmentData.mCumulativeContentBoxBSize +=
+        bSizeContributionIfNotFinalFragment;
+
+    // If we may need a next-in-flow, we'll need to skip block-end border and
+    // padding.
+    if (mayNeedNextInFlow && aReflowInput.mStyleBorder->mBoxDecorationBreak ==
+                                 StyleBoxDecorationBreak::Slice) {
+      borderPadding.BEnd(wm) = 0;
+    }
+  }
+
+  PopulateReflowOutput(aReflowOutput, aReflowInput, aStatus, contentBoxSize,
+                       borderPadding, consumedBSize, mayNeedNextInFlow,
+                       childrenBEndEdge, childrenStatus, axisTracker, flr);
+
+  if (wm.IsVerticalRL()) {
+    // If the final border-box block-size is different from the tentative one,
+    // adjust our children's position.
+    const nscoord deltaBCoord =
+        tentativeBorderBoxSize.BSize(wm) - aReflowOutput.Size(wm).BSize(wm);
+    if (deltaBCoord != 0) {
+      const LogicalPoint delta(wm, 0, deltaBCoord);
+      for (const FlexLine& line : flr.mLines) {
+        for (const FlexItem& item : line.Items()) {
+          item.Frame()->MovePositionBy(wm, delta);
+        }
+      }
+    }
+  }
+
+  // Overflow area = union(my overflow area, children's overflow areas)
+  aReflowOutput.SetOverflowAreasToDesiredBounds();
+  UnionInFlowChildOverflow(aReflowOutput.mOverflowAreas);
+
+  // Merge overflow container bounds and status.
+  aReflowOutput.mOverflowAreas.UnionWith(ocBounds);
+  aStatus.MergeCompletionStatusFrom(ocStatus);
+
+  FinishReflowWithAbsoluteFrames(PresContext(), aReflowOutput, aReflowInput,
+                                 aStatus);
+
+  // Finally update our line and item measurements in our containerInfo.
+  if (MOZ_UNLIKELY(containerInfo)) {
+    UpdateFlexLineAndItemInfo(*containerInfo, flr.mLines);
+  }
+
+  // If we are the first-in-flow, we want to store data for our next-in-flows,
+  // or clear the existing data if it is not needed.
+  if (!prevInFlow) {
+    SharedFlexData* sharedData = GetProperty(SharedFlexData::Prop());
+    if (!aStatus.IsFullyComplete()) {
+      if (!sharedData) {
+        sharedData = new SharedFlexData;
+        SetProperty(SharedFlexData::Prop(), sharedData);
+      }
+      sharedData->Update(std::move(flr));
+    } else if (sharedData && !GetNextInFlow()) {
+      // We are fully-complete, so no next-in-flow is needed. However, if we
+      // report SetInlineLineBreakBeforeAndReset() in an incremental reflow, our
+      // next-in-flow might still exist. It can be reflowed again before us if
+      // it is an overflow container. Delete the existing data only if we don't
+      // have a next-in-flow.
+      RemoveProperty(SharedFlexData::Prop());
+    }
+  }
+
+  PerFragmentFlexData* fragmentDataProp =
+      GetProperty(PerFragmentFlexData::Prop());
+  if (!aStatus.IsFullyComplete()) {
+    if (!fragmentDataProp) {
+      fragmentDataProp = new PerFragmentFlexData;
+      SetProperty(PerFragmentFlexData::Prop(), fragmentDataProp);
+    }
+    *fragmentDataProp = fragmentData;
+  } else if (fragmentDataProp && !GetNextInFlow()) {
+    // Similar to the condition to remove SharedFlexData, delete the
+    // existing data only if we don't have a next-in-flow.
+    RemoveProperty(PerFragmentFlexData::Prop());
+  }
+}
+
+Maybe<nscoord> nsFlexContainerFrame::GetNaturalBaselineBOffset(
+    WritingMode aWM, BaselineSharingGroup aBaselineGroup,
+    BaselineExportContext) const {
+  if (StyleDisplay()->IsContainLayout() ||
+      HasAnyStateBits(NS_STATE_FLEX_SYNTHESIZE_BASELINE)) {
+    return Nothing{};
+  }
+  return Some(aBaselineGroup == BaselineSharingGroup::First ? mFirstBaseline
+                                                            : mLastBaseline);
+}
+
+void nsFlexContainerFrame::UnionInFlowChildOverflow(
+    OverflowAreas& aOverflowAreas) {
+  // The CSS Overflow spec [1] requires that a scrollable container's
+  // scrollable overflow should include the following areas.
+  //
+  // a) "the box's own content and padding areas": we treat the *content* as
+  // the scrolled inner frame's theoretical content-box that's intrinsically
+  // sized to the union of all the flex items' margin boxes, _without_
+  // relative positioning applied. The *padding areas* is just inflation on
+  // top of the theoretical content-box by the flex container's padding.
+  //
+  // b) "the margin areas of grid item and flex item boxes for which the box
+  // establishes a containing block": a) already includes the flex items'
+  // normal-positioned margin boxes into the scrollable overflow, but their
+  // relative-positioned margin boxes should also be included because relpos
+  // children are still flex items.
+  //
+  // [1] https://drafts.csswg.org/css-overflow-3/#scrollable.
+  const bool isScrolledContent =
+      Style()->GetPseudoType() == PseudoStyleType::scrolledContent;
+  bool anyScrolledContentItem = false;
+  // Union of normal-positioned margin boxes for all the items.
+  nsRect itemMarginBoxes;
+  // Union of relative-positioned margin boxes for the relpos items only.
+  nsRect relPosItemMarginBoxes;
+  const bool useMozBoxCollapseBehavior =
+      StyleVisibility()->UseLegacyCollapseBehavior();
+  for (nsIFrame* f : mFrames) {
+    if (useMozBoxCollapseBehavior && f->StyleVisibility()->IsCollapse()) {
+      continue;
+    }
+    ConsiderChildOverflow(aOverflowAreas, f);
+    if (!isScrolledContent) {
+      continue;
+    }
+    if (f->IsPlaceholderFrame()) {
+      continue;
+    }
+    anyScrolledContentItem = true;
+    if (MOZ_UNLIKELY(f->IsRelativelyOrStickyPositioned())) {
+      const nsRect marginRect = f->GetMarginRectRelativeToSelf();
+      itemMarginBoxes =
+          itemMarginBoxes.Union(marginRect + f->GetNormalPosition());
+      relPosItemMarginBoxes =
+          relPosItemMarginBoxes.Union(marginRect + f->GetPosition());
+    } else {
+      itemMarginBoxes = itemMarginBoxes.Union(f->GetMarginRect());
+    }
+  }
+
+  if (anyScrolledContentItem) {
+    itemMarginBoxes.Inflate(GetUsedPadding());
+    aOverflowAreas.UnionAllWith(itemMarginBoxes);
+    aOverflowAreas.UnionAllWith(relPosItemMarginBoxes);
+  }
+}
+
+void nsFlexContainerFrame::UnionChildOverflow(OverflowAreas& aOverflowAreas) {
+  UnionInFlowChildOverflow(aOverflowAreas);
+  // Union with child frames, skipping the principal list since we already
+  // handled those above.
+  nsLayoutUtils::UnionChildOverflow(this, aOverflowAreas,
+                                    {FrameChildListID::Principal});
+}
+
+void nsFlexContainerFrame::CalculatePackingSpace(
+    uint32_t aNumThingsToPack, const StyleContentDistribution& aAlignVal,
+    nscoord* aFirstSubjectOffset, uint32_t* aNumPackingSpacesRemaining,
+    nscoord* aPackingSpaceRemaining) {
+  StyleAlignFlags val = aAlignVal.primary;
+  MOZ_ASSERT(val == StyleAlignFlags::SPACE_BETWEEN ||
+                 val == StyleAlignFlags::SPACE_AROUND ||
+                 val == StyleAlignFlags::SPACE_EVENLY,
+             "Unexpected alignment value");
+
+  MOZ_ASSERT(*aPackingSpaceRemaining >= 0,
+             "Should not be called with negative packing space");
+
+  // Note: In the aNumThingsToPack==1 case, the fallback behavior for
+  // 'space-between' depends on precise information about the axes that we
+  // don't have here. So, for that case, we just depend on the caller to
+  // explicitly convert 'space-{between,around,evenly}' keywords to the
+  // appropriate fallback alignment and skip this function.
+  MOZ_ASSERT(aNumThingsToPack > 1,
+             "Should not be called unless there's more than 1 thing to pack");
+
+  // Packing spaces between items:
+  *aNumPackingSpacesRemaining = aNumThingsToPack - 1;
+
+  if (val == StyleAlignFlags::SPACE_BETWEEN) {
+    // No need to reserve space at beginning/end, so we're done.
+    return;
+  }
+
+  // We need to add 1 or 2 packing spaces, split between beginning/end, for
+  // space-around / space-evenly:
+  size_t numPackingSpacesForEdges =
+      val == StyleAlignFlags::SPACE_AROUND ? 1 : 2;
+
+  // How big will each "full" packing space be:
+  nscoord packingSpaceSize =
+      *aPackingSpaceRemaining /
+      (*aNumPackingSpacesRemaining + numPackingSpacesForEdges);
+  // How much packing-space are we allocating to the edges:
+  nscoord totalEdgePackingSpace = numPackingSpacesForEdges * packingSpaceSize;
+
+  // Use half of that edge packing space right now:
+  *aFirstSubjectOffset += totalEdgePackingSpace / 2;
+  // ...but we need to subtract all of it right away, so that we won't
+  // hand out any of it to intermediate packing spaces.
+  *aPackingSpaceRemaining -= totalEdgePackingSpace;
+}
+
+ComputedFlexContainerInfo*
+nsFlexContainerFrame::CreateOrClearFlexContainerInfo() {
+  if (!HasAnyStateBits(NS_STATE_FLEX_COMPUTED_INFO)) {
+    return nullptr;
+  }
+
+  // The flag that sets ShouldGenerateComputedInfo() will never be cleared.
+  // That's acceptable because it's only set in a Chrome API invoked by
+  // devtools, and won't impact normal browsing.
+
+  // Re-use the ComputedFlexContainerInfo, if it exists.
+  ComputedFlexContainerInfo* info = GetProperty(FlexContainerInfo());
+  if (info) {
+    // We can reuse, as long as we clear out old data.
+    info->mLines.Clear();
+  } else {
+    info = new ComputedFlexContainerInfo();
+    SetProperty(FlexContainerInfo(), info);
+  }
+
+  return info;
+}
+
+nscoord nsFlexContainerFrame::FlexItemConsumedBSize(const FlexItem& aItem) {
+  nsSplittableFrame* f = do_QueryFrame(aItem.Frame());
+  return f ? ConsumedBSize(f) : 0;
+}
+
+void nsFlexContainerFrame::CreateFlexLineAndFlexItemInfo(
+    ComputedFlexContainerInfo& aContainerInfo,
+    const nsTArray<FlexLine>& aLines) {
+  for (const FlexLine& line : aLines) {
+    ComputedFlexLineInfo* lineInfo = aContainerInfo.mLines.AppendElement();
+    // Most of the remaining lineInfo properties will be filled out in
+    // UpdateFlexLineAndItemInfo (some will be provided by other functions),
+    // when we have real values. But we still add all the items here, so
+    // we can capture computed data for each item as we proceed.
+    for (const FlexItem& item : line.Items()) {
+      nsIFrame* frame = item.Frame();
+
+      // The frame may be for an element, or it may be for an
+      // anonymous flex item, e.g. wrapping one or more text nodes.
+      // DevTools wants the content node for the actual child in
+      // the DOM tree, so we descend through anonymous boxes.
+      nsIFrame* targetFrame = GetFirstNonAnonBoxInSubtree(frame);
+      nsIContent* content = targetFrame->GetContent();
+
+      // Skip over content that is only whitespace, which might
+      // have been broken off from a text node which is our real
+      // target.
+      while (content && content->TextIsOnlyWhitespace()) {
+        // If content is only whitespace, try the frame sibling.
+        targetFrame = targetFrame->GetNextSibling();
+        if (targetFrame) {
+          content = targetFrame->GetContent();
+        } else {
+          content = nullptr;
+        }
+      }
+
+      ComputedFlexItemInfo* itemInfo = lineInfo->mItems.AppendElement();
+
+      itemInfo->mNode = content;
+
+      // itemInfo->mMainBaseSize and mMainDeltaSize will be filled out
+      // in ResolveFlexibleLengths(). Other measurements will be captured in
+      // UpdateFlexLineAndItemInfo.
+    }
+  }
+}
+
+void nsFlexContainerFrame::ComputeFlexDirections(
+    ComputedFlexContainerInfo& aContainerInfo,
+    const FlexboxAxisTracker& aAxisTracker) {
+  auto ConvertPhysicalStartSideToFlexPhysicalDirection =
+      [](mozilla::Side aStartSide) {
+        switch (aStartSide) {
+          case eSideLeft:
+            return dom::FlexPhysicalDirection::Horizontal_lr;
+          case eSideRight:
+            return dom::FlexPhysicalDirection::Horizontal_rl;
+          case eSideTop:
+            return dom::FlexPhysicalDirection::Vertical_tb;
+          case eSideBottom:
+            return dom::FlexPhysicalDirection::Vertical_bt;
+        }
+
+        MOZ_ASSERT_UNREACHABLE("We should handle all sides!");
+        return dom::FlexPhysicalDirection::Horizontal_lr;
+      };
+
+  aContainerInfo.mMainAxisDirection =
+      ConvertPhysicalStartSideToFlexPhysicalDirection(
+          aAxisTracker.MainAxisPhysicalStartSide());
+  aContainerInfo.mCrossAxisDirection =
+      ConvertPhysicalStartSideToFlexPhysicalDirection(
+          aAxisTracker.CrossAxisPhysicalStartSide());
+}
+
+void nsFlexContainerFrame::UpdateFlexLineAndItemInfo(
+    ComputedFlexContainerInfo& aContainerInfo,
+    const nsTArray<FlexLine>& aLines) {
+  uint32_t lineIndex = 0;
+  for (const FlexLine& line : aLines) {
+    ComputedFlexLineInfo& lineInfo = aContainerInfo.mLines[lineIndex];
+
+    lineInfo.mCrossSize = line.LineCrossSize();
+    lineInfo.mFirstBaselineOffset = line.FirstBaselineOffset();
+    lineInfo.mLastBaselineOffset = line.LastBaselineOffset();
+
+    uint32_t itemIndex = 0;
+    for (const FlexItem& item : line.Items()) {
+      ComputedFlexItemInfo& itemInfo = lineInfo.mItems[itemIndex];
+      itemInfo.mFrameRect = item.Frame()->GetRect();
+      itemInfo.mMainMinSize = item.MainMinSize();
+      itemInfo.mMainMaxSize = item.MainMaxSize();
+      itemInfo.mCrossMinSize = item.CrossMinSize();
+      itemInfo.mCrossMaxSize = item.CrossMaxSize();
+      itemInfo.mClampState =
+          item.WasMinClamped()
+              ? mozilla::dom::FlexItemClampState::Clamped_to_min
+              : (item.WasMaxClamped()
+                     ? mozilla::dom::FlexItemClampState::Clamped_to_max
+                     : mozilla::dom::FlexItemClampState::Unclamped);
+      ++itemIndex;
+    }
+    ++lineIndex;
+  }
+}
+
+nsFlexContainerFrame* nsFlexContainerFrame::GetFlexFrameWithComputedInfo(
+    nsIFrame* aFrame) {
+  // Prepare a lambda function that we may need to call multiple times.
+  auto GetFlexContainerFrame = [](nsIFrame* aFrame) {
+    // Return the aFrame's content insertion frame, iff it is
+    // a flex container frame.
+    nsFlexContainerFrame* flexFrame = nullptr;
+
+    if (aFrame) {
+      nsIFrame* inner = aFrame;
+      if (MOZ_UNLIKELY(aFrame->IsFieldSetFrame())) {
+        inner = static_cast<nsFieldSetFrame*>(aFrame)->GetInner();
+      }
+      // Since "Get" methods like GetInner and GetContentInsertionFrame can
+      // return null, we check the return values before dereferencing. Our
+      // calling pattern makes this unlikely, but we're being careful.
+      nsIFrame* insertionFrame =
+          inner ? inner->GetContentInsertionFrame() : nullptr;
+      nsIFrame* possibleFlexFrame = insertionFrame ? insertionFrame : aFrame;
+      flexFrame = possibleFlexFrame->IsFlexContainerFrame()
+                      ? static_cast<nsFlexContainerFrame*>(possibleFlexFrame)
+                      : nullptr;
+    }
+    return flexFrame;
+  };
+
+  nsFlexContainerFrame* flexFrame = GetFlexContainerFrame(aFrame);
+  if (!flexFrame) {
+    return nullptr;
+  }
+  // Generate the FlexContainerInfo data, if it's not already there.
+  if (flexFrame->HasProperty(FlexContainerInfo())) {
+    return flexFrame;
+  }
+  // Trigger a reflow that generates additional flex property data.
+  // Hold onto aFrame while we do this, in case reflow destroys it.
+  AutoWeakFrame weakFrameRef(aFrame);
+
+  RefPtr<mozilla::PresShell> presShell = flexFrame->PresShell();
+  flexFrame->AddStateBits(NS_STATE_FLEX_COMPUTED_INFO);
+  presShell->FrameNeedsReflow(flexFrame, IntrinsicDirty::None,
+                              NS_FRAME_IS_DIRTY);
+  presShell->FlushPendingNotifications(FlushType::Layout);
+
+  // Since the reflow may have side effects, get the flex frame
+  // again. But if the weakFrameRef is no longer valid, then we
+  // must bail out.
+  if (!weakFrameRef.IsAlive()) {
+    return nullptr;
+  }
+
+  flexFrame = GetFlexContainerFrame(weakFrameRef.GetFrame());
+
+  NS_WARNING_ASSERTION(
+      !flexFrame || flexFrame->HasProperty(FlexContainerInfo()),
+      "The state bit should've made our forced-reflow "
+      "generate a FlexContainerInfo object");
+  return flexFrame;
+}
+
+/* static */
+bool nsFlexContainerFrame::IsItemInlineAxisMainAxis(nsIFrame* aFrame) {
+  MOZ_ASSERT(aFrame && aFrame->IsFlexItem(), "expecting arg to be a flex item");
+  const WritingMode flexItemWM = aFrame->GetWritingMode();
+  const nsIFrame* flexContainer = aFrame->GetParent();
+
+  if (IsLegacyBox(flexContainer)) {
+    // For legacy boxes, the main axis is determined by "box-orient", and we can
+    // just directly check if that's vertical, and compare that to whether the
+    // item's WM is also vertical:
+    bool boxOrientIsVertical =
+        flexContainer->StyleXUL()->mBoxOrient == StyleBoxOrient::Vertical;
+    return flexItemWM.IsVertical() == boxOrientIsVertical;
+  }
+
+  // For modern CSS flexbox, we get our return value by asking two questions
+  // and comparing their answers.
+  // Question 1: does aFrame have the same inline axis as its flex container?
+  bool itemInlineAxisIsParallelToParent =
+      !flexItemWM.IsOrthogonalTo(flexContainer->GetWritingMode());
+
+  // Question 2: is aFrame's flex container row-oriented? (This tells us
+  // whether the flex container's main axis is its inline axis.)
+  auto flexDirection = flexContainer->StylePosition()->mFlexDirection;
+  bool flexContainerIsRowOriented =
+      flexDirection == StyleFlexDirection::Row ||
+      flexDirection == StyleFlexDirection::RowReverse;
+
+  // aFrame's inline axis is its flex container's main axis IFF the above
+  // questions have the same answer.
+  return flexContainerIsRowOriented == itemInlineAxisIsParallelToParent;
+}
+
+/* static */
+bool nsFlexContainerFrame::IsUsedFlexBasisContent(
+    const StyleFlexBasis& aFlexBasis, const StyleSize& aMainSize) {
+  // We have a used flex-basis of 'content' if flex-basis explicitly has that
+  // value, OR if flex-basis is 'auto' (deferring to the main-size property)
+  // and the main-size property is also 'auto'.
+  // See https://drafts.csswg.org/css-flexbox-1/#valdef-flex-basis-auto
+  if (aFlexBasis.IsContent()) {
+    return true;
+  }
+  return aFlexBasis.IsAuto() && aMainSize.IsAuto();
+}
+
+nsFlexContainerFrame::FlexLayoutResult nsFlexContainerFrame::DoFlexLayout(
+    const ReflowInput& aReflowInput, const nscoord aTentativeContentBoxMainSize,
+    const nscoord aTentativeContentBoxCrossSize,
+    const FlexboxAxisTracker& aAxisTracker, nscoord aMainGapSize,
+    nscoord aCrossGapSize, nsTArray<StrutInfo>& aStruts,
+    ComputedFlexContainerInfo* const aContainerInfo) {
+  FlexLayoutResult flr;
+
+  GenerateFlexLines(aReflowInput, aTentativeContentBoxMainSize,
+                    aTentativeContentBoxCrossSize, aStruts, aAxisTracker,
+                    aMainGapSize, flr.mPlaceholders, flr.mLines,
+                    flr.mHasCollapsedItems);
+
+  if ((flr.mLines.Length() == 1 && flr.mLines[0].IsEmpty()) ||
+      aReflowInput.mStyleDisplay->IsContainLayout()) {
+    // We have no flex items, or we're layout-contained. So, we have no
+    // baseline, and our parent should synthesize a baseline if needed.
+    AddStateBits(NS_STATE_FLEX_SYNTHESIZE_BASELINE);
+  } else {
+    RemoveStateBits(NS_STATE_FLEX_SYNTHESIZE_BASELINE);
+  }
+
+  // Construct our computed info if we've been asked to do so. This is
+  // necessary to do now so we can capture some computed values for
+  // FlexItems during layout that would not otherwise be saved (like
+  // size adjustments). We'll later fix up the line properties,
+  // because the correct values aren't available yet.
+  if (aContainerInfo) {
+    MOZ_ASSERT(HasAnyStateBits(NS_STATE_FLEX_COMPUTED_INFO),
+               "We should only have the info struct if we should generate it");
+
+    if (!aStruts.IsEmpty()) {
+      // We restarted DoFlexLayout, and may have stale mLines to clear:
+      aContainerInfo->mLines.Clear();
+    } else {
+      MOZ_ASSERT(aContainerInfo->mLines.IsEmpty(), "Shouldn't have lines yet.");
+    }
+
+    CreateFlexLineAndFlexItemInfo(*aContainerInfo, flr.mLines);
+    ComputeFlexDirections(*aContainerInfo, aAxisTracker);
+  }
+
+  flr.mContentBoxMainSize = ComputeMainSize(
+      aReflowInput, aAxisTracker, aTentativeContentBoxMainSize, flr.mLines);
+
+  uint32_t lineIndex = 0;
+  for (FlexLine& line : flr.mLines) {
+    ComputedFlexLineInfo* lineInfo =
+        aContainerInfo ? &aContainerInfo->mLines[lineIndex] : nullptr;
+    line.ResolveFlexibleLengths(flr.mContentBoxMainSize, lineInfo);
+    ++lineIndex;
+  }
+
+  // Cross Size Determination - Flexbox spec section 9.4
+  // https://drafts.csswg.org/css-flexbox-1/#cross-sizing
+  // ===================================================
+  // Calculate the hypothetical cross size of each item:
+
+  // 'sumLineCrossSizes' includes the size of all gaps between lines. We
+  // initialize it with the sum of all the gaps, and add each line's cross size
+  // at the end of the following for-loop.
+  nscoord sumLineCrossSizes = aCrossGapSize * (flr.mLines.Length() - 1);
+  for (FlexLine& line : flr.mLines) {
+    for (FlexItem& item : line.Items()) {
+      // The item may already have the correct cross-size; only recalculate
+      // if the item's main size resolution (flexing) could have influenced it:
+      if (item.CanMainSizeInfluenceCrossSize()) {
+        StyleSizeOverrides sizeOverrides;
+        if (item.IsInlineAxisMainAxis()) {
+          sizeOverrides.mStyleISize.emplace(item.StyleMainSize());
+        } else {
+          sizeOverrides.mStyleBSize.emplace(item.StyleMainSize());
+        }
+        FLEX_LOG("Sizing flex item %p in cross axis", item.Frame());
+        FLEX_LOGV(" Main size override: %d", item.MainSize());
+
+        const WritingMode wm = item.GetWritingMode();
+        LogicalSize availSize = aReflowInput.ComputedSize(wm);
+        availSize.BSize(wm) = NS_UNCONSTRAINEDSIZE;
+        ReflowInput childReflowInput(PresContext(), aReflowInput, item.Frame(),
+                                     availSize, Nothing(), {}, sizeOverrides,
+                                     {ComputeSizeFlag::ShrinkWrap});
+        if (item.IsBlockAxisMainAxis() && item.TreatBSizeAsIndefinite()) {
+          childReflowInput.mFlags.mTreatBSizeAsIndefinite = true;
+        }
+
+        SizeItemInCrossAxis(childReflowInput, item);
+      }
+    }
+    // Now that we've finished with this line's items, size the line itself:
+    line.ComputeCrossSizeAndBaseline(aAxisTracker);
+    sumLineCrossSizes += line.LineCrossSize();
+  }
+
+  bool isCrossSizeDefinite;
+  flr.mContentBoxCrossSize = ComputeCrossSize(
+      aReflowInput, aAxisTracker, aTentativeContentBoxCrossSize,
+      sumLineCrossSizes, &isCrossSizeDefinite);
+
+  // Set up state for cross-axis alignment, at a high level (outside the
+  // scope of a particular flex line)
+  CrossAxisPositionTracker crossAxisPosnTracker(
+      flr.mLines, aReflowInput, flr.mContentBoxCrossSize, isCrossSizeDefinite,
+      aAxisTracker, aCrossGapSize);
+
+  // Now that we know the cross size of each line (including
+  // "align-content:stretch" adjustments, from the CrossAxisPositionTracker
+  // constructor), we can create struts for any flex items with
+  // "visibility: collapse" (and restart flex layout).
+  // Make sure to only do this if we had no struts.
+  if (aStruts.IsEmpty() && flr.mHasCollapsedItems &&
+      !StyleVisibility()->UseLegacyCollapseBehavior()) {
+    BuildStrutInfoFromCollapsedItems(flr.mLines, aStruts);
+    if (!aStruts.IsEmpty()) {
+      // Restart flex layout, using our struts.
+      return flr;
+    }
+  }
+
+  // If the flex container is row-oriented, it should derive its first/last
+  // baseline from the WM-relative startmost/endmost FlexLine if any items in
+  // the line participate in baseline alignment.
+  // https://drafts.csswg.org/css-flexbox-1/#flex-baselines
+  //
+  // Initialize the relevant variables here so that we can establish baselines
+  // while iterating FlexLine later (while crossAxisPosnTracker is conveniently
+  // pointing at the cross-start edge of that line, which the line's baseline
+  // offset is measured from).
+  const FlexLine* lineForFirstBaseline = nullptr;
+  const FlexLine* lineForLastBaseline = nullptr;
+  if (aAxisTracker.IsRowOriented()) {
+    lineForFirstBaseline = &StartmostLine(flr.mLines, aAxisTracker);
+    lineForLastBaseline = &EndmostLine(flr.mLines, aAxisTracker);
+  } else {
+    // For column-oriented flex container, use sentinel value to prompt us to
+    // get baselines from the startmost/endmost items.
+    flr.mAscent = nscoord_MIN;
+    flr.mAscentForLast = nscoord_MIN;
+  }
+
+  const auto justifyContent =
+      IsLegacyBox(aReflowInput.mFrame)
+          ? ConvertLegacyStyleToJustifyContent(StyleXUL())
+          : aReflowInput.mStylePosition->mJustifyContent;
+
+  lineIndex = 0;
+  for (FlexLine& line : flr.mLines) {
+    // Main-Axis Alignment - Flexbox spec section 9.5
+    // https://drafts.csswg.org/css-flexbox-1/#main-alignment
+    // ==============================================
+    line.PositionItemsInMainAxis(justifyContent, flr.mContentBoxMainSize,
+                                 aAxisTracker);
+
+    // See if we need to extract some computed info for this line.
+    if (MOZ_UNLIKELY(aContainerInfo)) {
+      ComputedFlexLineInfo& lineInfo = aContainerInfo->mLines[lineIndex];
+      lineInfo.mCrossStart = crossAxisPosnTracker.Position();
+    }
+
+    // Cross-Axis Alignment - Flexbox spec section 9.6
+    // https://drafts.csswg.org/css-flexbox-1/#cross-alignment
+    // ===============================================
+    line.PositionItemsInCrossAxis(crossAxisPosnTracker.Position(),
+                                  aAxisTracker);
+
+    // Flex Container Baselines - Flexbox spec section 8.5
+    // https://drafts.csswg.org/css-flexbox-1/#flex-baselines
+    auto ComputeAscentFromLine = [&](const FlexLine& aLine,
+                                     BaselineSharingGroup aBaselineGroup) {
+      MOZ_ASSERT(aAxisTracker.IsRowOriented(),
+                 "This makes sense only if we are row-oriented!");
+
+      // baselineOffsetInLine is a distance from the line's cross-start edge.
+      const nscoord baselineOffsetInLine =
+          aLine.ExtractBaselineOffset(aBaselineGroup);
+
+      if (baselineOffsetInLine == nscoord_MIN) {
+        // No "first baseline"-aligned or "last baseline"-aligned items in
+        // aLine. Return a sentinel value to prompt us to get baseline from the
+        // startmost or endmost FlexItem after we've reflowed it.
+        return nscoord_MIN;
+      }
+
+      // This "ascent" variable is a distance from the flex container's
+      // content-box block-start edge.
+      const nscoord ascent = aAxisTracker.LogicalAscentFromFlexRelativeAscent(
+          crossAxisPosnTracker.Position() + baselineOffsetInLine,
+          flr.mContentBoxCrossSize);
+
+      // Convert "ascent" variable to a distance from border-box start or end
+      // edge, per documentation for FlexLayoutResult ascent members.
+      const auto wm = aAxisTracker.GetWritingMode();
+      if (aBaselineGroup == BaselineSharingGroup::First) {
+        return ascent +
+               aReflowInput.ComputedLogicalBorderPadding(wm).BStart(wm);
+      }
+      return flr.mContentBoxCrossSize - ascent +
+             aReflowInput.ComputedLogicalBorderPadding(wm).BEnd(wm);
+    };
+
+    if (lineForFirstBaseline && lineForFirstBaseline == &line) {
+      flr.mAscent = ComputeAscentFromLine(line, BaselineSharingGroup::First);
+    }
+    if (lineForLastBaseline && lineForLastBaseline == &line) {
+      flr.mAscentForLast =
+          ComputeAscentFromLine(line, BaselineSharingGroup::Last);
+    }
+
+    crossAxisPosnTracker.TraverseLine(line);
+    crossAxisPosnTracker.TraversePackingSpace();
+
+    if (&line != &flr.mLines.LastElement()) {
+      crossAxisPosnTracker.TraverseGap();
+    }
+    ++lineIndex;
+  }
+
+  return flr;
+}
+
+// This data structure is used in fragmentation, storing the block coordinate
+// metrics when reflowing 1) the BStart-most line in each fragment of a
+// row-oriented flex container or, 2) the BStart-most item in each fragment of a
+// single-line column-oriented flex container.
+//
+// When we lay out a row-oriented flex container fragment, its first line might
+// contain one or more monolithic items that were pushed from the previous
+// fragment specifically to avoid having those monolithic items overlap the
+// page/column break. The situation is similar for single-row column-oriented
+// flex container fragments, but a bit simpler; only their first item might have
+// been pushed to avoid overlapping a page/column break.
+//
+// We'll have to place any such pushed items at the block-start edge of the
+// current fragment's content-box, which is as close as we can get them to their
+// theoretical/unfragmented position (without slicing them); but it does
+// represent a shift away from their theoretical/unfragmented position (which
+// was somewhere in the previous fragment).
+//
+// When that happens, we need to record the maximum such shift that we had to
+// perform so that we can apply the same block-endwards shift to "downstream"
+// items (items towards the block-end edge) that we could otherwise collide
+// with. We also potentially apply the same shift when computing the block-end
+// edge of this flex container fragment's content-box so that we don't
+// inadvertently shift the last item (or line-of-items) to overlap the flex
+// container's border, or content beyond the flex container.
+//
+// We use this structure to keep track of several metrics, in service of this
+// goal. This structure is also necessary to adjust PerFragmentFlexData at the
+// end of ReflowChildren().
+//
+// Note: "First" in the struct name means "BStart-most", not the order in the
+// flex line array or flex item array.
+struct FirstLineOrFirstItemBAxisMetrics final {
+  // This value stores the block-end edge shift for 1) the BStart-most line in
+  // the current fragment of a row-oriented flex container, or 2) the
+  // BStart-most item in the current fragment of a single-line column-oriented
+  // flex container. This number is non-negative.
+  //
+  // This value may become positive when any item is a first-in-flow and also
+  // satisfies either the above condition 1) or 2), since that's a hint that it
+  // could be monolithic or have a monolithic first descendant, and therefore an
+  // item that might incur a page/column-break-dodging position-shift that this
+  // variable needs to track.
+  //
+  // This value also stores the fragmentation-imposed growth in the block-size
+  // of a) the BStart-most line in the current fragment of a row-oriented flex
+  // container, or b) the BStart-most item in the current fragment of a
+  // single-line column-oriented flex container. This number is non-negative.
+  nscoord mBEndEdgeShift = 0;
+
+  // The first and second value in the pair store the max block-end edges for
+  // items before and after applying the per-item position-shift in the block
+  // axis. We only record the block-end edges for items with first-in-flow
+  // frames placed in the current flex container fragment. This is used only by
+  // row-oriented flex containers.
+  Maybe<std::pair<nscoord, nscoord>> mMaxBEndEdge;
+};
+
+std::tuple<nscoord, nsReflowStatus> nsFlexContainerFrame::ReflowChildren(
+    const ReflowInput& aReflowInput, const nsSize& aContainerSize,
+    const LogicalSize& aAvailableSizeForItems,
+    const LogicalMargin& aBorderPadding, const FlexboxAxisTracker& aAxisTracker,
+    FlexLayoutResult& aFlr, PerFragmentFlexData& aFragmentData) {
+  if (HidesContentForLayout()) {
+    return {0, nsReflowStatus()};
+  }
+
+  // Before giving each child a final reflow, calculate the origin of the
+  // flex container's content box (with respect to its border-box), so that
+  // we can compute our flex item's final positions.
+  WritingMode flexWM = aReflowInput.GetWritingMode();
+  const LogicalPoint containerContentBoxOrigin =
+      aBorderPadding.StartOffset(flexWM);
+
+  // The block-end of children is relative to the flex container's border-box.
+  nscoord maxBlockEndEdgeOfChildren = containerContentBoxOrigin.B(flexWM);
+
+  FirstLineOrFirstItemBAxisMetrics bAxisMetrics;
+  FrameHashtable pushedItems;
+  FrameHashtable incompleteItems;
+  FrameHashtable overflowIncompleteItems;
+
+  const bool isSingleLine =
+      StyleFlexWrap::Nowrap == aReflowInput.mStylePosition->mFlexWrap;
+
+  // FINAL REFLOW: Give each child frame another chance to reflow, now that
+  // we know its final size and position.
+  const FlexLine& startmostLine = StartmostLine(aFlr.mLines, aAxisTracker);
+  const FlexItem* startmostItem =
+      startmostLine.IsEmpty() ? nullptr
+                              : &startmostLine.StartmostItem(aAxisTracker);
+
+  const size_t numLines = aFlr.mLines.Length();
+  for (size_t lineIdx = 0; lineIdx < numLines; ++lineIdx) {
+    // Iterate flex lines from the startmost to endmost (relative to flex
+    // container's writing-mode).
+    const auto& line =
+        aFlr.mLines[aAxisTracker.IsCrossAxisReversed() ? numLines - lineIdx - 1
+                                                       : lineIdx];
+    MOZ_ASSERT(lineIdx != 0 || &line == &startmostLine,
+               "Logic for finding startmost line should be consistent!");
+
+    const size_t numItems = line.Items().Length();
+    for (size_t itemIdx = 0; itemIdx < numItems; ++itemIdx) {
+      // Iterate flex items from the startmost to endmost (relative to flex
+      // container's writing-mode).
+      const FlexItem& item = line.Items()[aAxisTracker.IsMainAxisReversed()
+                                              ? numItems - itemIdx - 1
+                                              : itemIdx];
+      MOZ_ASSERT(lineIdx != 0 || itemIdx != 0 || &item == startmostItem,
+                 "Logic for finding startmost item should be consistent!");
+
+      LogicalPoint framePos = aAxisTracker.LogicalPointFromFlexRelativePoint(
+          item.MainPosition(), item.CrossPosition(), aFlr.mContentBoxMainSize,
+          aFlr.mContentBoxCrossSize);
+      // This variable records the item's block-end edge before we give it a
+      // per-item-position-shift, if the item is a first-in-flow in the
+      // startmost line of a row-oriented flex container fragment. It is used to
+      // determine the block-end edge shift for the startmost line at the end of
+      // the outer loop.
+      Maybe<nscoord> frameBPosBeforePerItemShift;
+
+      if (item.Frame()->GetPrevInFlow()) {
+        // The item is a continuation. Lay it out at the beginning of the
+        // available space.
+        framePos.B(flexWM) = 0;
+      } else if (GetPrevInFlow()) {
+        // The item we're placing is not a continuation; though we're placing it
+        // into a flex container fragment which *is* a continuation. To compute
+        // the item's correct position in this fragment, we adjust the item's
+        // theoretical/unfragmented block-direction position by subtracting the
+        // cumulative content-box block-size for all the previous fragments and
+        // adding the cumulative block-end edge shift.
+        //
+        // Note that the item's position in this fragment has not been finalized
+        // yet. At this point, we've adjusted the item's
+        // theoretical/unfragmented position to be relative to the block-end
+        // edge of the previous container fragment's content-box. Later, we'll
+        // compute per-item position-shift to finalize its position.
+        framePos.B(flexWM) -= aFragmentData.mCumulativeContentBoxBSize;
+        framePos.B(flexWM) += aFragmentData.mCumulativeBEndEdgeShift;
+
+        // This helper gets the per-item position-shift in the block-axis.
+        auto GetPerItemPositionShiftToBEnd = [&]() {
+          if (framePos.B(flexWM) >= 0) {
+            // The item final position might be in current flex container
+            // fragment or in any of the later fragments. No adjustment needed.
+            return 0;
+          }
+
+          // The item's block position is negative, but we want to place it at
+          // the content-box block-start edge of this container fragment. To
+          // achieve this, return a negated (positive) value to make the final
+          // block position zero.
+          //
+          // This scenario occurs when fragmenting a row-oriented flex container
+          // where this item is pushed to this container fragment.
+          return -framePos.B(flexWM);
+        };
+
+        if (aAxisTracker.IsRowOriented()) {
+          if (&line == &startmostLine) {
+            frameBPosBeforePerItemShift.emplace(framePos.B(flexWM));
+            framePos.B(flexWM) += GetPerItemPositionShiftToBEnd();
+          } else {
+            // We've computed two things for the startmost line during the outer
+            // loop's first iteration: 1) how far the block-end edge had to
+            // shift and 2) how large the block-size needed to grow. Here, we
+            // just shift all items in the rest of the lines the same amount.
+            framePos.B(flexWM) += bAxisMetrics.mBEndEdgeShift;
+          }
+        } else {
+          MOZ_ASSERT(aAxisTracker.IsColumnOriented());
+          if (isSingleLine) {
+            if (&item == startmostItem) {
+              bAxisMetrics.mBEndEdgeShift = GetPerItemPositionShiftToBEnd();
+            }
+            framePos.B(flexWM) += bAxisMetrics.mBEndEdgeShift;
+          } else {
+            // Bug 1806717: We need a more sophisticated solution for multi-line
+            // column-oriented flex container when each line has a different
+            // position-shift value. For now, we don't shift them.
+          }
+        }
+      }
+
+      // Adjust available block-size for the item. (We compute it here because
+      // framePos is still relative to the container's content-box.)
+      //
+      // Note: The available block-size can become negative if item's
+      // block-direction position is below available space's block-end.
+      const nscoord availableBSizeForItem =
+          aAvailableSizeForItems.BSize(flexWM) == NS_UNCONSTRAINEDSIZE
+              ? NS_UNCONSTRAINEDSIZE
+              : aAvailableSizeForItems.BSize(flexWM) - framePos.B(flexWM);
+
+      // Adjust framePos to be relative to the container's border-box
+      // (i.e. its frame rect), instead of the container's content-box:
+      framePos += containerContentBoxOrigin;
+
+      // Check if we actually need to reflow the item -- if the item's position
+      // is below the available space's block-end, push it to our next-in-flow;
+      // if it does need a reflow, and we already reflowed it with the right
+      // content-box size.
+      const bool childBPosExceedAvailableSpaceBEnd =
+          availableBSizeForItem != NS_UNCONSTRAINEDSIZE &&
+          availableBSizeForItem <= 0;
+      bool itemInPushedItems = false;
+      if (childBPosExceedAvailableSpaceBEnd) {
+        // Note: Even if all of our items are beyond the available space & get
+        // pushed here, we'll be guaranteed to place at least one of them (and
+        // make progress) in one of the flex container's *next* fragment. It's
+        // because ComputeAvailableSizeForItems() always reserves at least 1px
+        // available block-size for its children, and we consume all available
+        // block-size and add it to
+        // PerFragmentFlexData::mCumulativeContentBoxBSize even if we are not
+        // laying out any child.
+        FLEX_LOG(
+            "[frag] Flex item %p needed to be pushed to container's "
+            "next-in-flow due to position below available space's block-end",
+            item.Frame());
+        pushedItems.Insert(item.Frame());
+        itemInPushedItems = true;
+      } else if (item.NeedsFinalReflow(aReflowInput)) {
+        // The available size must be in item's writing-mode.
+        const WritingMode itemWM = item.GetWritingMode();
+        const auto availableSize =
+            LogicalSize(flexWM, aAvailableSizeForItems.ISize(flexWM),
+                        availableBSizeForItem)
+                .ConvertTo(itemWM, flexWM);
+
+        const nsReflowStatus childReflowStatus =
+            ReflowFlexItem(aAxisTracker, aReflowInput, item, framePos,
+                           availableSize, aContainerSize);
+
+        const bool shouldPushItem = [&]() {
+          if (availableBSizeForItem == NS_UNCONSTRAINEDSIZE) {
+            // If the available block-size is unconstrained, then we're not
+            // fragmenting and we don't want to push the item.
+            return false;
+          }
+          if (framePos.B(flexWM) == containerContentBoxOrigin.B(flexWM)) {
+            // The flex item is adjacent with block-start of the container's
+            // content-box. Don't push it, or we'll trap in an infinite loop.
+            return false;
+          }
+          if (item.Frame()->BSize() <= availableBSizeForItem) {
+            return false;
+          }
+          if (aAxisTracker.IsColumnOriented() &&
+              item.Frame()->StyleDisplay()->mBreakBefore ==
+                  StyleBreakBetween::Avoid) {
+            return false;
+          }
+          return true;
+        }();
+        if (shouldPushItem) {
+          FLEX_LOG(
+              "[frag] Flex item %p needed to be pushed to container's "
+              "next-in-flow because its block-size is larger than the "
+              "available space",
+              item.Frame());
+          pushedItems.Insert(item.Frame());
+          itemInPushedItems = true;
+        } else if (childReflowStatus.IsIncomplete()) {
+          incompleteItems.Insert(item.Frame());
+        } else if (childReflowStatus.IsOverflowIncomplete()) {
+          overflowIncompleteItems.Insert(item.Frame());
+        }
+      } else {
+        MoveFlexItemToFinalPosition(item, framePos, aContainerSize);
+      }
+
+      if (!itemInPushedItems) {
+        const nscoord borderBoxBSize = item.Frame()->BSize(flexWM);
+        const nscoord bEndEdgeAfterPerItemShift =
+            framePos.B(flexWM) + borderBoxBSize;
+
+        // The item (or a fragment thereof) was placed in this flex container
+        // fragment. Update the max block-end edge with the item's block-end
+        // edge.
+        maxBlockEndEdgeOfChildren =
+            std::max(maxBlockEndEdgeOfChildren, bEndEdgeAfterPerItemShift);
+
+        if (frameBPosBeforePerItemShift) {
+          // Make the block-end edge relative to flex container's border-box
+          // because bEndEdgeAfterPerItemShift is relative to the border-box.
+          const nscoord bEndEdgeBeforePerItemShift =
+              containerContentBoxOrigin.B(flexWM) +
+              *frameBPosBeforePerItemShift + borderBoxBSize;
+
+          if (bAxisMetrics.mMaxBEndEdge) {
+            auto& [before, after] = *bAxisMetrics.mMaxBEndEdge;
+            before = std::max(before, bEndEdgeBeforePerItemShift);
+            after = std::max(after, bEndEdgeAfterPerItemShift);
+          } else {
+            bAxisMetrics.mMaxBEndEdge.emplace(bEndEdgeBeforePerItemShift,
+                                              bEndEdgeAfterPerItemShift);
+          }
+        }
+
+        if (item.Frame()->GetPrevInFlow()) {
+          // Items with a previous-continuation may experience some
+          // fragmentation-imposed growth in their block-size; we compute that
+          // here.
+          const nscoord bSizeOfThisFragment =
+              item.Frame()->ContentSize(flexWM).BSize(flexWM);
+          const nscoord consumedBSize = FlexItemConsumedBSize(item);
+          const nscoord unfragmentedBSize = item.BSize();
+          nscoord bSizeGrowthOfThisFragment = 0;
+
+          if (consumedBSize >= unfragmentedBSize) {
+            // The item's block-size has been grown to exceed the unfragmented
+            // block-size in the previous fragments.
+            bSizeGrowthOfThisFragment = bSizeOfThisFragment;
+          } else if (consumedBSize + bSizeOfThisFragment >= unfragmentedBSize) {
+            // The item's block-size just grows in the current fragment to
+            // exceed the unfragmented block-size.
+            bSizeGrowthOfThisFragment =
+                consumedBSize + bSizeOfThisFragment - unfragmentedBSize;
+          }
+
+          if (aAxisTracker.IsRowOriented()) {
+            if (&line == &startmostLine) {
+              bAxisMetrics.mBEndEdgeShift = std::max(
+                  bAxisMetrics.mBEndEdgeShift, bSizeGrowthOfThisFragment);
+            }
+          } else {
+            MOZ_ASSERT(aAxisTracker.IsColumnOriented());
+            if (isSingleLine) {
+              if (&item == startmostItem) {
+                MOZ_ASSERT(bAxisMetrics.mBEndEdgeShift == 0,
+                           "The item's frame is a continuation, so it "
+                           "shouldn't shift!");
+                bAxisMetrics.mBEndEdgeShift = bSizeGrowthOfThisFragment;
+              }
+            } else {
+              // Bug 1806717: We need a more sophisticated solution for
+              // multi-line column-oriented flex container when each line has a
+              // different block-size growth value. For now, we don't deal with
+              // them.
+            }
+          }
+        }
+      }
+
+      // If the item has auto margins, and we were tracking the UsedMargin
+      // property, set the property to the computed margin values.
+      if (item.HasAnyAutoMargin()) {
+        nsMargin* propValue =
+            item.Frame()->GetProperty(nsIFrame::UsedMarginProperty());
+        if (propValue) {
+          *propValue = item.PhysicalMargin();
+        }
+      }
+    }
+
+    // Now we've finished processing all the items in the startmost line.
+    // Determine the amount by which the startmost line's block-end edge has
+    // shifted, so we can apply the same shift for the remaining lines.
+    if (GetPrevInFlow() && aAxisTracker.IsRowOriented() &&
+        &line == &startmostLine && bAxisMetrics.mMaxBEndEdge) {
+      auto& [before, after] = *bAxisMetrics.mMaxBEndEdge;
+      bAxisMetrics.mBEndEdgeShift =
+          std::max(bAxisMetrics.mBEndEdgeShift, after - before);
+    }
+  }
+
+  if (!aFlr.mPlaceholders.IsEmpty()) {
+    ReflowPlaceholders(aReflowInput, aFlr.mPlaceholders,
+                       containerContentBoxOrigin, aContainerSize);
+  }
+
+  nsReflowStatus childrenStatus;
+  if (!pushedItems.IsEmpty() || !incompleteItems.IsEmpty()) {
+    childrenStatus.SetIncomplete();
+  } else if (!overflowIncompleteItems.IsEmpty()) {
+    childrenStatus.SetOverflowIncomplete();
+  }
+  PushIncompleteChildren(pushedItems, incompleteItems, overflowIncompleteItems);
+
+  // TODO: Try making this a fatal assertion after we fix bug 1751260.
+  NS_ASSERTION(childrenStatus.IsFullyComplete() ||
+                   aAvailableSizeForItems.BSize(flexWM) != NS_UNCONSTRAINEDSIZE,
+               "We shouldn't have any incomplete children if the available "
+               "block-size is unconstrained!");
+
+  if (!pushedItems.IsEmpty()) {
+    AddStateBits(NS_STATE_FLEX_DID_PUSH_ITEMS);
+  }
+
+  if (GetPrevInFlow()) {
+    aFragmentData.mCumulativeBEndEdgeShift += bAxisMetrics.mBEndEdgeShift;
+  }
+
+  return {maxBlockEndEdgeOfChildren, childrenStatus};
+}
+
+void nsFlexContainerFrame::PopulateReflowOutput(
+    ReflowOutput& aReflowOutput, const ReflowInput& aReflowInput,
+    nsReflowStatus& aStatus, const LogicalSize& aContentBoxSize,
+    const LogicalMargin& aBorderPadding, const nscoord aConsumedBSize,
+    const bool aMayNeedNextInFlow, const nscoord aMaxBlockEndEdgeOfChildren,
+    const nsReflowStatus& aChildrenStatus,
+    const FlexboxAxisTracker& aAxisTracker, FlexLayoutResult& aFlr) {
+  const WritingMode flexWM = aReflowInput.GetWritingMode();
+
+  // Compute flex container's desired size (in its own writing-mode).
+  LogicalSize desiredSizeInFlexWM(flexWM);
+  desiredSizeInFlexWM.ISize(flexWM) =
+      aContentBoxSize.ISize(flexWM) + aBorderPadding.IStartEnd(flexWM);
+
+  // Unconditionally skip adding block-end border and padding for now. We add it
+  // lower down, after we've established baseline and decided whether bottom
+  // border-padding fits (if we're fragmented).
+  const nscoord effectiveContentBSizeWithBStartBP =
+      aContentBoxSize.BSize(flexWM) - aConsumedBSize +
+      aBorderPadding.BStart(flexWM);
+  nscoord blockEndContainerBP = aBorderPadding.BEnd(flexWM);
+
+  if (aMayNeedNextInFlow) {
+    // We assume our status should be reported as incomplete because we may need
+    // a next-in-flow.
+    bool isStatusIncomplete = true;
+
+    const nscoord availableBSizeMinusBEndBP =
+        aReflowInput.AvailableBSize() - aBorderPadding.BEnd(flexWM);
+
+    if (aMaxBlockEndEdgeOfChildren <= availableBSizeMinusBEndBP) {
+      // Consume all the available block-size.
+      desiredSizeInFlexWM.BSize(flexWM) = availableBSizeMinusBEndBP;
+    } else {
+      // This case happens if we have some tall unbreakable children exceeding
+      // the available block-size.
+      desiredSizeInFlexWM.BSize(flexWM) = std::min(
+          effectiveContentBSizeWithBStartBP, aMaxBlockEndEdgeOfChildren);
+
+      if ((aReflowInput.ComputedBSize() != NS_UNCONSTRAINEDSIZE ||
+           aChildrenStatus.IsFullyComplete()) &&
+          aMaxBlockEndEdgeOfChildren >= effectiveContentBSizeWithBStartBP) {
+        // We have some tall unbreakable child that's sticking off the end of
+        // our fragment, *and* forcing us to consume all of our remaining
+        // content block-size and call ourselves complete.
+        //
+        // - If we have a definite block-size: we get here if the tall child
+        //   makes us reach that block-size.
+        // - If we have a content-based block-size: we get here if the tall
+        //   child makes us reach the content-based block-size from a
+        //   theoretical unfragmented layout, *and* all our children are
+        //   complete. (Note that if we have some incomplete child, then we
+        //   instead prefer to return an incomplete status, so we can get a
+        //   next-in-flow to include that child's requested next-in-flow, in the
+        //   spirit of having a block-size that fits the content.)
+        //
+        // TODO: the auto-height case might need more subtlety; see bug 1828977.
+        isStatusIncomplete = false;
+
+        // We also potentially need to get the unskipped block-end border and
+        // padding (if we assumed it'd be skipped as part of our tentative
+        // assumption that we'd be incomplete).
+        if (aReflowInput.mStyleBorder->mBoxDecorationBreak ==
+            StyleBoxDecorationBreak::Slice) {
+          blockEndContainerBP =
+              aReflowInput.ComputedLogicalBorderPadding(flexWM).BEnd(flexWM);
+        }
+      }
+    }
+
+    if (isStatusIncomplete) {
+      aStatus.SetIncomplete();
+    }
+  } else {
+    // Our own effective content-box block-size can fit within the available
+    // block-size.
+    desiredSizeInFlexWM.BSize(flexWM) = effectiveContentBSizeWithBStartBP;
+  }
+
+  // Now, we account for how the block-end border and padding (if any) impacts
+  // our desired size. If adding it pushes us over the available block-size,
+  // then we become incomplete (unless we already weren't asking for any
+  // block-size, in which case we stay complete to avoid looping forever).
+  //
+  // NOTE: If we have auto block-size, we allow our block-end border and padding
+  // to push us over the available block-size without requesting a continuation,
+  // for consistency with the behavior of "display:block" elements.
+  const nscoord effectiveContentBSizeWithBStartEndBP =
+      desiredSizeInFlexWM.BSize(flexWM) + blockEndContainerBP;
+
+  if (aReflowInput.AvailableBSize() != NS_UNCONSTRAINEDSIZE &&
+      effectiveContentBSizeWithBStartEndBP > aReflowInput.AvailableBSize() &&
+      desiredSizeInFlexWM.BSize(flexWM) != 0 &&
+      aReflowInput.ComputedBSize() != NS_UNCONSTRAINEDSIZE) {
+    // We couldn't fit with the block-end border and padding included, so we'll
+    // need a continuation.
+    aStatus.SetIncomplete();
+
+    if (aReflowInput.mStyleBorder->mBoxDecorationBreak ==
+        StyleBoxDecorationBreak::Slice) {
+      blockEndContainerBP = 0;
+    }
+  }
+
+  // The variable "blockEndContainerBP" now accurately reflects how much (if
+  // any) block-end border and padding we want for this frame, so we can proceed
+  // to add it in.
+  desiredSizeInFlexWM.BSize(flexWM) += blockEndContainerBP;
+
+  if (aStatus.IsComplete() && !aChildrenStatus.IsFullyComplete()) {
+    aStatus.SetOverflowIncomplete();
+    aStatus.SetNextInFlowNeedsReflow();
+  }
+
+  // If we are the first-in-flow and not fully complete (either our block-size
+  // or any of our flex items cannot fit in the available block-size), and the
+  // style requires us to avoid breaking inside, set the status to prompt our
+  // parent to push us to the next page/column.
+  if (!GetPrevInFlow() && !aStatus.IsFullyComplete() &&
+      ShouldAvoidBreakInside(aReflowInput)) {
+    aStatus.SetInlineLineBreakBeforeAndReset();
+    return;
+  }
+
+  // If we haven't established a baseline for the container yet, i.e. if we
+  // don't have any flex item in the startmost flex line that participates in
+  // baseline alignment, then use the startmost flex item to derive the
+  // container's baseline.
+  if (const FlexLine& line = StartmostLine(aFlr.mLines, aAxisTracker);
+      aFlr.mAscent == nscoord_MIN && !line.IsEmpty()) {
+    const FlexItem& item = line.StartmostItem(aAxisTracker);
+    aFlr.mAscent = item.Frame()
+                       ->GetLogicalPosition(
+                           flexWM, desiredSizeInFlexWM.GetPhysicalSize(flexWM))
+                       .B(flexWM) +
+                   item.ResolvedAscent(true);
+  }
+
+  // Likewise, if we don't have any flex item in the endmost flex line that
+  // participates in last baseline alignment, then use the endmost flex item to
+  // derived the container's last baseline.
+  if (const FlexLine& line = EndmostLine(aFlr.mLines, aAxisTracker);
+      aFlr.mAscentForLast == nscoord_MIN && !line.IsEmpty()) {
+    const FlexItem& item = line.EndmostItem(aAxisTracker);
+    const nscoord lastAscent =
+        item.Frame()
+            ->GetLogicalPosition(flexWM,
+                                 desiredSizeInFlexWM.GetPhysicalSize(flexWM))
+            .B(flexWM) +
+        item.ResolvedAscent(false);
+
+    aFlr.mAscentForLast = desiredSizeInFlexWM.BSize(flexWM) - lastAscent;
+  }
+
+  if (aFlr.mAscent == nscoord_MIN) {
+    // Still don't have our baseline set -- this happens if we have no
+    // children, if our children are huge enough that they have nscoord_MIN
+    // as their baseline, or our content is hidden in which case, we'll use the
+    // wrong baseline (but no big deal).
+    NS_WARNING_ASSERTION(
+        HidesContentForLayout() || aFlr.mLines[0].IsEmpty(),
+        "Have flex items but didn't get an ascent - that's odd (or there are "
+        "just gigantic sizes involved)");
+    // Per spec, synthesize baseline from the flex container's content box
+    // (i.e. use block-end side of content-box)
+    // XXXdholbert This only makes sense if parent's writing mode is
+    // horizontal (& even then, really we should be using the BSize in terms
+    // of the parent's writing mode, not ours). Clean up in bug 1155322.
+    aFlr.mAscent = effectiveContentBSizeWithBStartBP;
+  }
+
+  if (aFlr.mAscentForLast == nscoord_MIN) {
+    // Still don't have our last baseline set -- this happens if we have no
+    // children, if our children are huge enough that they have nscoord_MIN
+    // as their baseline, or our content is hidden in which case, we'll use the
+    // wrong baseline (but no big deal).
+    NS_WARNING_ASSERTION(
+        HidesContentForLayout() || aFlr.mLines[0].IsEmpty(),
+        "Have flex items but didn't get an ascent - that's odd (or there are "
+        "just gigantic sizes involved)");
+    // Per spec, synthesize baseline from the flex container's content box
+    // (i.e. use block-end side of content-box)
+    // XXXdholbert This only makes sense if parent's writing mode is
+    // horizontal (& even then, really we should be using the BSize in terms
+    // of the parent's writing mode, not ours). Clean up in bug 1155322.
+    aFlr.mAscentForLast = blockEndContainerBP;
+  }
+
+  if (HasAnyStateBits(NS_STATE_FLEX_SYNTHESIZE_BASELINE)) {
+    // This will force our parent to call GetLogicalBaseline, which will
+    // synthesize a margin-box baseline.
+    aReflowOutput.SetBlockStartAscent(ReflowOutput::ASK_FOR_BASELINE);
+  } else {
+    // XXXdholbert aFlr.mAscent needs to be in terms of our parent's
+    // writing-mode here. See bug 1155322.
+    aReflowOutput.SetBlockStartAscent(aFlr.mAscent);
+  }
+
+  // Cache the container baselines so that our parent can baseline-align us.
+  mFirstBaseline = aFlr.mAscent;
+  mLastBaseline = aFlr.mAscentForLast;
+
+  // Convert flex container's final desired size to parent's WM, for outparam.
+  aReflowOutput.SetSize(flexWM, desiredSizeInFlexWM);
+}
+
+void nsFlexContainerFrame::MoveFlexItemToFinalPosition(
+    const FlexItem& aItem, const LogicalPoint& aFramePos,
+    const nsSize& aContainerSize) {
+  const WritingMode outerWM = aItem.ContainingBlockWM();
+  const nsStyleDisplay* display = aItem.Frame()->StyleDisplay();
+  LogicalPoint pos(aFramePos);
+  if (display->IsRelativelyOrStickyPositionedStyle()) {
+    // If the item is relatively positioned, look up its offsets (cached from
+    // previous reflow). A sticky positioned item can pass a dummy
+    // logicalOffsets into ApplyRelativePositioning().
+    LogicalMargin logicalOffsets(outerWM);
+    if (display->IsRelativelyPositionedStyle()) {
+      nsMargin* cachedOffsets =
+          aItem.Frame()->GetProperty(nsIFrame::ComputedOffsetProperty());
+      MOZ_ASSERT(
+          cachedOffsets,
+          "relpos previously-reflowed frame should've cached its offsets");
+      logicalOffsets = LogicalMargin(outerWM, *cachedOffsets);
+    }
+    ReflowInput::ApplyRelativePositioning(aItem.Frame(), outerWM,
+                                          logicalOffsets, &pos, aContainerSize);
+  }
+
+  FLEX_LOG("Moving flex item %p to its desired position %s", aItem.Frame(),
+           ToString(pos).c_str());
+  aItem.Frame()->SetPosition(outerWM, pos, aContainerSize);
+  PositionFrameView(aItem.Frame());
+  PositionChildViews(aItem.Frame());
+}
+
+nsReflowStatus nsFlexContainerFrame::ReflowFlexItem(
+    const FlexboxAxisTracker& aAxisTracker, const ReflowInput& aReflowInput,
+    const FlexItem& aItem, const LogicalPoint& aFramePos,
+    const LogicalSize& aAvailableSize, const nsSize& aContainerSize) {
+  FLEX_LOG("Doing final reflow for flex item %p", aItem.Frame());
+
+  // Returns true if we should use 'auto' in block axis's StyleSizeOverrides to
+  // allow fragmentation-imposed block-size growth.
+  auto ComputeBSizeOverrideWithAuto = [&]() {
+    if (!aReflowInput.IsInFragmentedContext()) {
+      return false;
+    }
+    if (aItem.Frame()->IsReplaced()) {
+      // Disallow fragmentation-imposed block-size growth for replaced elements
+      // since they are monolithic, and cannot be fragmented.
+      return false;
+    }
+    if (aItem.HasAspectRatio()) {
+      // Aspect-ratio's automatic content-based minimum size doesn't work
+      // properly in a fragmented context (Bug 1868284) when we use 'auto'
+      // block-size to apply the fragmentation-imposed block-size growth.
+      // Disable it for now so that items with aspect-ratios can still use their
+      // known block-sizes (from flex layout algorithm) in final reflow.
+      return false;
+    }
+    if (aItem.IsBlockAxisMainAxis()) {
+      if (aItem.IsFlexBaseSizeContentBSize()) {
+        // The flex item resolved its indefinite flex-basis to the content
+        // block-size.
+        if (aItem.IsMainMinSizeContentBSize()) {
+          // The item's flex base size and main min-size are both content
+          // block-size. We interpret this content-based block-size as
+          // permission to apply fragmentation-imposed block-size growth.
+          return true;
+        }
+        if (aReflowInput.ComputedBSize() == NS_UNCONSTRAINEDSIZE) {
+          // The flex container has an indefinite block-size. We allow the
+          // item's to apply fragmentation-imposed block-size growth.
+          return true;
+        }
+      }
+      return false;
+    }
+
+    MOZ_ASSERT(aItem.IsBlockAxisCrossAxis());
+    MOZ_ASSERT(aItem.IsStretched(),
+               "No need to override block-size with 'auto' if the item is not "
+               "stretched in the cross axis!");
+
+    Maybe<nscoord> measuredBSize = aItem.MeasuredBSize();
+    if (measuredBSize && aItem.CrossSize() == *measuredBSize) {
+      // The item has a measured content-based block-size due to having an
+      // indefinite cross-size. If its cross-size is equal to the content-based
+      // block-size, then it is the tallest item that established the cross-size
+      // of the flex line. We allow it apply fragmentation-imposed block-size
+      // growth.
+      //
+      // Note: We only allow the tallest item to grow because it is likely to
+      // have the most impact on the overall flex container block-size growth.
+      // This is not a perfect solution since other shorter items in the same
+      // line might also have fragmentation-imposed block-size growth, but
+      // currently there is no reliable way to detect whether they will outgrow
+      // the tallest item.
+      return true;
+    }
+    return false;
+  };
+
+  StyleSizeOverrides sizeOverrides;
+  bool overrideBSizeWithAuto = false;
+
+  // Override flex item's main size.
+  if (aItem.IsInlineAxisMainAxis()) {
+    sizeOverrides.mStyleISize.emplace(aItem.StyleMainSize());
+    FLEX_LOGV(" Main size (inline-size) override: %d", aItem.MainSize());
+  } else {
+    overrideBSizeWithAuto = ComputeBSizeOverrideWithAuto();
+    if (overrideBSizeWithAuto) {
+      sizeOverrides.mStyleBSize.emplace(StyleSize::Auto());
+      FLEX_LOGV(" Main size (block-size) override: Auto");
+    } else {
+      sizeOverrides.mStyleBSize.emplace(aItem.StyleMainSize());
+      FLEX_LOGV(" Main size (block-size) override: %d", aItem.MainSize());
+    }
+  }
+
+  // Override flex item's cross size if it was stretched in the cross axis (in
+  // which case we're imposing a cross size).
+  if (aItem.IsStretched()) {
+    if (aItem.IsInlineAxisCrossAxis()) {
+      sizeOverrides.mStyleISize.emplace(aItem.StyleCrossSize());
+      FLEX_LOGV(" Cross size (inline-size) override: %d", aItem.CrossSize());
+    } else {
+      overrideBSizeWithAuto = ComputeBSizeOverrideWithAuto();
+      if (overrideBSizeWithAuto) {
+        sizeOverrides.mStyleBSize.emplace(StyleSize::Auto());
+        FLEX_LOGV(" Cross size (block-size) override: Auto");
+      } else {
+        sizeOverrides.mStyleBSize.emplace(aItem.StyleCrossSize());
+        FLEX_LOGV(" Cross size (block-size) override: %d", aItem.CrossSize());
+      }
+    }
+  }
+  if (sizeOverrides.mStyleBSize) {
+    // We are overriding the block-size. For robustness, we always assume that
+    // this represents a block-axis resize for the frame. This may be
+    // conservative, but we do capture all the conditions in the block-axis
+    // (checked in NeedsFinalReflow()) that make this item require a final
+    // reflow. This sets relevant flags in ReflowInput::InitResizeFlags().
+    aItem.Frame()->SetHasBSizeChange(true);
+  }
+
+  ReflowInput childReflowInput(PresContext(), aReflowInput, aItem.Frame(),
+                               aAvailableSize, Nothing(), {}, sizeOverrides,
+                               {ComputeSizeFlag::ShrinkWrap});
+  if (overrideBSizeWithAuto) {
+    // If we use 'auto' to override the item's block-size, set the item's
+    // original block-size to min-size as a lower bound.
+    childReflowInput.SetComputedMinBSize(aItem.BSize());
+
+    // Set the item's block-size as the percentage basis so that its children
+    // can resolve percentage sizes correctly.
+    childReflowInput.SetPercentageBasisInBlockAxis(aItem.BSize());
+  }
+
+  if (aItem.TreatBSizeAsIndefinite() && aItem.IsBlockAxisMainAxis()) {
+    childReflowInput.mFlags.mTreatBSizeAsIndefinite = true;
+  }
+
+  if (aItem.IsStretched() && aItem.IsBlockAxisCrossAxis()) {
+    // This item is stretched (in the cross axis), and that axis is its block
+    // axis.  That stretching effectively gives it a relative BSize.
+    // XXXdholbert This flag only makes a difference if we use the flex items'
+    // frame-state when deciding whether to reflow them -- and we don't, as of
+    // the changes in bug 851607. So this has no effect right now, but it might
+    // make a difference if we optimize to use dirty bits in the
+    // future. (Reftests flexbox-resizeviewport-1.xhtml and -2.xhtml are
+    // intended to catch any regressions here, if we end up relying on this bit
+    // & neglecting to set it.)
+    aItem.Frame()->AddStateBits(NS_FRAME_CONTAINS_RELATIVE_BSIZE);
+  }
+
+  // NOTE: Be very careful about doing anything else with childReflowInput
+  // after this point, because some of its methods (e.g. SetComputedWidth)
+  // internally call InitResizeFlags and stomp on mVResize & mHResize.
+
+  FLEX_LOG("Reflowing flex item %p at its desired position %s", aItem.Frame(),
+           ToString(aFramePos).c_str());
+
+  // CachedFlexItemData is stored in item's writing mode, so we pass
+  // aChildReflowInput into ReflowOutput's constructor.
+  ReflowOutput childReflowOutput(childReflowInput);
+  nsReflowStatus childReflowStatus;
+  WritingMode outerWM = aReflowInput.GetWritingMode();
+  ReflowChild(aItem.Frame(), PresContext(), childReflowOutput, childReflowInput,
+              outerWM, aFramePos, aContainerSize, ReflowChildFlags::Default,
+              childReflowStatus);
+
+  // XXXdholbert Perhaps we should call CheckForInterrupt here; see bug 1495532.
+
+  FinishReflowChild(aItem.Frame(), PresContext(), childReflowOutput,
+                    &childReflowInput, outerWM, aFramePos, aContainerSize,
+                    ReflowChildFlags::ApplyRelativePositioning);
+
+  aItem.SetAscent(childReflowOutput.BlockStartAscent());
+
+  // Update our cached flex item info:
+  if (auto* cached = aItem.Frame()->GetProperty(CachedFlexItemData::Prop())) {
+    cached->Update(childReflowInput, childReflowOutput,
+                   FlexItemReflowType::Final);
+  } else {
+    cached = new CachedFlexItemData(childReflowInput, childReflowOutput,
+                                    FlexItemReflowType::Final);
+    aItem.Frame()->SetProperty(CachedFlexItemData::Prop(), cached);
+  }
+
+  return childReflowStatus;
+}
+
+void nsFlexContainerFrame::ReflowPlaceholders(
+    const ReflowInput& aReflowInput, nsTArray<nsIFrame*>& aPlaceholders,
+    const LogicalPoint& aContentBoxOrigin, const nsSize& aContainerSize) {
+  WritingMode outerWM = aReflowInput.GetWritingMode();
+
+  // As noted in this method's documentation, we'll reflow every entry in
+  // |aPlaceholders| at the container's content-box origin.
+  for (nsIFrame* placeholder : aPlaceholders) {
+    MOZ_ASSERT(placeholder->IsPlaceholderFrame(),
+               "placeholders array should only contain placeholder frames");
+    WritingMode wm = placeholder->GetWritingMode();
+    LogicalSize availSize = aReflowInput.ComputedSize(wm);
+    ReflowInput childReflowInput(PresContext(), aReflowInput, placeholder,
+                                 availSize);
+    // No need to set the -webkit-line-clamp related flags when reflowing
+    // a placeholder.
+    ReflowOutput childReflowOutput(outerWM);
+    nsReflowStatus childReflowStatus;
+    ReflowChild(placeholder, PresContext(), childReflowOutput, childReflowInput,
+                outerWM, aContentBoxOrigin, aContainerSize,
+                ReflowChildFlags::Default, childReflowStatus);
+
+    FinishReflowChild(placeholder, PresContext(), childReflowOutput,
+                      &childReflowInput, outerWM, aContentBoxOrigin,
+                      aContainerSize, ReflowChildFlags::Default);
+
+    // Mark the placeholder frame to indicate that it's not actually at the
+    // element's static position, because we need to apply CSS Alignment after
+    // we determine the OOF's size:
+    placeholder->AddStateBits(PLACEHOLDER_STATICPOS_NEEDS_CSSALIGN);
+  }
+}
+
+nscoord nsFlexContainerFrame::IntrinsicISize(gfxContext* aRenderingContext,
+                                             IntrinsicISizeType aType) {
+  nscoord containerISize = 0;
+  const nsStylePosition* stylePos = StylePosition();
+  const FlexboxAxisTracker axisTracker(this);
+
+  nscoord mainGapSize;
+  if (axisTracker.IsRowOriented()) {
+    mainGapSize = nsLayoutUtils::ResolveGapToLength(stylePos->mColumnGap,
+                                                    NS_UNCONSTRAINEDSIZE);
+  } else {
+    mainGapSize = nsLayoutUtils::ResolveGapToLength(stylePos->mRowGap,
+                                                    NS_UNCONSTRAINEDSIZE);
+  }
+
+  const bool useMozBoxCollapseBehavior =
+      StyleVisibility()->UseLegacyCollapseBehavior();
+
+  // The loop below sets aside space for a gap before each item besides the
+  // first. This bool helps us handle that special-case.
+  bool onFirstChild = true;
+
+  for (nsIFrame* childFrame : mFrames) {
+    // Skip out-of-flow children because they don't participate in flex layout.
+    if (childFrame->IsPlaceholderFrame()) {
+      continue;
+    }
+
+    if (useMozBoxCollapseBehavior &&
+        childFrame->StyleVisibility()->IsCollapse()) {
+      // If we're using legacy "visibility:collapse" behavior, then we don't
+      // care about the sizes of any collapsed children.
+      continue;
+    }
+
+    nscoord childISize = nsLayoutUtils::IntrinsicForContainer(
+        aRenderingContext, childFrame, aType);
+
+    // * For a row-oriented single-line flex container, the intrinsic
+    // {min/pref}-isize is the sum of its items' {min/pref}-isizes and
+    // (n-1) column gaps.
+    // * For a column-oriented flex container, the intrinsic min isize
+    // is the max of its items' min isizes.
+    // * For a row-oriented multi-line flex container, the intrinsic
+    // pref isize is former (sum), and its min isize is the latter (max).
+    bool isSingleLine = (StyleFlexWrap::Nowrap == stylePos->mFlexWrap);
+    if (axisTracker.IsRowOriented() &&
+        (isSingleLine || aType == IntrinsicISizeType::PrefISize)) {
+      containerISize += childISize;
+      if (!onFirstChild) {
+        containerISize += mainGapSize;
+      }
+      onFirstChild = false;
+    } else {  // (col-oriented, or MinISize for multi-line row flex container)
+      containerISize = std::max(containerISize, childISize);
+    }
+  }
+
+  return containerISize;
+}
+
+/* virtual */
+nscoord nsFlexContainerFrame::GetMinISize(gfxContext* aRenderingContext) {
+  DISPLAY_MIN_INLINE_SIZE(this, mCachedMinISize);
+  if (mCachedMinISize == NS_INTRINSIC_ISIZE_UNKNOWN) {
+    if (Maybe<nscoord> containISize = ContainIntrinsicISize()) {
+      mCachedMinISize = *containISize;
+    } else {
+      mCachedMinISize =
+          IntrinsicISize(aRenderingContext, IntrinsicISizeType::MinISize);
+    }
+  }
+
+  return mCachedMinISize;
+}
+
+/* virtual */
+nscoord nsFlexContainerFrame::GetPrefISize(gfxContext* aRenderingContext) {
+  DISPLAY_PREF_INLINE_SIZE(this, mCachedPrefISize);
+  if (mCachedPrefISize == NS_INTRINSIC_ISIZE_UNKNOWN) {
+    if (Maybe<nscoord> containISize = ContainIntrinsicISize()) {
+      mCachedPrefISize = *containISize;
+    } else {
+      mCachedPrefISize =
+          IntrinsicISize(aRenderingContext, IntrinsicISizeType::PrefISize);
+    }
+  }
+
+  return mCachedPrefISize;
+}
+
+int32_t nsFlexContainerFrame::GetNumLines() const {
+  // TODO(emilio, bug 1793251): Treating all row oriented frames as single-lines
+  // might not be great for flex-wrap'd containers, consider trying to do
+  // better? We probably would need to persist more stuff than we do after
+  // layout.
+  return FlexboxAxisInfo(this).mIsRowOriented ? 1 : mFrames.GetLength();
+}
+
+bool nsFlexContainerFrame::IsLineIteratorFlowRTL() {
+  FlexboxAxisInfo info(this);
+  if (info.mIsRowOriented) {
+    const bool isRtl = StyleVisibility()->mDirection == StyleDirection::Rtl;
+    return info.mIsMainAxisReversed != isRtl;
+  }
+  return false;
+}
+
+Result<nsILineIterator::LineInfo, nsresult> nsFlexContainerFrame::GetLine(
+    int32_t aLineNumber) {
+  if (aLineNumber < 0 || aLineNumber >= GetNumLines()) {
+    return Err(NS_ERROR_FAILURE);
+  }
+  FlexboxAxisInfo info(this);
+  LineInfo lineInfo;
+  if (info.mIsRowOriented) {
+    lineInfo.mLineBounds = GetRect();
+    lineInfo.mFirstFrameOnLine = mFrames.FirstChild();
+    // This isn't quite ideal for multi-line row flexbox, see bug 1793251.
+    lineInfo.mNumFramesOnLine = mFrames.GetLength();
+  } else {
+    // TODO(emilio, bug 1793322): Deal with column-reverse (mIsMainAxisReversed)
+    nsIFrame* f = mFrames.FrameAt(aLineNumber);
+    lineInfo.mLineBounds = f->GetRect();
+    lineInfo.mFirstFrameOnLine = f;
+    lineInfo.mNumFramesOnLine = 1;
+  }
+  return lineInfo;
+}
+
+int32_t nsFlexContainerFrame::FindLineContaining(nsIFrame* aFrame,
+                                                 int32_t aStartLine) {
+  const int32_t index = mFrames.IndexOf(aFrame);
+  if (index < 0) {
+    return -1;
+  }
+  const FlexboxAxisInfo info(this);
+  if (info.mIsRowOriented) {
+    return 0;
+  }
+  if (index < aStartLine) {
+    return -1;
+  }
+  return index;
+}
+
+NS_IMETHODIMP
+nsFlexContainerFrame::CheckLineOrder(int32_t aLine, bool* aIsReordered,
+                                     nsIFrame** aFirstVisual,
+                                     nsIFrame** aLastVisual) {
+  *aIsReordered = false;
+  *aFirstVisual = nullptr;
+  *aLastVisual = nullptr;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsFlexContainerFrame::FindFrameAt(int32_t aLineNumber, nsPoint aPos,
+                                  nsIFrame** aFrameFound,
+                                  bool* aPosIsBeforeFirstFrame,
+                                  bool* aPosIsAfterLastFrame) {
+  const auto wm = GetWritingMode();
+  const LogicalPoint pos(wm, aPos, GetSize());
+  const FlexboxAxisInfo info(this);
+
+  *aFrameFound = nullptr;
+  *aPosIsBeforeFirstFrame = true;
+  *aPosIsAfterLastFrame = false;
+
+  if (!info.mIsRowOriented) {
+    nsIFrame* f = mFrames.FrameAt(aLineNumber);
+    if (!f) {
+      return NS_OK;
+    }
+
+    auto rect = f->GetLogicalRect(wm, GetSize());
+    *aFrameFound = f;
+    *aPosIsBeforeFirstFrame = pos.I(wm) < rect.IStart(wm);
+    *aPosIsAfterLastFrame = pos.I(wm) > rect.IEnd(wm);
+    return NS_OK;
+  }
+
+  LineFrameFinder finder(aPos, GetSize(), GetWritingMode(),
+                         IsLineIteratorFlowRTL());
+  for (nsIFrame* f : mFrames) {
+    finder.Scan(f);
+    if (finder.IsDone()) {
+      break;
+    }
+  }
+  finder.Finish(aFrameFound, aPosIsBeforeFirstFrame, aPosIsAfterLastFrame);
+  return NS_OK;
+}