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diff --git a/layout/generic/nsAbsoluteContainingBlock.h b/layout/generic/nsAbsoluteContainingBlock.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/*
+ * code for managing absolutely positioned children of a rendering
+ * object that is a containing block for them
+ */
+
+#ifndef nsAbsoluteContainingBlock_h___
+#define nsAbsoluteContainingBlock_h___
+
+#include "nsFrameList.h"
+#include "nsIFrame.h"
+#include "mozilla/TypedEnumBits.h"
+
+class nsContainerFrame;
+class nsPresContext;
+
+/**
+ * This class contains the logic for being an absolute containing block.  This
+ * class is used within viewport frames (for frames representing content with
+ * fixed position) and blocks (for frames representing absolutely positioned
+ * content), since each set of frames is absolutely positioned with respect to
+ * its parent.
+ *
+ * There is no principal child list, just a named child list which contains
+ * the absolutely positioned frames (FrameChildListID::Absolute or
+ * FrameChildListID::Fixed).
+ *
+ * All functions include as the first argument the frame that is delegating
+ * the request.
+ */
+class nsAbsoluteContainingBlock {
+  using ReflowInput = mozilla::ReflowInput;
+
+ public:
+  explicit nsAbsoluteContainingBlock(mozilla::FrameChildListID aChildListID)
+#ifdef DEBUG
+      : mChildListID(aChildListID)
+#endif
+  {
+    MOZ_ASSERT(mChildListID == mozilla::FrameChildListID::Absolute ||
+                   mChildListID == mozilla::FrameChildListID::Fixed,
+               "should either represent position:fixed or absolute content");
+  }
+
+  const nsFrameList& GetChildList() const { return mAbsoluteFrames; }
+  void AppendChildList(nsTArray<mozilla::FrameChildList>* aLists,
+                       mozilla::FrameChildListID aListID) const {
+    NS_ASSERTION(aListID == mChildListID, "wrong list ID");
+    GetChildList().AppendIfNonempty(aLists, aListID);
+  }
+
+  void SetInitialChildList(nsIFrame* aDelegatingFrame,
+                           mozilla::FrameChildListID aListID,
+                           nsFrameList&& aChildList);
+  void AppendFrames(nsIFrame* aDelegatingFrame,
+                    mozilla::FrameChildListID aListID,
+                    nsFrameList&& aFrameList);
+  void InsertFrames(nsIFrame* aDelegatingFrame,
+                    mozilla::FrameChildListID aListID, nsIFrame* aPrevFrame,
+                    nsFrameList&& aFrameList);
+  void RemoveFrame(mozilla::FrameDestroyContext&, mozilla::FrameChildListID,
+                   nsIFrame*);
+
+  enum class AbsPosReflowFlags {
+    ConstrainHeight = 0x1,
+    CBWidthChanged = 0x2,
+    CBHeightChanged = 0x4,
+    CBWidthAndHeightChanged = CBWidthChanged | CBHeightChanged,
+    IsGridContainerCB = 0x8,
+  };
+
+  /**
+   * Called by the delegating frame after it has done its reflow first. This
+   * function will reflow any absolutely positioned child frames that need to
+   * be reflowed, e.g., because the absolutely positioned child frame has
+   * 'auto' for an offset, or a percentage based width or height.
+   *
+   * @param aOverflowAreas, if non-null, is unioned with (in the local
+   * coordinate space) the overflow areas of the absolutely positioned
+   * children.
+   *
+   * @param aReflowStatus is assumed to be already-initialized, e.g. with the
+   * status of the delegating frame's main reflow. This function merges in the
+   * statuses of the absolutely positioned children's reflows.
+   *
+   * @param aFlags zero or more AbsPosReflowFlags
+   */
+  void Reflow(nsContainerFrame* aDelegatingFrame, nsPresContext* aPresContext,
+              const ReflowInput& aReflowInput, nsReflowStatus& aReflowStatus,
+              const nsRect& aContainingBlock, AbsPosReflowFlags aFlags,
+              mozilla::OverflowAreas* aOverflowAreas);
+
+  using DestroyContext = nsIFrame::DestroyContext;
+  void DestroyFrames(DestroyContext&);
+
+  bool HasAbsoluteFrames() const { return mAbsoluteFrames.NotEmpty(); }
+
+  /**
+   * Mark our size-dependent absolute frames with NS_FRAME_HAS_DIRTY_CHILDREN
+   * so that we'll make sure to reflow them.
+   */
+  void MarkSizeDependentFramesDirty();
+
+  /**
+   * Mark all our absolute frames with NS_FRAME_IS_DIRTY.
+   */
+  void MarkAllFramesDirty();
+
+ protected:
+  /**
+   * Returns true if the position of aFrame depends on the position of
+   * its placeholder or if the position or size of aFrame depends on a
+   * containing block dimension that changed.
+   */
+  bool FrameDependsOnContainer(nsIFrame* aFrame, bool aCBWidthChanged,
+                               bool aCBHeightChanged);
+
+  /**
+   * After an abspos child's size is known, this method can be used to
+   * resolve size-dependent values in the ComputedLogicalOffsets on its
+   * reflow input. (This may involve resolving the inline dimension of
+   * aLogicalCBSize, too; hence, that variable is an in/outparam.)
+   *
+   * aKidSize, aMargin, aOffsets, and aLogicalCBSize are all expected to be
+   * represented in terms of the absolute containing block's writing-mode.
+   */
+  void ResolveSizeDependentOffsets(nsPresContext* aPresContext,
+                                   ReflowInput& aKidReflowInput,
+                                   const mozilla::LogicalSize& aKidSize,
+                                   const mozilla::LogicalMargin& aMargin,
+                                   mozilla::LogicalMargin* aOffsets,
+                                   mozilla::LogicalSize* aLogicalCBSize);
+
+  /**
+   * For frames that have intrinsic block sizes, since we want to use the
+   * frame's actual instrinsic block-size, we don't compute margins in
+   * InitAbsoluteConstraints because the block-size isn't computed yet. This
+   * method computes the margins for them after layout.
+   * aMargin and aOffsets are both outparams (though we only touch aOffsets if
+   * the position is overconstrained)
+   */
+  void ResolveAutoMarginsAfterLayout(ReflowInput& aKidReflowInput,
+                                     const mozilla::LogicalSize* aLogicalCBSize,
+                                     const mozilla::LogicalSize& aKidSize,
+                                     mozilla::LogicalMargin& aMargin,
+                                     mozilla::LogicalMargin& aOffsets);
+
+  void ReflowAbsoluteFrame(nsIFrame* aDelegatingFrame,
+                           nsPresContext* aPresContext,
+                           const ReflowInput& aReflowInput,
+                           const nsRect& aContainingBlockRect,
+                           AbsPosReflowFlags aFlags, nsIFrame* aKidFrame,
+                           nsReflowStatus& aStatus,
+                           mozilla::OverflowAreas* aOverflowAreas);
+
+  /**
+   * Mark our absolute frames dirty.
+   * @param aMarkAllDirty if true, all will be marked with NS_FRAME_IS_DIRTY.
+   * Otherwise, the size-dependant ones will be marked with
+   * NS_FRAME_HAS_DIRTY_CHILDREN.
+   */
+  void DoMarkFramesDirty(bool aMarkAllDirty);
+
+ protected:
+  nsFrameList mAbsoluteFrames;  // additional named child list
+
+#ifdef DEBUG
+  mozilla::FrameChildListID const
+      mChildListID;  // FrameChildListID::Fixed or FrameChildListID::Absolute
+#endif
+};
+
+namespace mozilla {
+MOZ_MAKE_ENUM_CLASS_BITWISE_OPERATORS(
+    nsAbsoluteContainingBlock::AbsPosReflowFlags)
+}
+#endif /* nsnsAbsoluteContainingBlock_h___ */