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diff --git a/layout/generic/nsSplittableFrame.h b/layout/generic/nsSplittableFrame.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/. */
+
+/*
+ * base class for rendering objects that can be split across lines,
+ * columns, or pages
+ */
+
+#ifndef nsSplittableFrame_h___
+#define nsSplittableFrame_h___
+
+#include "mozilla/Attributes.h"
+#include "nsIFrame.h"
+
+// Derived class that allows splitting
+class nsSplittableFrame : public nsIFrame {
+ public:
+  NS_DECL_ABSTRACT_FRAME(nsSplittableFrame)
+
+  void Init(nsIContent* aContent, nsContainerFrame* aParent,
+            nsIFrame* aPrevInFlow) override;
+
+  void Destroy(DestroyContext&) override;
+
+  /*
+   * Frame continuations can be either fluid or non-fluid.
+   *
+   * Fluid continuations ("in-flows") are the result of line breaking,
+   * column breaking, or page breaking.
+   *
+   * Non-fluid continuations can be the result of BiDi frame splitting,
+   * column-span splitting, or <col span="N"> where N > 1.
+   *
+   * A "flow" is a chain of fluid continuations.
+   *
+   * For more information, see https://wiki.mozilla.org/Gecko:Continuation_Model
+   */
+
+  // Get the previous/next continuation, regardless of its type (fluid or
+  // non-fluid).
+  nsIFrame* GetPrevContinuation() const final;
+  nsIFrame* GetNextContinuation() const final;
+
+  // Set a previous/next non-fluid continuation.
+  void SetPrevContinuation(nsIFrame*) final;
+  void SetNextContinuation(nsIFrame*) final;
+
+  // Get the first/last continuation for this frame.
+  nsIFrame* FirstContinuation() const override;
+  nsIFrame* LastContinuation() const final;
+
+#ifdef DEBUG
+  // Can aFrame2 be reached from aFrame1 by following prev/next continuations?
+  static bool IsInPrevContinuationChain(nsIFrame* aFrame1, nsIFrame* aFrame2);
+  static bool IsInNextContinuationChain(nsIFrame* aFrame1, nsIFrame* aFrame2);
+#endif
+
+  // Get the previous/next continuation, only if it is fluid (an "in-flow").
+  nsIFrame* GetPrevInFlow() const final;
+  nsIFrame* GetNextInFlow() const final;
+
+  // Set a previous/next fluid continuation.
+  void SetPrevInFlow(nsIFrame*) final;
+  void SetNextInFlow(nsIFrame*) final;
+
+  // Get the first/last frame in the current flow.
+  nsIFrame* FirstInFlow() const final;
+  nsIFrame* LastInFlow() const final;
+
+  // Remove the frame from the flow. Connects the frame's prev-in-flow
+  // and its next-in-flow. This should only be called in frame Destroy()
+  // methods.
+  static void RemoveFromFlow(nsIFrame* aFrame);
+
+ protected:
+  nsSplittableFrame(ComputedStyle* aStyle, nsPresContext* aPresContext,
+                    ClassID aID)
+      : nsIFrame(aStyle, aPresContext, aID),
+        mPrevContinuation(nullptr),
+        mNextContinuation(nullptr) {}
+
+  /**
+   * Return the sum of the block-axis content size of our previous
+   * continuations.
+   *
+   * Classes that call this are _required_ to call this at least once for each
+   * reflow (unless you're the first continuation, in which case you can skip
+   * it, because as an optimization we don't cache it there).
+   *
+   * This guarantees that the internal cache works, by refreshing it. Calling it
+   * multiple times in the same reflow is wasteful, but not an error.
+   */
+  nscoord CalcAndCacheConsumedBSize();
+
+  /**
+   * Retrieve the effective computed block size of this frame, which is the
+   * computed block size, minus the block size consumed by any previous
+   * continuations.
+   */
+  nscoord GetEffectiveComputedBSize(const ReflowInput& aReflowInput,
+                                    nscoord aConsumed) const;
+
+  /**
+   * @see nsIFrame::GetLogicalSkipSides()
+   */
+  LogicalSides GetLogicalSkipSides() const override {
+    return GetBlockLevelLogicalSkipSides(true);
+  }
+
+  LogicalSides GetBlockLevelLogicalSkipSides(bool aAfterReflow) const;
+
+  /**
+   * A version of GetLogicalSkipSides() that is intended to be used inside
+   * Reflow before it's known if |this| frame will be COMPLETE or not.
+   * It returns a result that assumes this fragment is the last and thus
+   * should apply the block-end border/padding etc (except for "true" overflow
+   * containers which always skip block sides).  You're then expected to
+   * recalculate the block-end side (as needed) when you know |this| frame's
+   * reflow status is INCOMPLETE.
+   * This method is intended for frames that break in the block axis.
+   */
+  LogicalSides PreReflowBlockLevelLogicalSkipSides() const {
+    return GetBlockLevelLogicalSkipSides(false);
+  };
+
+  nsIFrame* mPrevContinuation;
+  nsIFrame* mNextContinuation;
+};
+
+#endif /* nsSplittableFrame_h___ */