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diff --git a/layout/generic/ColumnSetWrapperFrame.h b/layout/generic/ColumnSetWrapperFrame.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 https://mozilla.org/MPL/2.0/. */
+
+// A frame for CSS multi-column layout that wraps nsColumnSetFrames and
+// column-span frames.
+
+#ifndef mozilla_ColumnSetWrapperFrame_h
+#define mozilla_ColumnSetWrapperFrame_h
+
+#include "nsBlockFrame.h"
+
+namespace mozilla {
+
+class PresShell;
+
+// This class is a wrapper for nsColumnSetFrames and column-span frame.
+// Essentially, we divide the *original* nsColumnSetFrame into multiple
+// nsColumnSetFrames on the basis of the number and position of spanning
+// elements.
+//
+// This wrapper is necessary for implementing column-span as it allows us to
+// maintain each nsColumnSetFrame as an independent set of columns, and each
+// column-span element then becomes just a block level element.
+//
+class ColumnSetWrapperFrame final : public nsBlockFrame {
+ public:
+ NS_DECL_FRAMEARENA_HELPERS(ColumnSetWrapperFrame)
+ NS_DECL_QUERYFRAME
+
+ friend nsBlockFrame* ::NS_NewColumnSetWrapperFrame(
+ mozilla::PresShell* aPresShell, ComputedStyle* aStyle,
+ nsFrameState aStateFlags);
+
+ void Init(nsIContent* aContent, nsContainerFrame* aParent,
+ nsIFrame* aPrevInFlow) override;
+
+ nsContainerFrame* GetContentInsertionFrame() override;
+
+ void AppendDirectlyOwnedAnonBoxes(nsTArray<OwnedAnonBox>& aResult) override;
+
+#ifdef DEBUG_FRAME_DUMP
+ nsresult GetFrameName(nsAString& aResult) const override;
+#endif
+
+ void AppendFrames(ChildListID aListID, nsFrameList&& aFrameList) override;
+
+ void InsertFrames(ChildListID aListID, nsIFrame* aPrevFrame,
+ const nsLineList::iterator* aPrevFrameLine,
+ nsFrameList&& aFrameList) override;
+
+ void RemoveFrame(DestroyContext&, ChildListID, nsIFrame*) override;
+
+ void MarkIntrinsicISizesDirty() override;
+
+ nscoord GetMinISize(gfxContext* aRenderingContext) override;
+
+ nscoord GetPrefISize(gfxContext* aRenderingContext) override;
+
+ Maybe<nscoord> GetNaturalBaselineBOffset(
+ WritingMode aWM, BaselineSharingGroup aBaselineGroup,
+ BaselineExportContext aExportContext) const override;
+
+ private:
+ explicit ColumnSetWrapperFrame(ComputedStyle* aStyle,
+ nsPresContext* aPresContext);
+ ~ColumnSetWrapperFrame() override = default;
+
+#ifdef DEBUG
+ static void AssertColumnSpanWrapperSubtreeIsSane(const nsIFrame* aFrame);
+
+ // True if frame constructor has finished building this frame and all of
+ // its descendants.
+ bool mFinishedBuildingColumns = false;
+#endif
+
+ template <typename Iterator>
+ Maybe<nscoord> GetBaselineBOffset(Iterator aStart, Iterator aEnd,
+ WritingMode aWM,
+ BaselineSharingGroup aBaselineGroup,
+ BaselineExportContext aExportContext) const;
+};
+
+} // namespace mozilla
+
+#endif // mozilla_ColumnSetWrapperFrame_h