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diff --git a/layout/generic/nsPageSequenceFrame.h b/layout/generic/nsPageSequenceFrame.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/. */
+#ifndef nsPageSequenceFrame_h___
+#define nsPageSequenceFrame_h___
+
+#include "mozilla/Attributes.h"
+#include "mozilla/UniquePtr.h"
+#include "nsContainerFrame.h"
+#include "nsIPrintSettings.h"
+
+namespace mozilla {
+
+class PresShell;
+class PrintedSheetFrame;
+
+namespace dom {
+
+class HTMLCanvasElement;
+
+}  // namespace dom
+}  // namespace mozilla
+
+//-----------------------------------------------
+// This class is used to manage some static data about the layout
+// characteristics of our various "Pages Per Sheet" options.
+struct nsPagesPerSheetInfo {
+  static const nsPagesPerSheetInfo& LookupInfo(int32_t aPPS);
+
+  uint16_t mNumPages;
+
+  // This is the larger of the row-count vs. column-count for this layout
+  // (if they aren't the same). We'll aim to stack this number of pages
+  // in the sheet's longer axis.
+  uint16_t mLargerNumTracks;
+};
+
+/**
+ * This class maintains various shared data that is used by printing-related
+ * frames. The nsPageSequenceFrame strongly owns an instance of this class,
+ * which lives for as long as the nsPageSequenceFrame does.
+ */
+class nsSharedPageData {
+ public:
+  nsString mDateTimeStr;
+  nsString mPageNumFormat;
+  nsString mPageNumAndTotalsFormat;
+  nsString mDocTitle;
+  nsString mDocURL;
+  nsFont mHeadFootFont;
+
+  // Total number of pages (populated by PrintedSheetFrame when it determines
+  // that it's reflowed the final page):
+  int32_t mRawNumPages = 0;
+
+  // If there's more than one page-range, then its components are stored here
+  // as pairs of (start,end).  They're stored in the order provided (not
+  // necessarily in ascending order).
+  nsTArray<int32_t> mPageRanges;
+
+  // Margin for headers and footers; it defaults to 4/100 of an inch on UNIX
+  // and 0 elsewhere; I think it has to do with some inconsistency in page size
+  // computations
+  nsMargin mEdgePaperMargin;
+
+  nsCOMPtr<nsIPrintSettings> mPrintSettings;
+
+  // The scaling ratio we need to apply to make all pages fit horizontally. It's
+  // the minimum "ComputedWidth / OverflowWidth" ratio of all page content
+  // frames that overflowed.  It's 1.0 if none overflowed horizontally.
+  float mShrinkToFitRatio = 1.0f;
+
+  // Lazy getter, to look up our pages-per-sheet info based on mPrintSettings
+  // (if it's available).  The result is stored in our mPagesPerSheetInfo
+  // member-var to speed up subsequent lookups.
+  // This API is infallible; in failure cases, it just returns the info struct
+  // that corresponds to 1 page per sheet.
+  const nsPagesPerSheetInfo* PagesPerSheetInfo();
+
+ private:
+  const nsPagesPerSheetInfo* mPagesPerSheetInfo = nullptr;
+};
+
+// Page sequence frame class. Manages a series of pages, in paginated mode.
+// (Strictly speaking, this frame's direct children are PrintedSheetFrame
+// instances, and each of those will usually contain one nsPageFrame, depending
+// on the "pages-per-sheet" setting and whether the print operation is
+// restricted to a custom page range.)
+class nsPageSequenceFrame final : public nsContainerFrame {
+  using LogicalSize = mozilla::LogicalSize;
+
+ public:
+  friend nsPageSequenceFrame* NS_NewPageSequenceFrame(
+      mozilla::PresShell* aPresShell, ComputedStyle* aStyle);
+
+  NS_DECL_QUERYFRAME
+  NS_DECL_FRAMEARENA_HELPERS(nsPageSequenceFrame)
+
+  // nsIFrame
+  void Reflow(nsPresContext* aPresContext, ReflowOutput& aReflowOutput,
+              const ReflowInput& aReflowInput,
+              nsReflowStatus& aStatus) override;
+
+  void BuildDisplayList(nsDisplayListBuilder* aBuilder,
+                        const nsDisplayListSet& aLists) override;
+
+  // For Shrink To Fit
+  float GetSTFPercent() const { return mPageData->mShrinkToFitRatio; }
+
+  // Gets the final print preview scale that we're applying to the previewed
+  // sheets of paper.
+  float GetPrintPreviewScale() const;
+
+  // Async Printing
+  nsresult StartPrint(nsPresContext* aPresContext,
+                      nsIPrintSettings* aPrintSettings,
+                      const nsAString& aDocTitle, const nsAString& aDocURL);
+  nsresult PrePrintNextSheet(nsITimerCallback* aCallback, bool* aDone);
+  nsresult PrintNextSheet();
+  void ResetPrintCanvasList();
+
+  uint32_t GetCurrentSheetIdx() const { return mCurrentSheetIdx; }
+
+  int32_t GetRawNumPages() const { return mPageData->mRawNumPages; }
+
+  uint32_t GetPagesInFirstSheet() const;
+
+  nsresult DoPageEnd();
+
+  ComputeTransformFunction GetTransformGetter() const override;
+
+#ifdef DEBUG_FRAME_DUMP
+  nsresult GetFrameName(nsAString& aResult) const override;
+#endif
+
+ protected:
+  nsPageSequenceFrame(ComputedStyle*, nsPresContext*);
+  virtual ~nsPageSequenceFrame();
+
+  void SetPageNumberFormat(const char* aPropName, const char* aDefPropVal,
+                           bool aPageNumOnly);
+
+  // SharedPageData Helper methods
+  void SetDateTimeStr(const nsAString& aDateTimeStr);
+  void SetPageNumberFormat(const nsAString& aFormatStr, bool aForPageNumOnly);
+
+  // Print scaling is applied in this function.
+  void PopulateReflowOutput(ReflowOutput&, const ReflowInput&);
+
+  // Helper function to compute the offset needed to center a child
+  // page-frame's margin-box inside our content-box.
+  nscoord ComputeCenteringMargin(nscoord aContainerContentBoxWidth,
+                                 nscoord aChildPaddingBoxWidth,
+                                 const nsMargin& aChildPhysicalMargin);
+
+  mozilla::PrintedSheetFrame* GetCurrentSheetFrame();
+
+  nsSize mSize;
+
+  // These next two LogicalSize members are used when we're in print-preview to
+  // ensure that each previewed sheet will fit in the print-preview scrollport:
+  // -------
+
+  // Each component of this LogicalSize represents the maximum length of all
+  // our print-previewed sheets in that axis, plus a little extra for the
+  // print-preview margin.  Note that this LogicalSize doesn't necessarily
+  // correspond to any one particular sheet's size (especially if our sheets
+  // have different sizes), since the components are tracked independently such
+  // that we end up storing the maximum in each dimension.
+  LogicalSize mMaxSheetSize;
+  // The size of the scrollport where we're print-previewing sheets.
+  LogicalSize mScrollportSize;
+
+  // Data shared by all the nsPageFrames:
+  mozilla::UniquePtr<nsSharedPageData> mPageData;
+
+  // The zero-based index of the PrintedSheetFrame child that is being printed
+  // (or about-to-be-printed), in an async print operation.
+  // This is an index into our PrincipalChildList, effectively.
+  uint32_t mCurrentSheetIdx = 0;
+
+  nsTArray<RefPtr<mozilla::dom::HTMLCanvasElement> > mCurrentCanvasList;
+
+  bool mCalledBeginPage;
+
+  bool mCurrentCanvasListSetup;
+};
+
+#endif /* nsPageSequenceFrame_h___ */