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diff --git a/layout/generic/PrintedSheetFrame.h b/layout/generic/PrintedSheetFrame.h
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+++ b/layout/generic/PrintedSheetFrame.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/. */
+
+/* Rendering object for a printed or print-previewed sheet of paper */
+
+#ifndef LAYOUT_GENERIC_PRINTEDSHEETFRAME_H_
+#define LAYOUT_GENERIC_PRINTEDSHEETFRAME_H_
+
+#include "mozilla/gfx/Point.h"
+#include "nsContainerFrame.h"
+#include "nsHTMLParts.h"
+
+class nsSharedPageData;
+
+namespace mozilla {
+
+class PrintedSheetFrame final : public nsContainerFrame {
+ public:
+  using IntSize = mozilla::gfx::IntSize;
+
+  NS_DECL_QUERYFRAME
+  NS_DECL_FRAMEARENA_HELPERS(PrintedSheetFrame)
+
+  friend PrintedSheetFrame* ::NS_NewPrintedSheetFrame(
+      mozilla::PresShell* aPresShell, ComputedStyle* aStyle);
+
+  void SetSharedPageData(nsSharedPageData* aPD) { mPD = aPD; }
+
+  // XXX: this needs a better name, since it also updates style.
+  // Invokes MoveOverflowToChildList.
+  // This is intended for use by callers that need to be able to get our first/
+  // only nsPageFrame from our child list to examine its computed style just
+  // **prior** to us being reflowed. (If our first nsPageFrame will come from
+  // our prev-in-flow, we won't otherwise take ownership of it until we are
+  // reflowed.)
+  void ClaimPageFrameFromPrevInFlow();
+
+  // nsIFrame overrides
+  void Reflow(nsPresContext* aPresContext, ReflowOutput& aReflowOutput,
+              const ReflowInput& aReflowInput,
+              nsReflowStatus& aStatus) override;
+
+  void BuildDisplayList(nsDisplayListBuilder* aBuilder,
+                        const nsDisplayListSet& aLists) override;
+
+#ifdef DEBUG_FRAME_DUMP
+  nsresult GetFrameName(nsAString& aResult) const override;
+#endif
+
+  uint32_t GetNumPages() const { return mNumPages; }
+
+  // These methods provide information about the grid that pages should be
+  // placed into in the case that there are multiple pages-per-sheet.
+  uint32_t GetGridNumCols() const { return mGridNumCols; }
+  nsPoint GetGridOrigin() const { return mGridOrigin; }
+  nscoord GetGridCellWidth() const { return mGridCellWidth; }
+  nscoord GetGridCellHeight() const { return mGridCellHeight; }
+
+  nsSize ComputeSheetSize(const nsPresContext* aPresContext);
+
+  /**
+   * When we're printing one page-per-sheet and `page-orientation` on our
+   * single nsPageFrame child should cause the page to rotate, then we want to
+   * essentially rotate the sheet. We implement that by switching the
+   * dimensions of this sheet (changing its orientation), sizing the
+   * nsPageFrame to the original dimensions, and then applying the rotation to
+   * the nsPageFrame child.
+   *
+   * This returns the dimensions that this frame would have without any
+   * dimension swap we may have done to implement `page-orientation`. If
+   * there is no rotation caused by `page-orientation`, then the value returned
+   * and mRect.Size() are identical.
+   */
+  nsSize GetSizeForChildren() const { return mSizeForChildren; }
+
+  /**
+   * This method returns the dimensions of the physical page that the target
+   * [pseudo-]printer should create. This may be different from our own
+   * dimensions in the case where CSS `page-orientation` causes us to be
+   * rotated, but we only support that if the PrintTarget backend supports
+   * different page sizes/orientations. That's only the case for our Save-to-PDF
+   * backends (possibly other save-to-file outputs in future).
+   *
+   * The dimensions returned are expected to be passed to
+   * nsDeviceContext::BeginPage, which will pass them on to
+   * PrintTarget::BeginPage to use as the physical dimensions of the page.
+   */
+  IntSize GetPrintTargetSizeInPoints(
+      const int32_t aAppUnitsPerPhysicalInch) const;
+
+ private:
+  // Private construtor & destructor, to avoid accidental (non-FrameArena)
+  // instantiation/deletion:
+  PrintedSheetFrame(ComputedStyle* aStyle, nsPresContext* aPresContext)
+      : nsContainerFrame(aStyle, aPresContext, kClassID) {}
+  ~PrintedSheetFrame() = default;
+
+  // Helper function to populate some pages-per-sheet metrics in our
+  // nsSharedPageData.
+  // XXXjwatt: We should investigate sharing this function for the single
+  // page-per-sheet case (bug 1835782). The logic for that case
+  // (nsPageFrame::ComputePageSizeScale) is somewhat different though, since
+  // that case uses no sheet margins and uses the user/CSS specified margins on
+  // the page, with any page scaling reverted to keep the margins unchanged.
+  // We, on the other hand, use the unwriteable margins for the sheet, unscaled,
+  // and use the user/CSS margins on the pages and allow them to be scaled
+  // along with any pages-per-sheet scaling. (This behavior makes maximum use
+  // of the sheet and, by scaling the default on the pages, results in a
+  // a sensible amount of spacing between pages.)
+  void ComputePagesPerSheetGridMetrics(const nsSize& aSheetSize);
+
+  // See GetSizeForChildren.
+  nsSize mSizeForChildren;
+
+  // Note: this will be set before reflow, and it's strongly owned by our
+  // nsPageSequenceFrame, which outlives us.
+  nsSharedPageData* mPD = nullptr;
+
+  // The number of visible pages in this sheet.
+  uint32_t mNumPages = 0;
+
+  // Number of "columns" in our pages-per-sheet layout. For example: if we're
+  // printing with 6 pages-per-sheet, then this could be either 3 or 2,
+  // depending on whether we're printing portrait-oriented pages onto a
+  // landscape-oriented sheet (3 cols) vs. if we're printing landscape-oriented
+  // pages onto a portrait-oriented sheet (2 cols).
+  uint32_t mGridNumCols = 1;
+
+  // The offset of the start of the multiple pages-per-sheet grid from the
+  // top-left of the sheet.
+  nsPoint mGridOrigin;
+
+  // The size of each cell on the sheet into which pages are to be placed.
+  // (The default values are arbitrary.)
+  nscoord mGridCellWidth = 1;
+  nscoord mGridCellHeight = 1;
+};
+
+}  // namespace mozilla
+
+#endif /* LAYOUT_GENERIC_PRINTEDSHEETFRAME_H_ */