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diff --git a/layout/style/nsComputedDOMStyle.h b/layout/style/nsComputedDOMStyle.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/. */
+
+/* DOM object returned from element.getComputedStyle() */
+
+#ifndef nsComputedDOMStyle_h__
+#define nsComputedDOMStyle_h__
+
+#include "mozilla/Attributes.h"
+#include "mozilla/StyleColorInlines.h"
+#include "mozilla/UniquePtr.h"
+#include "nsCOMPtr.h"
+#include "nsContentUtils.h"
+#include "nscore.h"
+#include "nsDOMCSSDeclaration.h"
+#include "mozilla/ComputedStyle.h"
+#include "nsIWeakReferenceUtils.h"
+#include "mozilla/gfx/Types.h"
+#include "nsCoord.h"
+#include "nsColor.h"
+#include "nsStubMutationObserver.h"
+#include "nsStyleStruct.h"
+#include "mozilla/WritingModes.h"
+
+// XXX Avoid including this here by moving function bodies to the cpp file
+#include "mozilla/dom/Element.h"
+
+namespace mozilla {
+enum class FlushType : uint8_t;
+enum class PseudoStyleType : uint8_t;
+
+namespace dom {
+class DocGroup;
+class Element;
+}  // namespace dom
+class PresShell;
+struct ComputedGridTrackInfo;
+}  // namespace mozilla
+
+struct ComputedStyleMap;
+struct nsCSSKTableEntry;
+class nsIFrame;
+class nsDOMCSSValueList;
+struct nsMargin;
+class nsROCSSPrimitiveValue;
+class nsStyleGradient;
+
+class nsComputedDOMStyle final : public nsDOMCSSDeclaration,
+                                 public nsStubMutationObserver {
+ private:
+  // Convenience typedefs:
+  template <typename T>
+  using Span = mozilla::Span<T>;
+  using KTableEntry = nsCSSKTableEntry;
+  using CSSValue = mozilla::dom::CSSValue;
+  using StyleGeometryBox = mozilla::StyleGeometryBox;
+  using Element = mozilla::dom::Element;
+  using Document = mozilla::dom::Document;
+  using PseudoStyleType = mozilla::PseudoStyleType;
+  using LengthPercentage = mozilla::LengthPercentage;
+  using LengthPercentageOrAuto = mozilla::LengthPercentageOrAuto;
+  using ComputedStyle = mozilla::ComputedStyle;
+
+ public:
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_SKIPPABLE_WRAPPERCACHE_CLASS_AMBIGUOUS(
+      nsComputedDOMStyle, nsICSSDeclaration)
+
+  NS_DECL_NSIDOMCSSSTYLEDECLARATION_HELPER
+  nsresult GetPropertyValue(const nsCSSPropertyID aPropID,
+                            nsACString& aValue) override;
+  void SetPropertyValue(const nsCSSPropertyID aPropID, const nsACString& aValue,
+                        nsIPrincipal* aSubjectPrincipal,
+                        mozilla::ErrorResult& aRv) override;
+
+  void IndexedGetter(uint32_t aIndex, bool& aFound,
+                     nsACString& aPropName) final;
+
+  enum class StyleType : uint8_t {
+    DefaultOnly,  // Only includes UA and user sheets
+    All           // Includes all stylesheets
+  };
+
+  // In some cases, for legacy reasons, we forcefully return an empty style.
+  enum class AlwaysReturnEmptyStyle : bool { No, Yes };
+
+  nsComputedDOMStyle(Element*, PseudoStyleType, Document*, StyleType,
+                     AlwaysReturnEmptyStyle = AlwaysReturnEmptyStyle::No);
+
+  nsINode* GetAssociatedNode() const override { return mElement; }
+  nsINode* GetParentObject() const override { return mElement; }
+
+  static already_AddRefed<const ComputedStyle> GetComputedStyle(
+      Element* aElement, PseudoStyleType = PseudoStyleType::NotPseudo,
+      StyleType = StyleType::All);
+
+  static already_AddRefed<const ComputedStyle> GetComputedStyleNoFlush(
+      const Element* aElement,
+      PseudoStyleType aPseudo = PseudoStyleType::NotPseudo,
+      StyleType aStyleType = StyleType::All) {
+    return DoGetComputedStyleNoFlush(
+        aElement, aPseudo, nsContentUtils::GetPresShellForContent(aElement),
+        aStyleType);
+  }
+
+  static already_AddRefed<const ComputedStyle>
+  GetUnanimatedComputedStyleNoFlush(
+      Element*, PseudoStyleType = PseudoStyleType::NotPseudo);
+
+  // Helper for nsDOMWindowUtils::GetVisitedDependentComputedStyle
+  void SetExposeVisitedStyle(bool aExpose) {
+    NS_ASSERTION(aExpose != mExposeVisitedStyle, "should always be changing");
+    mExposeVisitedStyle = aExpose;
+  }
+
+  void GetCSSImageURLs(const nsACString& aPropertyName,
+                       nsTArray<nsCString>& aImageURLs,
+                       mozilla::ErrorResult& aRv) final;
+
+  // nsDOMCSSDeclaration abstract methods which should never be called
+  // on a nsComputedDOMStyle object, but must be defined to avoid
+  // compile errors.
+  mozilla::DeclarationBlock* GetOrCreateCSSDeclaration(
+      Operation aOperation, mozilla::DeclarationBlock** aCreated) final;
+  virtual nsresult SetCSSDeclaration(mozilla::DeclarationBlock*,
+                                     mozilla::MutationClosureData*) override;
+  virtual mozilla::dom::Document* DocToUpdate() override;
+
+  nsDOMCSSDeclaration::ParsingEnvironment GetParsingEnvironment(
+      nsIPrincipal* aSubjectPrincipal) const final;
+
+  static already_AddRefed<nsROCSSPrimitiveValue> MatrixToCSSValue(
+      const mozilla::gfx::Matrix4x4& aMatrix);
+
+  static void RegisterPrefChangeCallbacks();
+  static void UnregisterPrefChangeCallbacks();
+
+  // nsIMutationObserver
+  NS_DECL_NSIMUTATIONOBSERVER_PARENTCHAINCHANGED
+
+ private:
+  nsresult GetPropertyValue(const nsCSSPropertyID aPropID,
+                            const nsACString& aMaybeCustomPropertyNme,
+                            nsACString& aValue);
+
+  virtual ~nsComputedDOMStyle();
+
+  void AssertFlushedPendingReflows() {
+    NS_ASSERTION(mFlushedPendingReflows,
+                 "property getter should have been marked layout-dependent");
+  }
+
+  nsMargin GetAdjustedValuesForBoxSizing();
+
+  // This indicates error by leaving mComputedStyle null.
+  void UpdateCurrentStyleSources(nsCSSPropertyID);
+  void ClearCurrentStyleSources();
+
+  // Helper functions called by UpdateCurrentStyleSources.
+  void ClearComputedStyle();
+  void SetResolvedComputedStyle(RefPtr<const ComputedStyle>&& aContext,
+                                uint64_t aGeneration);
+  void SetFrameComputedStyle(ComputedStyle* aStyle, uint64_t aGeneration);
+
+  static already_AddRefed<const ComputedStyle> DoGetComputedStyleNoFlush(
+      const Element*, PseudoStyleType, mozilla::PresShell*, StyleType);
+
+#define STYLE_STRUCT(name_)                \
+  const nsStyle##name_* Style##name_() {   \
+    return mComputedStyle->Style##name_(); \
+  }
+#include "nsStyleStructList.h"
+#undef STYLE_STRUCT
+
+  /**
+   * A method to get a percentage base for a percentage value.  Returns true
+   * if a percentage base value was determined, false otherwise.
+   */
+  typedef bool (nsComputedDOMStyle::*PercentageBaseGetter)(nscoord&);
+
+  already_AddRefed<CSSValue> GetOffsetWidthFor(mozilla::Side);
+  already_AddRefed<CSSValue> GetAbsoluteOffset(mozilla::Side);
+  nscoord GetUsedAbsoluteOffset(mozilla::Side);
+  already_AddRefed<CSSValue> GetNonStaticPositionOffset(
+      mozilla::Side aSide, bool aResolveAuto, PercentageBaseGetter aWidthGetter,
+      PercentageBaseGetter aHeightGetter);
+
+  already_AddRefed<CSSValue> GetStaticOffset(mozilla::Side aSide);
+
+  already_AddRefed<CSSValue> GetPaddingWidthFor(mozilla::Side aSide);
+
+  already_AddRefed<CSSValue> GetBorderWidthFor(mozilla::Side aSide);
+
+  already_AddRefed<CSSValue> GetMarginFor(mozilla::Side aSide);
+
+  already_AddRefed<CSSValue> GetTransformValue(const mozilla::StyleTransform&);
+
+  already_AddRefed<nsROCSSPrimitiveValue> GetGridTrackSize(
+      const mozilla::StyleTrackSize&);
+  already_AddRefed<nsROCSSPrimitiveValue> GetGridTrackBreadth(
+      const mozilla::StyleTrackBreadth&);
+  void SetValueToTrackBreadth(nsROCSSPrimitiveValue*,
+                              const mozilla::StyleTrackBreadth&);
+  already_AddRefed<CSSValue> GetGridTemplateColumnsRows(
+      const mozilla::StyleGridTemplateComponent& aTrackList,
+      const mozilla::ComputedGridTrackInfo& aTrackInfo);
+
+  bool GetLineHeightCoord(nscoord& aCoord);
+
+  bool ShouldHonorMinSizeAutoInAxis(mozilla::PhysicalAxis aAxis);
+
+  /* Properties queryable as CSSValues.
+   * To avoid a name conflict with nsIDOM*CSS2Properties, these are all
+   * DoGetXXX instead of GetXXX.
+   */
+
+  /* Box properties */
+
+  already_AddRefed<CSSValue> DoGetWidth();
+  already_AddRefed<CSSValue> DoGetHeight();
+  already_AddRefed<CSSValue> DoGetMaxHeight();
+  already_AddRefed<CSSValue> DoGetMaxWidth();
+  already_AddRefed<CSSValue> DoGetMinHeight();
+  already_AddRefed<CSSValue> DoGetMinWidth();
+  already_AddRefed<CSSValue> DoGetLeft();
+  already_AddRefed<CSSValue> DoGetTop();
+  already_AddRefed<CSSValue> DoGetRight();
+  already_AddRefed<CSSValue> DoGetBottom();
+
+  /* Font properties */
+  already_AddRefed<CSSValue> DoGetMozOsxFontSmoothing();
+
+  /* Grid properties */
+  already_AddRefed<CSSValue> DoGetGridTemplateColumns();
+  already_AddRefed<CSSValue> DoGetGridTemplateRows();
+
+  /* StyleImageLayer properties */
+  already_AddRefed<CSSValue> DoGetImageLayerPosition(
+      const nsStyleImageLayers& aLayers);
+
+  /* Padding properties */
+  already_AddRefed<CSSValue> DoGetPaddingTop();
+  already_AddRefed<CSSValue> DoGetPaddingBottom();
+  already_AddRefed<CSSValue> DoGetPaddingLeft();
+  already_AddRefed<CSSValue> DoGetPaddingRight();
+
+  /* Table Properties */
+  already_AddRefed<CSSValue> DoGetBorderSpacing();
+
+  /* Border Properties */
+  already_AddRefed<CSSValue> DoGetBorderTopWidth();
+  already_AddRefed<CSSValue> DoGetBorderBottomWidth();
+  already_AddRefed<CSSValue> DoGetBorderLeftWidth();
+  already_AddRefed<CSSValue> DoGetBorderRightWidth();
+
+  /* Margin Properties */
+  already_AddRefed<CSSValue> DoGetMarginTop();
+  already_AddRefed<CSSValue> DoGetMarginBottom();
+  already_AddRefed<CSSValue> DoGetMarginLeft();
+  already_AddRefed<CSSValue> DoGetMarginRight();
+
+  /* Display properties */
+  already_AddRefed<CSSValue> DoGetTransform();
+  already_AddRefed<CSSValue> DoGetTransformOrigin();
+  already_AddRefed<CSSValue> DoGetPerspectiveOrigin();
+
+  // For working around a MSVC bug. See related comment in
+  // GenerateComputedDOMStyleGenerated.py.
+  already_AddRefed<CSSValue> DummyGetter();
+
+  /* Helper functions */
+  void SetValueToPosition(const mozilla::Position& aPosition,
+                          nsDOMCSSValueList* aValueList);
+  void SetValueToURLValue(const mozilla::StyleComputedUrl* aURL,
+                          nsROCSSPrimitiveValue* aValue);
+
+  void SetValueFromFitContentFunction(nsROCSSPrimitiveValue* aValue,
+                                      const mozilla::LengthPercentage&);
+
+  void SetValueToSize(nsROCSSPrimitiveValue* aValue, const mozilla::StyleSize&);
+
+  void SetValueToLengthPercentageOrAuto(nsROCSSPrimitiveValue* aValue,
+                                        const LengthPercentageOrAuto&,
+                                        bool aClampNegativeCalc);
+
+  void SetValueToLengthPercentage(nsROCSSPrimitiveValue* aValue,
+                                  const LengthPercentage&,
+                                  bool aClampNegativeCalc);
+
+  void SetValueToMaxSize(nsROCSSPrimitiveValue* aValue,
+                         const mozilla::StyleMaxSize&);
+
+  bool GetCBContentWidth(nscoord& aWidth);
+  bool GetCBContentHeight(nscoord& aHeight);
+  bool GetCBPaddingRectWidth(nscoord& aWidth);
+  bool GetCBPaddingRectHeight(nscoord& aHeight);
+  bool GetScrollFrameContentWidth(nscoord& aWidth);
+  bool GetScrollFrameContentHeight(nscoord& aHeight);
+  bool GetFrameBorderRectWidth(nscoord& aWidth);
+  bool GetFrameBorderRectHeight(nscoord& aHeight);
+
+  // Find out if we can safely skip flushing (i.e. pending restyles do not
+  // affect our element).
+  bool NeedsToFlushStyle(nsCSSPropertyID) const;
+  // Find out if we need to flush layout of the document, depending on the
+  // property that was requested.
+  bool NeedsToFlushLayout(nsCSSPropertyID) const;
+  // Flushes the given document, which must be our document, and potentially the
+  // mElement's document.
+  void Flush(Document&, mozilla::FlushType);
+  nsIFrame* GetOuterFrame() const;
+
+  static ComputedStyleMap* GetComputedStyleMap();
+
+  // We don't really have a good immutable representation of "presentation".
+  // Given the way GetComputedStyle is currently used, we should just grab the
+  // presshell, if any, from the document.
+  nsWeakPtr mDocumentWeak;
+  RefPtr<Element> mElement;
+
+  /**
+   * Strong reference to the ComputedStyle we access data from.  This can be
+   * either a ComputedStyle we resolved ourselves or a ComputedStyle we got
+   * from our frame.
+   *
+   * If we got the ComputedStyle from the frame, we clear out mComputedStyle
+   * in ClearCurrentStyleSources.  If we resolved one ourselves, then
+   * ClearCurrentStyleSources leaves it in mComputedStyle for use the next
+   * time this nsComputedDOMStyle object is queried.  UpdateCurrentStyleSources
+   * in this case will check that the ComputedStyle is still valid to be used,
+   * by checking whether flush styles results in any restyles having been
+   * processed.
+   */
+  RefPtr<const ComputedStyle> mComputedStyle;
+
+  /*
+   * While computing style data, the primary frame for mContent --- named
+   * "outer" because we should use it to compute positioning data.  Null
+   * otherwise.
+   */
+  nsIFrame* mOuterFrame;
+  /*
+   * While computing style data, the "inner frame" for mContent --- the frame
+   * which we should use to compute margin, border, padding and content data.
+   * Null otherwise.
+   */
+  nsIFrame* mInnerFrame;
+  /*
+   * While computing style data, the presshell we're working with.  Null
+   * otherwise.
+   */
+  mozilla::PresShell* mPresShell;
+
+  PseudoStyleType mPseudo;
+
+  /* The kind of styles we should be returning. */
+  StyleType mStyleType;
+
+  /* Whether for legacy reasons we return an empty style (when an unknown
+   * pseudo-element is specified) */
+  AlwaysReturnEmptyStyle mAlwaysReturnEmpty;
+
+  /**
+   * The nsComputedDOMStyle generation at the time we last resolved a style
+   * context and stored it in mComputedStyle, and the pres shell we got the
+   * style from. Should only be used together.
+   */
+  uint64_t mComputedStyleGeneration = 0;
+
+  uint32_t mPresShellId = 0;
+
+  bool mExposeVisitedStyle = false;
+
+  /**
+   * Whether we resolved a ComputedStyle last time we called
+   * UpdateCurrentStyleSources.  Initially false.
+   */
+  bool mResolvedComputedStyle = false;
+
+#ifdef DEBUG
+  bool mFlushedPendingReflows = false;
+#endif
+
+  friend struct ComputedStyleMap;
+};
+
+already_AddRefed<nsComputedDOMStyle> NS_NewComputedDOMStyle(
+    mozilla::dom::Element*, const nsAString& aPseudoElt,
+    mozilla::dom::Document*, nsComputedDOMStyle::StyleType,
+    mozilla::ErrorResult&);
+
+#endif /* nsComputedDOMStyle_h__ */