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diff --git a/dom/events/ContentEventHandler.h b/dom/events/ContentEventHandler.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 mozilla_ContentEventHandler_h_
+#define mozilla_ContentEventHandler_h_
+
+#include "js/GCAPI.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/EventForwards.h"
+#include "mozilla/RangeBoundary.h"
+#include "mozilla/dom/Selection.h"
+#include "mozilla/dom/Text.h"
+#include "nsCOMPtr.h"
+#include "nsIFrame.h"
+#include "nsINode.h"
+
+class nsPresContext;
+class nsRange;
+
+struct nsRect;
+
+namespace mozilla {
+
+namespace dom {
+class Element;
+}  // namespace dom
+
+enum LineBreakType { LINE_BREAK_TYPE_NATIVE, LINE_BREAK_TYPE_XP };
+
+/*
+ * Query Content Event Handler
+ *   ContentEventHandler is a helper class for EventStateManager.
+ *   The platforms request some content informations, e.g., the selected text,
+ *   the offset of the selected text and the text for specified range.
+ *   This class answers to NS_QUERY_* events from actual contents.
+ */
+
+class MOZ_STACK_CLASS ContentEventHandler {
+ private:
+  /**
+   * SimpleRangeBase is a helper template class of ContentEventHandler class
+   * that stores 2 DOM points as a range without observing the mutation.  I.e.,
+   * similar to dom::StaticRange, but can only be on the stack and does not have
+   * unnecessary features for ContentEventHandler so it is fast.
+   * Therefore, initializers are responsible for making sure the start/end nodes
+   * are in document order. This is enforced by assertions in DEBUG builds.
+   */
+  template <typename NodeType, typename RangeBoundaryType>
+  class MOZ_STACK_CLASS SimpleRangeBase final {
+   public:
+    SimpleRangeBase();
+    SimpleRangeBase(SimpleRangeBase<NodeType, RangeBoundaryType>&&) noexcept;
+    template <typename OtherNodeType, typename OtherRangeBoundaryType>
+    explicit SimpleRangeBase(
+        const SimpleRangeBase<OtherNodeType, OtherRangeBoundaryType>& aOther);
+#ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+    ~SimpleRangeBase();
+#endif
+
+    void Clear() {
+      mRoot = nullptr;
+      mStart = {};
+      mEnd = {};
+    }
+
+    bool IsPositioned() const { return mStart.IsSet() && mEnd.IsSet(); }
+    bool Collapsed() const { return mStart == mEnd && IsPositioned(); }
+    nsINode* GetStartContainer() const { return mStart.Container(); }
+    nsINode* GetEndContainer() const { return mEnd.Container(); }
+    uint32_t StartOffset() const {
+      return *mStart.Offset(
+          RangeBoundaryType::OffsetFilter::kValidOrInvalidOffsets);
+    }
+    uint32_t EndOffset() const {
+      return *mEnd.Offset(
+          RangeBoundaryType::OffsetFilter::kValidOrInvalidOffsets);
+    }
+    nsIContent* StartRef() const { return mStart.Ref(); }
+    nsIContent* EndRef() const { return mEnd.Ref(); }
+
+    const RangeBoundaryType& Start() const { return mStart; }
+    const RangeBoundaryType& End() const { return mEnd; }
+
+    nsINode* GetRoot() const { return mRoot; }
+
+    // XXX: Make these use RangeBoundaries...
+    nsresult CollapseTo(const RawRangeBoundary& aBoundary) {
+      return SetStartAndEnd(aBoundary, aBoundary);
+    }
+    nsresult SetStart(const RawRangeBoundary& aStart);
+    nsresult SetEnd(const RawRangeBoundary& aEnd);
+
+    // NOTE: These helpers can hide performance problems, as they perform a
+    // search to find aStartOffset in aStartContainer.
+    nsresult SetStart(nsINode* aStartContainer, uint32_t aStartOffset) {
+      return SetStart(RawRangeBoundary(aStartContainer, aStartOffset));
+    }
+    nsresult SetEnd(nsINode* aEndContainer, uint32_t aEndOffset) {
+      return SetEnd(RawRangeBoundary(aEndContainer, aEndOffset));
+    }
+
+    nsresult SetEndAfter(nsINode* aEndContainer);
+    void SetStartAndEnd(const nsRange* aRange);
+    nsresult SetStartAndEnd(const RawRangeBoundary& aStart,
+                            const RawRangeBoundary& aEnd);
+
+    nsresult SelectNodeContents(const nsINode* aNodeToSelectContents);
+
+   private:
+    inline void AssertStartIsBeforeOrEqualToEnd();
+
+    NodeType mRoot;
+
+    RangeBoundaryType mStart;
+    RangeBoundaryType mEnd;
+
+#ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+    nsMutationGuard mMutationGuard;
+    Maybe<JS::AutoAssertNoGC> mAssertNoGC;
+#endif
+  };
+
+  using SimpleRange = SimpleRangeBase<RefPtr<nsINode>, RangeBoundary>;
+  using UnsafeSimpleRange = SimpleRangeBase<nsINode*, RawRangeBoundary>;
+
+ public:
+  using Element = dom::Element;
+  using Selection = dom::Selection;
+
+  explicit ContentEventHandler(nsPresContext* aPresContext);
+
+  // Handle aEvent in the current process.
+  MOZ_CAN_RUN_SCRIPT nsresult
+  HandleQueryContentEvent(WidgetQueryContentEvent* aEvent);
+
+  // eQuerySelectedText event handler
+  MOZ_CAN_RUN_SCRIPT nsresult
+  OnQuerySelectedText(WidgetQueryContentEvent* aEvent);
+  // eQueryTextContent event handler
+  MOZ_CAN_RUN_SCRIPT nsresult
+  OnQueryTextContent(WidgetQueryContentEvent* aEvent);
+  // eQueryCaretRect event handler
+  MOZ_CAN_RUN_SCRIPT nsresult OnQueryCaretRect(WidgetQueryContentEvent* aEvent);
+  // eQueryTextRect event handler
+  MOZ_CAN_RUN_SCRIPT nsresult OnQueryTextRect(WidgetQueryContentEvent* aEvent);
+  // eQueryTextRectArray event handler
+  MOZ_CAN_RUN_SCRIPT nsresult
+  OnQueryTextRectArray(WidgetQueryContentEvent* aEvent);
+  // eQueryEditorRect event handler
+  MOZ_CAN_RUN_SCRIPT nsresult
+  OnQueryEditorRect(WidgetQueryContentEvent* aEvent);
+  // eQueryContentState event handler
+  MOZ_CAN_RUN_SCRIPT nsresult
+  OnQueryContentState(WidgetQueryContentEvent* aEvent);
+  // eQuerySelectionAsTransferable event handler
+  MOZ_CAN_RUN_SCRIPT nsresult
+  OnQuerySelectionAsTransferable(WidgetQueryContentEvent* aEvent);
+  // eQueryCharacterAtPoint event handler
+  MOZ_CAN_RUN_SCRIPT nsresult
+  OnQueryCharacterAtPoint(WidgetQueryContentEvent* aEvent);
+  // eQueryDOMWidgetHittest event handler
+  MOZ_CAN_RUN_SCRIPT nsresult
+  OnQueryDOMWidgetHittest(WidgetQueryContentEvent* aEvent);
+
+  // NS_SELECTION_* event
+  MOZ_CAN_RUN_SCRIPT nsresult OnSelectionEvent(WidgetSelectionEvent* aEvent);
+
+ protected:
+  RefPtr<dom::Document> mDocument;
+  // mSelection is typically normal selection but if OnQuerySelectedText()
+  // is called, i.e., handling eQuerySelectedText, it's the specified selection
+  // by WidgetQueryContentEvent::mInput::mSelectionType.
+  RefPtr<Selection> mSelection;
+  // mFirstSelectedSimpleRange is initialized from the first range of
+  // mSelection, if it exists.  Otherwise, it is reset by Clear().
+  SimpleRange mFirstSelectedSimpleRange;
+  RefPtr<Element> mRootElement;
+
+  MOZ_CAN_RUN_SCRIPT nsresult Init(WidgetQueryContentEvent* aEvent);
+  MOZ_CAN_RUN_SCRIPT nsresult Init(WidgetSelectionEvent* aEvent);
+
+  nsresult InitBasic(bool aRequireFlush = true);
+  MOZ_CAN_RUN_SCRIPT nsresult
+  InitCommon(EventMessage aEventMessage,
+             SelectionType aSelectionType = SelectionType::eNormal,
+             bool aRequireFlush = true);
+  /**
+   * InitRootContent() computes the root content of current focused editor.
+   *
+   * @param aNormalSelection    This must be a Selection instance whose type is
+   *                            SelectionType::eNormal.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult
+  InitRootContent(const Selection& aNormalSelection);
+
+ public:
+  // FlatText means the text that is generated from DOM tree. The BR elements
+  // are replaced to native linefeeds. Other elements are ignored.
+
+  // RawNodePosition stores a pair of node and offset in the node.
+  // When mNode is an element and mOffset is 0, the start position means after
+  // the open tag of mNode.
+  // This is useful to receive one or more sets of them instead of nsRange.
+  // This type is intended to be used for short-lived operations, and is thus
+  // marked MOZ_STACK_CLASS.
+  struct MOZ_STACK_CLASS RawNodePosition : public RawRangeBoundary {
+    // Only when mNode is an element node and mOffset is 0, mAfterOpenTag is
+    // referred.
+    bool mAfterOpenTag = true;
+
+    RawNodePosition() = default;
+    explicit RawNodePosition(const RawNodePosition& aOther)
+        : RawRangeBoundary(aOther),
+          mAfterOpenTag(aOther.mAfterOpenTag)
+    // Don't use the copy constructor of mAssertNoGC.
+    {}
+
+    RawNodePosition(nsINode* aContainer, uint32_t aOffset)
+        : RawRangeBoundary(aContainer, aOffset) {}
+
+    RawNodePosition(nsINode* aContainer, nsIContent* aRef)
+        : RawRangeBoundary(aContainer, aRef) {}
+
+    explicit RawNodePosition(const nsIFrame::ContentOffsets& aContentOffsets)
+        : RawRangeBoundary(aContentOffsets.content, aContentOffsets.offset) {}
+
+#ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+    ~RawNodePosition() { MOZ_DIAGNOSTIC_ASSERT(!mMutationGuard.Mutated(0)); }
+#endif  // #ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+
+   public:
+    const RawNodePosition& operator=(const RawNodePosition& aOther) {
+      if (this != &aOther) {
+        RawRangeBoundary::operator=(aOther);
+        mAfterOpenTag = aOther.mAfterOpenTag;
+      }
+      return *this;
+    }
+
+    bool operator==(const RawNodePosition& aOther) const {
+      return RawRangeBoundary::operator==(aOther) &&
+             mAfterOpenTag == aOther.mAfterOpenTag;
+    }
+
+    bool IsBeforeOpenTag() const {
+      return IsSet() && Container()->IsElement() && !Ref() && !mAfterOpenTag;
+    }
+    bool IsImmediatelyAfterOpenTag() const {
+      return IsSet() && Container()->IsElement() && !Ref() && mAfterOpenTag;
+    }
+
+#ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+   private:
+    nsMutationGuard mMutationGuard;
+    JS::AutoAssertNoGC mAssertNoGC;
+#endif  // #ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+  };
+
+  // RawNodePositionBefore isn't good name if Container() isn't an element node
+  // nor Offset() is not 0, though, when Container() is an element node and
+  // mOffset is 0, this is treated as before the open tag of Container().
+  struct MOZ_STACK_CLASS RawNodePositionBefore final : public RawNodePosition {
+    RawNodePositionBefore(nsINode* aContainer, uint32_t aOffset)
+        : RawNodePosition(aContainer, aOffset) {
+      mAfterOpenTag = false;
+    }
+
+    RawNodePositionBefore(nsINode* aContainer, nsIContent* aRef)
+        : RawNodePosition(aContainer, aRef) {
+      mAfterOpenTag = false;
+    }
+  };
+
+  // Get the flatten text length in the range.
+  // @param aStartPosition      Start node and offset in the node of the range.
+  // @param aEndPosition        End node and offset in the node of the range.
+  // @param aRootElement        The root element of the editor or document.
+  //                            aRootElement won't cause any text including
+  //                            line breaks.
+  // @param aLength             The result of the flatten text length of the
+  //                            range.
+  // @param aLineBreakType      Whether this computes flatten text length with
+  //                            native line breakers on the platform or
+  //                            with XP line breaker (\n).
+  // @param aIsRemovingNode     Should be true only when this is called from
+  //                            nsIMutationObserver::ContentRemoved().
+  //                            When this is true, aStartPosition.mNode should
+  //                            be the root node of removing nodes and mOffset
+  //                            should be 0 and aEndPosition.mNode should be
+  //                            same as aStartPosition.mNode and mOffset should
+  //                            be number of the children of mNode.
+  static nsresult GetFlatTextLengthInRange(
+      const RawNodePosition& aStartPosition,
+      const RawNodePosition& aEndPosition, const Element* aRootElement,
+      uint32_t* aLength, LineBreakType aLineBreakType,
+      bool aIsRemovingNode = false);
+  // Computes the native text length between aStartOffset and aEndOffset of
+  // aTextNode.
+  static uint32_t GetNativeTextLength(const dom::Text& aTextNode,
+                                      uint32_t aStartOffset,
+                                      uint32_t aEndOffset);
+  // Get the native text length of aTextNode.
+  static uint32_t GetNativeTextLength(const dom::Text& aTextNode,
+                                      uint32_t aMaxLength = UINT32_MAX);
+
+  static uint32_t GetNativeTextLength(const nsAString& aText);
+
+ protected:
+  // Get the text length of aTextNode.
+  static uint32_t GetTextLength(const dom::Text& aTextNode,
+                                LineBreakType aLineBreakType,
+                                uint32_t aMaxLength = UINT32_MAX);
+  // Get the text length of a given range of a content node in
+  // the given line break type.
+  static uint32_t GetTextLengthInRange(const dom::Text& aTextNode,
+                                       uint32_t aXPStartOffset,
+                                       uint32_t aXPEndOffset,
+                                       LineBreakType aLineBreakType);
+  // Get the contents in aElement (meaning all children of aElement) as plain
+  // text.  E.g., specifying mRootElement gets whole text in it.
+  // Note that the result is not same as .textContent.  The result is
+  // optimized for native IMEs.  For example, <br> element and some block
+  // elements causes "\n" (or "\r\n"), see also ShouldBreakLineBefore().
+  nsresult GenerateFlatTextContent(const Element* aElement, nsString& aString,
+                                   LineBreakType aLineBreakType);
+  // Get the contents of aRange as plain text.
+  template <typename NodeType, typename RangeBoundaryType>
+  nsresult GenerateFlatTextContent(
+      const SimpleRangeBase<NodeType, RangeBoundaryType>& aSimpleRange,
+      nsString& aString, LineBreakType aLineBreakType);
+  // Get offset of start of aRange.  Note that the result includes the length
+  // of line breaker caused by the start of aContent because aRange never
+  // includes the line breaker caused by its start node.
+  template <typename SimpleRangeType>
+  nsresult GetStartOffset(const SimpleRangeType& aSimpleRange,
+                          uint32_t* aOffset, LineBreakType aLineBreakType);
+  // Check if we should insert a line break before aContent.
+  // This should return false only when aContent is an html element which
+  // is typically used in a paragraph like <em>.
+  static bool ShouldBreakLineBefore(const nsIContent& aContent,
+                                    const Element* aRootElement);
+  // Get the line breaker length.
+  static inline uint32_t GetBRLength(LineBreakType aLineBreakType);
+  static LineBreakType GetLineBreakType(WidgetQueryContentEvent* aEvent);
+  static LineBreakType GetLineBreakType(WidgetSelectionEvent* aEvent);
+  static LineBreakType GetLineBreakType(bool aUseNativeLineBreak);
+  // Returns focused content (including its descendant documents).
+  nsIContent* GetFocusedContent();
+  // QueryContentRect() sets the rect of aContent's frame(s) to aEvent.
+  nsresult QueryContentRect(nsIContent* aContent,
+                            WidgetQueryContentEvent* aEvent);
+
+  template <typename RangeType, typename TextNodeType>
+  struct MOZ_STACK_CLASS DOMRangeAndAdjustedOffsetInFlattenedTextBase {
+    bool RangeStartsFromLastTextNode() const {
+      return mLastTextNode && mRange.GetStartContainer() == mLastTextNode;
+    }
+    bool RangeStartsFromEndOfContainer() const {
+      return mRange.GetStartContainer() &&
+             mRange.GetStartContainer()->Length() == mRange.StartOffset();
+    }
+    bool RangeStartsFromContent() const {
+      return mRange.GetStartContainer() &&
+             mRange.GetStartContainer()->IsContent();
+    }
+
+    // The range in the DOM tree.
+    RangeType mRange;
+    // Actual start offset of the range in the flattened text.  If aOffset
+    // of ConvertFlatTextOffsetToDOMRange() is middle of a surrogate pair,
+    // a CRLF or a complex character of some languages, this may be set to
+    // different offset.
+    uint32_t mAdjustedOffset = 0;
+    // The last text node which is found while walking the tree.
+    // If the range ends in a text node, this is the text node.  Otherwise,
+    // the last found text node before the end container of mRange.
+    TextNodeType mLastTextNode = nullptr;
+  };
+  using DOMRangeAndAdjustedOffsetInFlattenedText =
+      DOMRangeAndAdjustedOffsetInFlattenedTextBase<SimpleRange,
+                                                   RefPtr<dom::Text>>;
+  using UnsafeDOMRangeAndAdjustedOffsetInFlattenedText =
+      DOMRangeAndAdjustedOffsetInFlattenedTextBase<UnsafeSimpleRange,
+                                                   dom::Text*>;
+
+  /**
+   * Scans the DOM tree and set mRange as same as from aOffset to aOffset +
+   * aLength in the flattened text.
+   * NOTE: Use ConvertFlatTextOffsetToDOMRange() or
+   * ConvertFlatTextOffsetToUnsafeDOMRange() instead of
+   * ConvertFlatTextOffsetToDOMRangeBase<RangeType, TextNodeType>().
+   */
+  template <typename RangeType, typename TextNodeType>
+  Result<DOMRangeAndAdjustedOffsetInFlattenedTextBase<RangeType, TextNodeType>,
+         nsresult>
+  ConvertFlatTextOffsetToDOMRangeBase(uint32_t aOffset, uint32_t aLength,
+                                      LineBreakType aLineBreakType,
+                                      bool aExpandToClusterBoundaries);
+  MOZ_ALWAYS_INLINE Result<DOMRangeAndAdjustedOffsetInFlattenedText, nsresult>
+  ConvertFlatTextOffsetToDOMRange(uint32_t aOffset, uint32_t aLength,
+                                  LineBreakType aLineBreakType,
+                                  bool aExpandToClusterBoundaries) {
+    return ConvertFlatTextOffsetToDOMRangeBase<SimpleRange, RefPtr<dom::Text>>(
+        aOffset, aLength, aLineBreakType, aExpandToClusterBoundaries);
+  }
+  MOZ_ALWAYS_INLINE
+  Result<UnsafeDOMRangeAndAdjustedOffsetInFlattenedText, nsresult>
+  ConvertFlatTextOffsetToUnsafeDOMRange(uint32_t aOffset, uint32_t aLength,
+                                        LineBreakType aLineBreakType,
+                                        bool aExpandToClusterBoundaries) {
+    return ConvertFlatTextOffsetToDOMRangeBase<UnsafeSimpleRange, dom::Text*>(
+        aOffset, aLength, aLineBreakType, aExpandToClusterBoundaries);
+  }
+
+  // If the aSimpleRange isn't in text node but next to a text node,
+  // this method modifies it in the text node.  Otherwise, not modified.
+  // Note that aSimpleRange must be collapsed.
+  nsresult AdjustCollapsedRangeMaybeIntoTextNode(SimpleRange& aSimpleRange);
+  // Convert the frame relative offset to be relative to the root frame of the
+  // root presContext (but still measured in appUnits of aFrame's presContext).
+  nsresult ConvertToRootRelativeOffset(nsIFrame* aFrame, nsRect& aRect);
+  // Expand aXPOffset to the nearest offset in cluster boundary. aForward is
+  // true, it is expanded to forward.
+  // FYI: Due to `nsFrameSelection::GetFrameForNodeOffset()`, this cannot
+  //      take `const dom::Text&`.
+  nsresult ExpandToClusterBoundary(dom::Text& aTextNode, bool aForward,
+                                   uint32_t* aXPOffset) const;
+
+  using FontRangeArray = nsTArray<mozilla::FontRange>;
+  static void AppendFontRanges(FontRangeArray& aFontRanges,
+                               const dom::Text& aTextNode, uint32_t aBaseOffset,
+                               uint32_t aXPStartOffset, uint32_t aXPEndOffset,
+                               LineBreakType aLineBreakType);
+  nsresult GenerateFlatFontRanges(const UnsafeSimpleRange& aSimpleRange,
+                                  FontRangeArray& aFontRanges,
+                                  uint32_t& aLength,
+                                  LineBreakType aLineBreakType);
+  nsresult QueryTextRectByRange(const SimpleRange& aSimpleRange,
+                                LayoutDeviceIntRect& aRect,
+                                WritingMode& aWritingMode);
+
+  struct MOZ_STACK_CLASS FrameAndNodeOffset final {
+    // mFrame is safe since this can live in only stack class and
+    // ContentEventHandler doesn't modify layout after
+    // ContentEventHandler::Init() flushes pending layout.  In other words,
+    // this struct shouldn't be used before calling
+    // ContentEventHandler::Init().
+    nsIFrame* mFrame;
+    // offset in the node of mFrame
+    int32_t mOffsetInNode;
+
+    FrameAndNodeOffset() : mFrame(nullptr), mOffsetInNode(-1) {}
+
+    FrameAndNodeOffset(nsIFrame* aFrame, int32_t aStartOffsetInNode)
+        : mFrame(aFrame), mOffsetInNode(aStartOffsetInNode) {}
+
+    nsIFrame* operator->() { return mFrame; }
+    const nsIFrame* operator->() const { return mFrame; }
+    operator nsIFrame*() { return mFrame; }
+    operator const nsIFrame*() const { return mFrame; }
+    bool IsValid() const { return mFrame && mOffsetInNode >= 0; }
+  };
+  // Get first frame after the start of the given range for computing text rect.
+  // This returns invalid FrameAndNodeOffset if there is no content which
+  // should affect to computing text rect in the range.  mOffsetInNode is start
+  // offset in the frame.
+  template <typename NodeType, typename RangeBoundaryType>
+  FrameAndNodeOffset GetFirstFrameInRangeForTextRect(
+      const SimpleRangeBase<NodeType, RangeBoundaryType>& aSimpleRange);
+
+  // Get last frame before the end of the given range for computing text rect.
+  // This returns invalid FrameAndNodeOffset if there is no content which
+  // should affect to computing text rect in the range.  mOffsetInNode is end
+  // offset in the frame.
+  template <typename NodeType, typename RangeBoundaryType>
+  FrameAndNodeOffset GetLastFrameInRangeForTextRect(
+      const SimpleRangeBase<NodeType, RangeBoundaryType>& aSimpleRange);
+
+  struct MOZ_STACK_CLASS FrameRelativeRect final {
+    // mRect is relative to the mBaseFrame's position.
+    nsRect mRect;
+    nsIFrame* mBaseFrame;
+
+    FrameRelativeRect() : mBaseFrame(nullptr) {}
+
+    explicit FrameRelativeRect(nsIFrame* aBaseFrame) : mBaseFrame(aBaseFrame) {}
+
+    FrameRelativeRect(const nsRect& aRect, nsIFrame* aBaseFrame)
+        : mRect(aRect), mBaseFrame(aBaseFrame) {}
+
+    bool IsValid() const { return mBaseFrame != nullptr; }
+
+    // Returns an nsRect relative to aBaseFrame instead of mBaseFrame.
+    nsRect RectRelativeTo(nsIFrame* aBaseFrame) const;
+  };
+
+  // Returns a rect for line breaker before the node of aFrame (If aFrame is
+  // a <br> frame or a block level frame, it causes a line break at its
+  // element's open tag, see also ShouldBreakLineBefore()).  Note that this
+  // doesn't check if aFrame should cause line break in non-debug build.
+  FrameRelativeRect GetLineBreakerRectBefore(nsIFrame* aFrame);
+
+  // Returns a line breaker rect after aTextNode as there is a line breaker
+  // immediately after aTextNode.  This is useful when following block
+  // element causes a line break before it and it needs to compute the line
+  // breaker's rect.  For example, if there is |<p>abc</p><p>def</p>|, the
+  // rect of 2nd <p>'s line breaker should be at right of "c" in the first
+  // <p>, not the start of 2nd <p>.  The result is relative to the last text
+  // frame which represents the last character of aTextNode.
+  FrameRelativeRect GuessLineBreakerRectAfter(const dom::Text& aTextNode);
+
+  // Returns a guessed first rect.  I.e., it may be different from actual
+  // caret when selection is collapsed at start of aFrame.  For example, this
+  // guess the caret rect only with the content box of aFrame and its font
+  // height like:
+  // +-aFrame----------------- (border box)
+  // |
+  // |  +--------------------- (content box)
+  // |  | I
+  //      ^ guessed caret rect
+  // However, actual caret is computed with more information like line-height,
+  // child frames of aFrame etc.  But this does not emulate actual caret
+  // behavior exactly for simpler and faster code because it's difficult and
+  // we're not sure it's worthwhile to do it with complicated implementation.
+  FrameRelativeRect GuessFirstCaretRectIn(nsIFrame* aFrame);
+
+  // Make aRect non-empty.  If width and/or height is 0, these methods set them
+  // to 1.  Note that it doesn't set nsRect's width nor height to one device
+  // pixel because using nsRect::ToOutsidePixels() makes actual width or height
+  // to 2 pixels because x and y may not be aligned to device pixels.
+  void EnsureNonEmptyRect(nsRect& aRect) const;
+  void EnsureNonEmptyRect(LayoutDeviceIntRect& aRect) const;
+
+  /**
+   * Compute caret rect before or after a character rect.
+   */
+  static LayoutDeviceIntRect GetCaretRectBefore(
+      const LayoutDeviceIntRect& aCharRect, const WritingMode& aWritingMode);
+  static LayoutDeviceIntRect GetCaretRectAfter(
+      const LayoutDeviceIntRect& aCharRect, const WritingMode& aWritingMode);
+  static nsRect GetCaretRectBefore(const nsRect& aCharRect,
+                                   const WritingMode& aWritingMode);
+  static nsRect GetCaretRectAfter(nsPresContext& aPresContext,
+                                  const nsRect& aCharRect,
+                                  const WritingMode& aWritingMode);
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_ContentEventHandler_h_