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diff --git a/layout/base/AccessibleCaret.h b/layout/base/AccessibleCaret.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 AccessibleCaret_h__
+#define AccessibleCaret_h__
+
+#include "mozilla/Attributes.h"
+#include "mozilla/dom/AnonymousContent.h"
+#include "nsCOMPtr.h"
+#include "nsIDOMEventListener.h"
+#include "nsIFrame.h"  // for WeakFrame only
+#include "nsISupports.h"
+#include "nsISupportsImpl.h"
+#include "nsLiteralString.h"
+#include "nsRect.h"
+#include "mozilla/RefPtr.h"
+#include "nsString.h"
+
+class nsIFrame;
+struct nsPoint;
+
+namespace mozilla {
+class PresShell;
+namespace dom {
+class Element;
+class Event;
+}  // namespace dom
+
+// -----------------------------------------------------------------------------
+// Upon the creation of AccessibleCaret, it will insert DOM Element as an
+// anonymous content containing the caret image. The caret appearance and
+// position can be controlled by SetAppearance() and SetPosition().
+//
+// All the rect or point are relative to root frame except being specified
+// explicitly.
+//
+// None of the methods in AccessibleCaret will flush layout or style. To ensure
+// that SetPosition() works correctly, the caller must make sure the layout is
+// up to date.
+//
+// Please see the wiki page for more information.
+// https://wiki.mozilla.org/AccessibleCaret
+//
+class AccessibleCaret {
+ public:
+  explicit AccessibleCaret(PresShell* aPresShell);
+  virtual ~AccessibleCaret();
+
+  // This enumeration representing the visibility and visual style of an
+  // AccessibleCaret.
+  //
+  // Use SetAppearance() to change the appearance, and use GetAppearance() to
+  // get the current appearance.
+  enum class Appearance : uint8_t {
+    // Do not display the caret at all.
+    None,
+
+    // Display the caret in default style.
+    Normal,
+
+    // The caret should be displayed logically but it is kept invisible to the
+    // user. This enum is the only difference between "logically visible" and
+    // "visually visible". It can be used for reasons such as:
+    // 1. Out of scroll port.
+    // 2. For UX requirement such as hide a caret in an empty text area.
+    NormalNotShown,
+
+    // Display the caret which is tilted to the left.
+    Left,
+
+    // Display the caret which is tilted to the right.
+    Right
+  };
+
+  friend std::ostream& operator<<(std::ostream& aStream,
+                                  const Appearance& aAppearance);
+
+  Appearance GetAppearance() const { return mAppearance; }
+
+  virtual void SetAppearance(Appearance aAppearance);
+
+  // Return true if current appearance is either Normal, NormalNotShown, Left,
+  // or Right.
+  bool IsLogicallyVisible() const { return mAppearance != Appearance::None; }
+
+  // Return true if current appearance is either Normal, Left, or Right.
+  bool IsVisuallyVisible() const {
+    return (mAppearance != Appearance::None) &&
+           (mAppearance != Appearance::NormalNotShown);
+  }
+
+  // This enum represents the result returned by SetPosition().
+  enum class PositionChangedResult : uint8_t {
+    // Both position and the zoom level are not changed.
+    NotChanged,
+
+    // The position is changed. (The zoom level may or may not be changed.)
+    Position,
+
+    // Only the zoom level is changed. The position is *not* changed.
+    Zoom,
+
+    // The position is out of scroll port.
+    Invisible
+  };
+
+  friend std::ostream& operator<<(std::ostream& aStream,
+                                  const PositionChangedResult& aResult);
+
+  virtual PositionChangedResult SetPosition(nsIFrame* aFrame, int32_t aOffset);
+
+  // Does two AccessibleCarets overlap?
+  bool Intersects(const AccessibleCaret& aCaret) const;
+
+  // Is the point within the caret's rect? The point should be relative to root
+  // frame.
+  enum class TouchArea {
+    Full,  // Contains both text overlay and caret image.
+    CaretImage
+  };
+  bool Contains(const nsPoint& aPoint, TouchArea aTouchArea) const;
+
+  // The geometry center of the imaginary caret (nsCaret) to which this
+  // AccessibleCaret is attached. It is needed when dragging the caret.
+  nsPoint LogicalPosition() const { return mImaginaryCaretRect.Center(); }
+
+  // Element for 'Intersects' test. This is the container of the caret image
+  // and text-overlay elements. See CreateCaretElement() for the content
+  // structure.
+  dom::Element& CaretElement() const {
+    return mCaretElementHolder->ContentNode();
+  }
+
+  // Ensures that the caret element is made "APZ aware" so that the APZ code
+  // doesn't scroll the page when the user is trying to drag the caret.
+  void EnsureApzAware();
+
+  bool IsInPositionFixedSubtree() const;
+
+ protected:
+  // Argument aRect should be relative to CustomContentContainerFrame().
+  void SetCaretElementStyle(const nsRect& aRect, float aZoomLevel);
+  void SetTextOverlayElementStyle(const nsRect& aRect, float aZoomLevel);
+  void SetCaretImageElementStyle(const nsRect& aRect, float aZoomLevel);
+
+  // Get current zoom level.
+  float GetZoomLevel();
+
+  // Element which contains the text overly for the 'Contains' test.
+  dom::Element* TextOverlayElement() const {
+    return mCaretElementHolder->GetElementById(sTextOverlayElementId);
+  }
+
+  // Element which contains the caret image for 'Contains' test.
+  dom::Element* CaretImageElement() const {
+    return mCaretElementHolder->GetElementById(sCaretImageElementId);
+  }
+
+  nsIFrame* RootFrame() const;
+
+  nsIFrame* CustomContentContainerFrame() const;
+
+  // Transform Appearance to CSS id used in ua.css.
+  static nsAutoString AppearanceString(Appearance aAppearance);
+
+  already_AddRefed<dom::Element> CreateCaretElement(dom::Document*) const;
+
+  // Inject caret element into custom content container.
+  void InjectCaretElement(dom::Document*);
+
+  // Remove caret element from custom content container.
+  void RemoveCaretElement(dom::Document*);
+
+  // Clear the cached rects and zoom level.
+  void ClearCachedData();
+
+  // The top-center of the imaginary caret to which this AccessibleCaret is
+  // attached.
+  static nsPoint CaretElementPosition(const nsRect& aRect) {
+    return aRect.TopLeft() + nsPoint(aRect.width / 2, 0);
+  }
+
+  class DummyTouchListener final : public nsIDOMEventListener {
+   public:
+    NS_DECL_ISUPPORTS
+    NS_IMETHOD HandleEvent(mozilla::dom::Event* aEvent) override {
+      return NS_OK;
+    }
+
+   private:
+    virtual ~DummyTouchListener() = default;
+  };
+
+  // Member variables
+  Appearance mAppearance = Appearance::None;
+
+  // AccessibleCaretManager owns us by a UniquePtr. When it's terminated by
+  // AccessibleCaretEventHub::Terminate() which is called in
+  // PresShell::Destroy(), it frees us automatically. No need to worry if we
+  // outlive mPresShell.
+  PresShell* const MOZ_NON_OWNING_REF mPresShell = nullptr;
+
+  RefPtr<dom::AnonymousContent> mCaretElementHolder;
+
+  // This cached rect is relative to the root frame, and is used in
+  // LogicalPosition() when dragging a caret.
+  nsRect mImaginaryCaretRect;
+
+  // This cached rect is relative to the custom content container, and is used
+  // in SetPosition() to check whether the caret position has changed.
+  nsRect mImaginaryCaretRectInContainerFrame;
+
+  // The reference frame we used to calculate mImaginaryCaretRect and
+  // mImaginaryCaretRectInContainerFrame.
+  WeakFrame mImaginaryCaretReferenceFrame;
+
+  // Cache current zoom level to determine whether position is changed.
+  float mZoomLevel = 0.0f;
+
+  // A no-op touch-start listener which prevents APZ from panning when dragging
+  // the caret.
+  RefPtr<DummyTouchListener> mDummyTouchListener{new DummyTouchListener()};
+
+  // Static class variables
+  static const nsLiteralString sTextOverlayElementId;
+  static const nsLiteralString sCaretImageElementId;
+
+};  // class AccessibleCaret
+
+std::ostream& operator<<(std::ostream& aStream,
+                         const AccessibleCaret::Appearance& aAppearance);
+
+std::ostream& operator<<(std::ostream& aStream,
+                         const AccessibleCaret::PositionChangedResult& aResult);
+
+}  // namespace mozilla
+
+#endif  // AccessibleCaret_h__