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diff --git a/layout/generic/nsILineIterator.h b/layout/generic/nsILineIterator.h
new file mode 100644
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+++ b/layout/generic/nsILineIterator.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 nsILineIterator_h___
+#define nsILineIterator_h___
+
+#include "nscore.h"
+#include "nsINode.h"
+#include "nsRect.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/Result.h"
+#include "mozilla/WritingModes.h"
+
+class nsIFrame;
+
+/**
+ * Line iterator API.
+ *
+ * Lines are numbered from 0 to N, where 0 is the top line and N is
+ * the bottom line.
+ *
+ * Obtain this interface from frames via nsIFrame::GetLineIterator.
+ * This iterator belongs to the frame from which it was obtained, and should
+ * not be deleted by the caller.
+ * Note that any modification of the frame will invalidate the iterator!
+ * Users must get a new iterator any time the target may have been touched.
+ */
+class nsILineIterator {
+ protected:
+  ~nsILineIterator() = default;
+
+ public:
+  /**
+   * The number of lines in the block
+   */
+  virtual int32_t GetNumLines() const = 0;
+
+  /**
+   * Returns whether our lines are rtl.
+   */
+  virtual bool IsLineIteratorFlowRTL() = 0;
+
+  struct LineInfo {
+    /** The first frame on the line. */
+    nsIFrame* mFirstFrameOnLine = nullptr;
+    /** The numbers of frames on the line. */
+    int32_t mNumFramesOnLine = 0;
+    /**
+     * The bounding box of the line (which is based on the in-flow position of
+     * the frames on the line; if a frame was moved because of relative
+     * positioning then its coordinates may be outside the line bounds)
+     */
+    nsRect mLineBounds;
+    /** Whether the line is wrapped at the end */
+    bool mIsWrapped = false;
+  };
+
+  // Return miscellaneous information about a line.
+  virtual mozilla::Result<LineInfo, nsresult> GetLine(int32_t aLineNumber) = 0;
+
+  /**
+   * Given a frame that's a child of the block, find which line its on
+   * and return that line index, as long as it's at least as big as
+   * aStartLine.  Returns -1 if the frame cannot be found on lines
+   * starting with aStartLine.
+   */
+  virtual int32_t FindLineContaining(nsIFrame* aFrame,
+                                     int32_t aStartLine = 0) = 0;
+
+  // Given a line number and a coordinate, find the frame on the line
+  // that is nearest to aPos along the inline axis. (The block-axis coord
+  // of aPos is irrelevant.)
+  // The aPosIsBeforeFirstFrame and aPosIsAfterLastFrame flags are updated
+  // appropriately.
+  NS_IMETHOD FindFrameAt(int32_t aLineNumber, nsPoint aPos,
+                         nsIFrame** aFrameFound, bool* aPosIsBeforeFirstFrame,
+                         bool* aPosIsAfterLastFrame) = 0;
+
+  // Check whether visual and logical order of frames within a line are
+  // identical.
+  //  If not, return the first and last visual frames
+  NS_IMETHOD CheckLineOrder(int32_t aLine, bool* aIsReordered,
+                            nsIFrame** aFirstVisual,
+                            nsIFrame** aLastVisual) = 0;
+};
+
+namespace mozilla {
+
+// Helper struct for FindFrameAt.
+struct LineFrameFinder {
+  LineFrameFinder(const nsPoint& aPos, const nsSize& aContainerSize,
+                  WritingMode aWM, bool aIsReversed)
+      : mPos(aWM, aPos, aContainerSize),
+        mContainerSize(aContainerSize),
+        mWM(aWM),
+        mIsReversed(aIsReversed) {}
+
+  void Scan(nsIFrame*);
+  void Finish(nsIFrame**, bool* aPosIsBeforeFirstFrame,
+              bool* aPosIsAfterLastFrame);
+
+  const LogicalPoint mPos;
+  const nsSize mContainerSize;
+  const WritingMode mWM;
+  const bool mIsReversed;
+
+  bool IsDone() const { return mDone; }
+
+ private:
+  bool mDone = false;
+  nsIFrame* mFirstFrame = nullptr;
+  nsIFrame* mClosestFromStart = nullptr;
+  nsIFrame* mClosestFromEnd = nullptr;
+};
+
+}  // namespace mozilla
+
+/**
+ * Helper intended to be used in a scope where we're using an nsILineIterator
+ * and want to verify that no DOM mutations (which would invalidate the
+ * iterator) occur while we're using it.
+ */
+class MOZ_STACK_CLASS AutoAssertNoDomMutations final {
+#ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+  nsMutationGuard mGuard;
+#endif
+ public:
+  ~AutoAssertNoDomMutations() { MOZ_DIAGNOSTIC_ASSERT(!mGuard.Mutated(0)); }
+};
+
+#endif /* nsILineIterator_h___ */