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diff --git a/layout/generic/ScrollOrigin.h b/layout/generic/ScrollOrigin.h
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+/* 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_ScrollOrigin_h_
+#define mozilla_ScrollOrigin_h_
+
+namespace mozilla {
+
+// A scroll origin is a bit of a combination of "what part of the code caused
+// this scroll" and "what special properties does this scroll have, in the
+// context of what caused it". See specific comments below.
+enum class ScrollOrigin : uint8_t {
+  // This is used as an initial value for the "LastScrollOrigin" property on
+  // scrollable frames. It is not intended to be an actual scroll origin, but
+  // a sentinel value that indicates that there was no "last scroll". It is
+  // used similarly for the "LastSmoothScrollOrigin" property, to indicate
+  // no smooth scroll is in progress.
+  None,
+
+  // This is a default value that we use when we don't know of a more specific
+  // value that we can use.
+  NotSpecified,
+  // The scroll came from APZ code.
+  Apz,
+  // The scroll came from an attempt at restoring a scroll position saved in
+  // the bfcache or history.
+  Restore,
+  // The scroll came from a "relative" scroll method like ScrollBy, where the
+  // scroll destination is indicated by a delta from the current position
+  // instead of an absolute value.
+  Relative,
+  // The scroll came from an attempt by the main thread to re-clamp the scroll
+  // position after a reflow.
+  Clamp,
+
+  // The following scroll origins also are associated with prefs of the form
+  //   general.smoothScroll.<origin>(.*)
+  // e.g. general.smoothScroll.lines indicates whether or not a scroll with
+  // origin Lines will be animated smoothly, and e.g. subprefs like
+  // general.smoothScroll.lines.durationMinMS control some of the animation
+  // timing behavior.
+
+  // The scroll came from some sort of input that's not one of the above or
+  // below values. Generally this means it came from a content-exposed API,
+  // like window.scrollTo, but may also be from other sources that don't need
+  // any particular special handling.
+  Other,
+  // The scroll was originated by pixel-scrolling input device (e.g. precision
+  // mouse wheel).
+  Pixels,
+  // The scroll was originated by a line-scrolling input device (e.g. up/down
+  // keyboard buttons).
+  Lines,
+  // The scroll was originated by a page-scrolling input device (e.g. pgup/
+  // pgdn keyboard buttons).
+  Pages,
+  // The scroll was originated by a mousewheel that scrolls by lines.
+  MouseWheel,
+  // The scroll was originated by moving the scrollbars.
+  Scrollbars,
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_ScrollOrigin_h_