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diff --git a/layout/generic/Visibility.h b/layout/generic/Visibility.h
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+++ b/layout/generic/Visibility.h
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+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* 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/. */
+
+/**
+ * Declares visibility-related types. @Visibility is an enumeration of the
+ * possible visibility states of a frame. @OnNonvisible is an enumeration that
+ * allows callers to request a specific action when a frame transitions from
+ * visible to nonvisible.
+ */
+
+#ifndef mozilla_layout_generic_Visibility_h
+#define mozilla_layout_generic_Visibility_h
+
+namespace mozilla {
+
+// Visibility states for frames.
+enum class Visibility : uint8_t
+{
+  // Indicates that we're not tracking visibility for this frame.
+  UNTRACKED,
+
+  // Indicates that the frame is probably nonvisible. Visible frames *may* be
+  // APPROXIMATELY_NONVISIBLE because approximate visibility is not updated
+  // synchronously. Some truly nonvisible frames may be marked
+  // APPROXIMATELY_VISIBLE instead if our heuristics lead us to think they may
+  // be visible soon.
+  APPROXIMATELY_NONVISIBLE,
+
+  // Indicates that the frame is either visible now or is likely to be visible
+  // soon according to our heuristics. As with APPROXIMATELY_NONVISIBLE, it's
+  // important to note that approximately visibility is not updated
+  // synchronously, so this information may be out of date.
+  APPROXIMATELY_VISIBLE
+};
+
+// Requested actions when frames transition to the nonvisible state.
+enum class OnNonvisible : uint8_t
+{
+  DISCARD_IMAGES  // Discard images associated with the frame.
+};
+
+} // namespace mozilla
+
+#endif // mozilla_layout_generic_Visibility_h