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diff --git a/gfx/layers/apz/src/ScrollThumbUtils.h b/gfx/layers/apz/src/ScrollThumbUtils.h
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+++ b/gfx/layers/apz/src/ScrollThumbUtils.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_layers_ScrollThumbUtils_h
+#define mozilla_layers_ScrollThumbUtils_h
+
+#include "LayersTypes.h"
+#include "Units.h"
+
+namespace mozilla {
+namespace layers {
+
+class AsyncPanZoomController;
+
+struct FrameMetrics;
+struct ScrollbarData;
+
+namespace apz {
+/**
+ * Compute the updated shadow transform for a scroll thumb layer that
+ * reflects async scrolling of the associated scroll frame.
+ *
+ * @param aCurrentTransform The current shadow transform on the scroll thumb
+ *    layer, as returned by Layer::GetLocalTransform() or similar.
+ * @param aScrollableContentTransform The current content transform on the
+ *    scrollable content, as returned by Layer::GetTransform().
+ * @param aApzc The APZC that scrolls the scroll frame.
+ * @param aMetrics The metrics associated with the scroll frame, reflecting
+ *    the last paint of the associated content. Note: this metrics should
+ *    NOT reflect async scrolling, i.e. they should be the layer tree's
+ *    copy of the metrics, or APZC's last-content-paint metrics.
+ * @param aScrollbarData The scrollbar data for the the scroll thumb layer.
+ * @param aScrollbarIsDescendant True iff. the scroll thumb layer is a
+ *    descendant of the layer bearing the scroll frame's metrics.
+ * @return The new shadow transform for the scroll thumb layer, including
+ *    any pre- or post-scales.
+ */
+LayerToParentLayerMatrix4x4 ComputeTransformForScrollThumb(
+    const LayerToParentLayerMatrix4x4& aCurrentTransform,
+    const gfx::Matrix4x4& aScrollableContentTransform,
+    AsyncPanZoomController* aApzc, const FrameMetrics& aMetrics,
+    const ScrollbarData& aScrollbarData, bool aScrollbarIsDescendant);
+
+}  // namespace apz
+}  // namespace layers
+}  // namespace mozilla
+
+#endif  // mozilla_layers_ScrollThumbUtils_h