import FIREFOX_52_6_0esr_RELEASE from mozilla-esr52 hg repo

layout/generic/ScrollSnap.h

@@ -0,0 +1,41 @@
+/* -*- 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/. */
+
+#ifndef mozilla_layout_ScrollSnap_h_
+#define mozilla_layout_ScrollSnap_h_
+
+namespace mozilla {
+
+namespace layers {
+struct ScrollSnapInfo;
+}
+
+struct ScrollSnapUtils {
+  /**
+   * GetSnapPointForDestination determines which point to snap to after
+   * scrolling. |aStartPos| gives the position before scrolling and
+   * |aDestination| gives the position after scrolling, with no snapping.
+   * Behaviour is dependent on the value of |aUnit|.
+   * |aSnapInfo|, |aScrollPortSize|, and |aScrollRange| are characteristics
+   * of the scroll frame for which snapping is being performed.
+   * If a suitable snap point could be found, it is returned. Otherwise, an
+   * empty Maybe is returned.
+   * IMPORTANT NOTE: This function is designed to be called both on and off
+   *                 the main thread. If modifying its implementation, be sure
+   *                 not to touch main-thread-only data structures without
+   *                 appropriate locking.
+   */
+  static Maybe<nsPoint> GetSnapPointForDestination(
+      const layers::ScrollSnapInfo& aSnapInfo,
+      nsIScrollableFrame::ScrollUnit aUnit,
+      const nsSize& aScrollPortSize,
+      const nsRect& aScrollRange,
+      const nsPoint& aStartPos,
+      const nsPoint& aDestination);
+};
+
+} // namespace mozilla
+
+#endif // mozilla_layout_ScrollSnap_h_