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diff --git a/layout/generic/ScrollSnap.h b/layout/generic/ScrollSnap.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_layout_ScrollSnap_h_
+#define mozilla_layout_ScrollSnap_h_
+
+#include <memory>
+#include "mozilla/ScrollTypes.h"
+#include "mozilla/ScrollSnapTargetId.h"
+#include "mozilla/Maybe.h"
+
+class nsIContent;
+class nsIFrame;
+struct nsPoint;
+struct nsRect;
+struct nsSize;
+
+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| 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<SnapTarget> GetSnapPointForDestination(
+ const layers::ScrollSnapInfo& aSnapInfo, ScrollUnit aUnit,
+ ScrollSnapFlags aSnapFlags, const nsRect& aScrollRange,
+ const nsPoint& aStartPos, const nsPoint& aDestination);
+
+ /**
+ * Similar to above GetSnapPointForDestination but for re-snapping.
+ *
+ * |aCurrentPosition| are the snap point(s) last time when we scrolled.
+ * |aLastSnapTargetIds| are the snap point(s) last time when we scrolled if
+ * exists.
+ * |aFocusedContent| is the focused content in the document if exists.
+ * Other parameters are same as GetSnapPointForDestination.
+ */
+
+ static mozilla::Maybe<mozilla::SnapTarget> GetSnapPointForResnap(
+ const layers::ScrollSnapInfo& aSnapInfo, const nsRect& aScrollRange,
+ const nsPoint& aCurrentPosition,
+ const UniquePtr<ScrollSnapTargetIds>& aLastSnapTargetIds,
+ const nsIContent* aFocusedContent);
+
+ static ScrollSnapTargetId GetTargetIdFor(const nsIFrame* aFrame);
+
+ // Post a pending re-snap request if the given |aFrame| is one of the snap
+ // points on the last scroll operation.
+ static void PostPendingResnapIfNeededFor(nsIFrame* aFrame);
+
+ // Similar to above PostPendingResnapIfNeededFor but post a pending re-snap
+ // request even if the given |aFrame| is not one of the last snap point.
+ // This is basically used for cases there was no valid snap point on the last
+ // scroll operation but the given |aFrame| might be a valid snap point now,
+ // e.g changing the scroll-snap-align property from `none` to something.
+ static void PostPendingResnapFor(nsIFrame* aFrame);
+
+ // Returns true if the writing-mode of the snap target element needs to be
+ // respected to resolve scroll-snap-align property.
+ // Note that usually the scroll container's writing-mode is used for resolving
+ // the property but there's a special case defined in the CSS scroll snap
+ // spec.
+ static bool NeedsToRespectTargetWritingMode(const nsSize& aSnapAreaSize,
+ const nsSize& aSnapportSize);
+
+ // Returns the scroll snap area for the snap target frame |aFrame| inside the
+ // nearest scroll container |aScrolledFrame| and its scrolled rect
+ // |aScrolledRect|.
+ static nsRect GetSnapAreaFor(const nsIFrame* aFrame,
+ const nsIFrame* aScrolledFrame,
+ const nsRect& aScrolledRect);
+};
+
+} // namespace mozilla
+
+#endif // mozilla_layout_ScrollSnap_h_