1 files changed, 51 insertions, 0 deletions
diff --git a/layout/base/RelativeTo.h b/layout/base/RelativeTo.h
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index 0000000000..e82802e2e8
--- /dev/null
+++ b/layout/base/RelativeTo.h
@@ -0,0 +1,51 @@
+/* -*- 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_RelativeTo_h
+#define mozilla_RelativeTo_h
+
+#include <ostream>
+
+class nsIFrame;
+
+namespace mozilla {
+
+// A flag that can be used to annotate a frame to distinguish coordinates
+// relative to the viewport frame as being in layout or visual coordinates.
+enum class ViewportType { Layout, Visual };
+
+// A struct that combines a frame with a ViewportType annotation. The
+// combination completely describes what a set of coordinates is "relative to".
+// Notes on expected usage:
+//  - The boundary between visual and layout coordinates is approximately
+//    at the root content document (RCD)'s ViewportFrame, which we'll
+//    call "RCD-VF".
+//  - Coordinates relative to the RCD-VF's descendants (other than the
+//    RCD's viewport scrollbar frames) should be in layout coordinates.
+//  - Coordinates relative to the RCD-VF's ancestors should be in visual
+//    coordinates (note that in an e10s setup, the RCD-VF doesn't
+//    typically have in-process ancestors).
+//  - Coordinates relative to the RCD-VF itself can be in either layout
+//    or visual coordinates.
+struct RelativeTo {
+  const nsIFrame* mFrame = nullptr;
+  // Choose ViewportType::Layout as the default as this is what the vast
+  // majority of layout code deals with.
+  ViewportType mViewportType = ViewportType::Layout;
+  bool operator==(const RelativeTo& aOther) const {
+    return mFrame == aOther.mFrame && mViewportType == aOther.mViewportType;
+  }
+  friend std::ostream& operator<<(std::ostream& aOs, const RelativeTo& aR) {
+    return aOs << "{" << aR.mFrame << ", "
+               << (aR.mViewportType == ViewportType::Visual ? "visual"
+                                                            : "layout")
+               << "}";
+  }
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_RelativeTo_h