1 files changed, 48 insertions, 0 deletions
diff --git a/layout/base/ScrollStyles.cpp b/layout/base/ScrollStyles.cpp
new file mode 100644
index 0000000000..6ce21aad8c
--- /dev/null
+++ b/layout/base/ScrollStyles.cpp
@@ -0,0 +1,48 @@
+/* -*- 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/. */
+
+#include "mozilla/ScrollStyles.h"
+#include "nsStyleStruct.h"  // for nsStyleDisplay
+
+namespace mozilla {
+
+// https://drafts.csswg.org/css-overflow/#overflow-propagation
+// "If `visible` is applied to the viewport, it must be interpreted as `auto`.
+//  If `clip` is applied to the viewport, it must be interpreted as `hidden`."
+static StyleOverflow MapOverflowValueForViewportPropagation(
+    StyleOverflow aOverflow) {
+  switch (aOverflow) {
+    case StyleOverflow::Visible:
+      return StyleOverflow::Auto;
+    case StyleOverflow::Clip:
+      return StyleOverflow::Hidden;
+    default:
+      return aOverflow;
+  }
+}
+
+ScrollStyles::ScrollStyles(StyleOverflow aH, StyleOverflow aV)
+    : mHorizontal(aH), mVertical(aV) {
+  MOZ_ASSERT(mHorizontal == StyleOverflow::Auto ||
+             mHorizontal == StyleOverflow::Hidden ||
+             mHorizontal == StyleOverflow::Scroll);
+  MOZ_ASSERT(mVertical == StyleOverflow::Auto ||
+             mVertical == StyleOverflow::Hidden ||
+             mVertical == StyleOverflow::Scroll);
+}
+
+ScrollStyles::ScrollStyles(const nsStyleDisplay& aDisplay,
+                           MapOverflowToValidScrollStyleTag)
+    : ScrollStyles(
+          MapOverflowValueForViewportPropagation(aDisplay.mOverflowX),
+          MapOverflowValueForViewportPropagation(aDisplay.mOverflowY)) {}
+
+bool ScrollStyles::IsHiddenInBothDirections() const {
+  return mHorizontal == StyleOverflow::Hidden &&
+         mVertical == StyleOverflow::Hidden;
+}
+
+}  // namespace mozilla