1 files changed, 36 insertions, 0 deletions
diff --git a/toolkit/themes/shared/menu-scrolling.css b/toolkit/themes/shared/menu-scrolling.css
new file mode 100644
index 0000000000..d0a62d130d
--- /dev/null
+++ b/toolkit/themes/shared/menu-scrolling.css
@@ -0,0 +1,36 @@
+/* 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/. */
+
+@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
+
+/* Scroll buttons */
+
+/* Hide arrow buttons when there's nothing to scroll in that direction */
+.menupopup-arrowscrollbox[scrolledtostart="true"]::part(scrollbutton-up),
+.menupopup-arrowscrollbox[scrolledtoend="true"]::part(scrollbutton-down) {
+  display: none;
+}
+
+/* Prevent the scrolled contents of the menupopup from jumping vertically when
+ * the arrow buttons appear / disappear, by positioning ::part(scrollbox) in
+ * such a way that its edges are at the same position as the edges of
+ * arrowscrollbox regardless of scroll button visibility.
+ */
+.menupopup-arrowscrollbox:not([scrolledtostart="true"])::part(scrollbox) {
+  /* scrollbutton-up is visible; shift our top edge up by its height. */
+  margin-top: -16px;
+}
+
+.menupopup-arrowscrollbox:not([scrolledtoend="true"])::part(scrollbox) {
+  /* scrollbutton-down is visible; shift our bottom edge down by its height. */
+  margin-bottom: -16px;
+}
+
+.menupopup-arrowscrollbox::part(scrollbox-clip) {
+  /* In the space where the arrow buttons overlap the scrollbox, clip away the
+   * scrollbox so that nothing is shown behind the arrow button even if the
+   * button is transparent.
+   */
+  overflow: clip;
+}