Add Mac hack for autorepeatbutton inside popups (untested)

src/chrome/os_target/mac/global.css

@@ -2,6 +2,46 @@
  * 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/. */
 
+/* autorepeatbuttons in menus */
+
+.popup-internal-box > autorepeatbutton {
+  height: 15px;
+  position: relative;
+  list-style-image: none;
+  /* Here we're using a little magic.
+   * The arrow button is supposed to overlay the scrollbox, blocking
+   * everything under it from reaching the screen. However, the menu background
+   * is slightly transparent, so how can we block something completely without
+   * messing up the transparency? It's easy: The native theming of the
+   * "menuitem" appearance uses CGContextClearRect before drawing, which
+   * clears everything under it.
+   * Without help from native theming this effect wouldn't be achievable.
+   */
+  -moz-appearance: menuitem;
+}
+
+.popup-internal-box > .autorepeatbutton-up {
+  padding-top: 1px; /* 4px padding-top from the .popup-internal-box. */
+  margin-bottom: -15px;
+}
+
+.popup-internal-box > .autorepeatbutton-up > .autorepeatbutton-icon {
+  -moz-appearance: button-arrow-up;
+}
+
+.popup-internal-box > .autorepeatbutton-down {
+  padding-top: 5px;
+  margin-top: -15px;
+}
+
+.popup-internal-box > .autorepeatbutton-down > .autorepeatbutton-icon {
+  -moz-appearance: button-arrow-down;
+}
+
+.popup-internal-box > autorepeatbutton[disabled="true"] {
+  visibility: collapse;
+}
+
 /* :::::: Close button icons ::::: */
 
 @media (min-resolution: 2dppx) {