Bug 874792 - [Australis] Make the PanelUI popup slide instead of flip when it doesn't fit on the screen. r=Gijs

browser/components/customizableui/content/panelUI.inc.xul

@@ -6,6 +6,7 @@
        role="group"
        type="arrow"
        hidden="true"
+       flip="slide"
        noautofocus="true">
   <panelmultiview id="PanelUI-multiView" mainViewId="PanelUI-mainView">
     <panelview id="PanelUI-mainView" context="customizationPanelContextMenu">