Other: Update ButtonDropdown styles to PostCSS.

Author: jodator

src/buttongroup/buttongroupview.js

@@ -12,6 +12,8 @@ import FocusTracker from '@ckeditor/ckeditor5-utils/src/focustracker';
 import FocusCycler from '../focuscycler';
 import KeystrokeHandler from '@ckeditor/ckeditor5-utils/src/keystrokehandler';
 
+import '../../theme/components/buttongroup.css';
+
 /**
  * The button group view class.
  *

src/dropdown/button/createbuttondropdown.js

@@ -12,6 +12,8 @@ import createDropdown from '../createdropdown';
 import ButtonGroupView from '../../buttongroup/buttongroupview';
 import { closeDropdownOnBlur, closeDropdownOnExecute, openDropdownOnArrows } from '../utils';
 
+import '../../../theme/components/dropdown/buttondropdown.css';
+
 /**
  * Creates an instance of {@link module:ui/dropdown/button/buttondropdownview~ButtonDropdownView} class using
  * a provided {@link module:ui/dropdown/button/buttondropdownmodel~ButtonDropdownModel}.

theme/components/buttongroup.scss renamed to theme/components/buttongroup.css

@@ -0,0 +1,10 @@
+/*
+ * Copyright (c) 2003-2017, CKSource - Frederico Knabben. All rights reserved.
+ * For licensing, see LICENSE.md.
+ */
+
+.ck-dropdown .ck-button-group .ck-button {
+	&:focus {
+		z-index: calc(var(--ck-z-default) + 1);
+	}
+}