Prevent dropdown arrow from flipping in modern Vector

Bug: T314669
Change-Id: Icaae181cac1b399d9c0e90c55c7ee36982b638d3

resources/common/components/MenuDropdownCheckbox.less

@@ -81,11 +81,6 @@
 		display: block;
 	}
 
-	// flips the chevron so it points up when the dropdown is open.
-	&:checked + .vector-menu-heading:after {
-		transform: scaleY( -1 );
-	}
-
 	// Add focus state to menu dropdown buttons (i.e. #p-variants, #p-cactions)
 	&:focus + .vector-menu-heading {
 		// Simulate browser focus ring

resources/skins.vector.styles.legacy/components/MenuDropdown.less

@@ -2,6 +2,11 @@
 @import 'mediawiki.mixins.less';
 @import '../../common/mixins.less';
 
+// Flips the chevron so it points up when the dropdown is open.
+.vector-menu-checkbox:checked + .vector-menu-heading:after {
+	transform: scaleY( -1 );
+}
+
 /**
  * Dropdown menus that only appear in the tab bar in legacy Vector.
  */