(less) Fixes twbs#13079: Scope .open class in button variant mixin to immediate children

Author: mdo

mixins/buttons.less

@@ -12,14 +12,14 @@
   &:focus,
   &:active,
   &.active,
-  .open .dropdown-toggle& {
+  .open > .dropdown-toggle& {
     color: @color;
     background-color: darken(@background, 10%);
         border-color: darken(@border, 12%);
   }
   &:active,
   &.active,
-  .open .dropdown-toggle& {
+  .open > .dropdown-toggle& {
     background-image: none;
   }
   &.disabled,