use the classic naming scheme

src/components/mobile_post_status_modal/mobile_post_status_modal.vue

@@ -20,7 +20,7 @@
     </div>
     <button
       class="new-status-button"
-      :class="{ 'new-status-button--hidden': isHidden }"
+      :class="{ 'new-status-button_hidden': isHidden }"
       @click="openPostForm"
     >
       <i class="icon-edit" />
@@ -69,7 +69,7 @@
   transition: 0.35s transform;
   transition-timing-function: cubic-bezier(0, 1, 0.5, 1);
 
-  &--hidden {
+  &_hidden {
     transform: translateY(150%);
   }