Fixes z-index bug when hovering in virtual scroller

This fixes a bug where if the user hovers the mouse over a different file
in the virtual scroller then the z-index bug for the more actions
dropdown appears again.

app/assets/javascripts/diffs/components/diff_file_header.vue

@@ -213,9 +213,9 @@ export default {
         const el = this.$el.closest('.vue-recycle-scroller__item-view');
 
         if (this.glFeatures.diffsVirtualScrolling && el) {
-          // We can't add a class with Vue because of the way the virtual
+          // We can't add a style with Vue because of the way the virtual
           // scroller library renders the diff files
-          el.classList.toggle('gl-z-index-1', val);
+          el.style.zIndex = val ? '1' : null;
         }
       },
     },