[#2410] Fixed editor field fullscreen z-index

ui/src/scss/_form.scss

@@ -1210,6 +1210,7 @@ select {
     }
     .form-field label ~ & {
         position: relative;
+        z-index: auto; // reset default z-index
         padding: 5px 2px 2px 2px;
         // blank placeholder
         &:before {
@@ -1252,4 +1253,9 @@ body {
             display: none;
         }
     }
+    &.tox-fullscreen {
+        .overlay-panel-section {
+            overflow: hidden;
+        }
+    }
 }