#tinymce { box-sizing: border-box; }
#tinymce * { box-sizing: inherit; }
#tinymce .editor-col-1-2,
#tinymce .editor-col-1-3,
#tinymce .editor-col-2-3 {
   border: 1px dashed #ccc;
   padding-top: 15px;
   padding-bottom: 15px;
}

.editor-grid {   
   margin-left: -15px;
   margin-right: -15px;
   overflow: hidden;
}

.editor-col-1-2,
.editor-col-1-3,
.editor-col-2-3 {
   float: left;
   padding-left: 15px;
   padding-right: 15px;
}

.editor-col-1-2 { width: 50%; }
.editor-col-1-3 { width: 33.333%; }
.editor-col-2-3 { width: 66.666%; }

@media (max-width: 767px) {
   .editor-col-1-2,
   .editor-col-1-3,
   .editor-col-2-3 { 
      width: 100%;      
   }
}