fix stuck loading tip

customize.dist/src/less/loading.less

@@ -36,6 +36,9 @@
     left: 0;
     right: 0;
     text-align: center;
+
+    transition: opacity 750ms;
+    transition-delay: 3000ms;
     @media screen and (max-height: @media-medium-screen) {
         display: none;
     }